Photography 101 – Light and the Pinhole Camera

By: Neil Creek 

Welcome to the first lesson in Photography 101 – A Basic Course on the Camera. In this series, we cover all the basics of camera design and use. We talk about the ‘exposure triangle’: shutter speed, aperture and ISO. We talk about focus, depth of field and sharpness, as well as how lenses work, what focal lengths mean and how they put light on the sensor. We also look at the camera itself, how it works, what all the options mean and how they affect your photos.
This week’s lesson is Light and the Pinhole Camera

About Light

Light is everywhere, even where you can’t see it. Without delving too deeply into the mysterious physics of light, there are some basics that are helpful to know as a photographer:

• We only see light when it reflects from something, or we look directly at the source
• Light can be bounced (reflected) or bent (refracted) and always does so in predictable ways
• Reflected light scatters depending on the “smoothness” of the surface
• White light is made up of all of the visible colours
• Different coloured light has different energy levels or “wavelengths”
• Shadows occur when something comes between a light source and another surface
• Light varies hugely in brightness, and our eyes very cleverly adapt to see clearly in a wide variety of brightnesses
• Cameras are far less capable of “seeing” clearly in as wide a variety of lighting conditions

Fig 1.1.1 Light hitting a textured surface scatters in all directions, like water splashing from a thrown water balloon. Click for clearer version.

These observations are pretty basic, and most of them are either obvious or should have been taught in the science classroom. As straightforward as they are, these basic points are at the heart of photography, and understanding them is very important. Throughout this course, and through your adventures with the camera, you will be working with some or all of these essential principles of light. If you are unclear about any of the above points, it would be helpful to do a little bit of independent research. You’ll find some good links for further reading at the end of today’s lesson, and I encourage you to explore further by searching the ‘net.
The central point of photography is turning light into an image. An image is actually just an illusion. Anything that is a representation in light of a real thing is an image. So how do we turn light, which scatters randomly around the universe into an image that we can recognise?

The Camera Obscura

Fig 1.1.2 – Imagine a typical outdoor scene with the sun shining brightly on a tree. The light from the sun travels in parallel rays, shown here in black. When they hit the tree, the light is scattered in all directions, shown in grey. This is known as “diffuse” light. Because of diffuse light you can see the pretty tree, as some of this scattered light hits your eyes.



Fig 1.1.3 – Now lets imagine we pitch a tent with perfect light blocking material and a tiny hole in one wall. To keep it simple, a single ray hitting the tree will diffuse in all directions, but only a very tiny sliver of that light will go through the small hole. If you were sitting in this tent in the middle of the floor and you closed one eye and looked through the hole from there, you would only see a very tiny part of the tree at once. If you move to the right a bit, you will see the left of the tree. If you move up, you will see further down the tree. Your view of the tree is opposite to the direction of your movement.



Fig 1.1.4 – Of course there is more than one ray of light hitting the tree. These rays reflect diffusely in all directions. This is why you can see more of the tree by moving around inside the tent. If we were to set up a screen in the tent opposite the hole, which is made of nice bright white material, we would actually be able to see an image of the tree, projected onto it! For the reason that we saw parts of the tree in the opposite direction when we moved, the projected image of the tree appears upside down.

Fig 1.1.5 A camera obscura made by blocking a window. Photo by brighterorange, used under creative commons.

The room we just made in our imagination is called a “camera obscura” and you can make one just as I have described. Instead of a tent, you could find a room in your house with one window, and cover the whole window with thick card or foil. Then make a tiny hole (1-5mm) in the cover and look at the opposite wall. You should see a very dim, upside down image of the world outside.
The image will be dim because only a small amount of light can pass through the hole. If you make the hole bigger, the image will be brighter, but less sharp. Why is this? A small hole is very good at restricting the direction from which rays of light can enter the room. If the hole is bigger, then more light can get in, but that extra light comes from a bigger range of angles, and it overlaps nearby parts of the projected image.

Fig 1.1.6 – A small hole through which the light passes, restricts the possible angles of light reflecting from a certain part of the tree. The overlap of light rays on the projection is very small and the result is a sharp image. The trade-off is that the image is dim.

Fig 1.1.7 – A large hole allows a greater variety of angles of reflected light from a particular part of the tree to pass through onto the projection. This means that much light from neighbouring parts of the tree overlap each other. This results in lower contrast and a blurry image. The benefit however, is that more light can get through and the projected image is brighter.

How is it relevant?
The trade-off between sharpness and brightness should be a familiar one to many photographers. The “hole” of the camera obscura is the aperture of a modern camera. When you open up the aperture you let more light into the camera, but the depth of field narrows, and more of the image drops out of focus. We’ll talk more about this in a future lesson.

The Pinhole Camera

The camera obscura is a very old idea, first built around 1000AD in what is now Iraq. A variation on the concept is the pinhole camera. This shrinks the room of the camera obscura into a handheld box, and is the simplest possible camera. Pinhole cameras can easily be made of almost anything. All it needs is a light-proof box, something to project the image onto, and a tiny hole. There are countless projects online to make your own pinhole camera, or make a pinhole “lens” for your DSLR. Due to the tiny hole used in these cameras, one advantage is that they have is an enormous depth of field.

Homework

To help you get a feel for the issues discussed in each lesson, I’m going to be assigning some homework. Naturally it’s completely voluntary, but you learn by doing, and practical exercises will certainly help you improve your photography. I list a few assignments below, and you are welcome to do any or all of them. Please post links to your completed homework assignments in the comments on this post.

• Make a camera obscura – Whether it be a tent or a room of your house, build and photograph a camera obscura in action.
• Visit a camera obscura – There are a handful of publicly accessible camera obscuras around the world as tourist attractions. Visit one and take photographs.
• Make a pinhole camera – There are many projects online to build pinhole cameras from matchboxes, sardine tins, or various other materials. Make one yourself and share photos you take with it.
• Make a pinhole attachment for your DSLR – By drilling hole in a body cap for your DSLR camera, putting some foil over the hole and making a tiny hole in the foil, you can turn any DSLR into a pinhole camera. Show us your modification and photos you took with it. Have a look at my attempt at this.
• Project the sun – This technique for viewing a solar eclipse can work all year round. Put a pin hole in a piece of card, and project the sun onto another piece of card. You will be able to see clouds pass in front of the sun, and maybe even observe sunspots. Share photos of your experiments.

Resources

• Specular vs Diffuse Reflection
• Camera obscura on wikipedia
• Pinhole photography on photo.net
• How to make a pinhole Polaroid camera on Make Magazine
• Making a pinhole “lens” for your DSLR
• Pinhole photography group on Flickr

Next Week

Photography 101 – Lenses and Focus.
In addition to posting his Project 365 photos to his blog, Neil also runs a monthly photography project. This month’s topic is Iron Chef Photography – The Fork.



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Photography 101 – Lenses and Focus

By: Neil Creek

Welcome to the second lesson in Photography 101 – A Basic Course on the Camera. In this series, we cover all the basics of camera design and use. We talk about the ‘exposure triangle’: shutter speed, aperture and ISO. We talk about focus, depth of field and sharpness, as well as how lenses work, what focal lengths mean and how they put light on the sensor. We also look at the camera itself, how it works, what all the options mean and how they affect your photos.
This week’s lesson is Lenses and Focus

Bending Light

Last week we discussed how we can use a tiny hole to direct light so that it forms an image. All that a pinhole camera does is excludes all the light that doesn’t make an image. As we learned, however, the problem with that technique, is that it results in very dim images. As photographers we want bright images, and although that may seem obvious, we’ll discuss why in detail in a later lesson. Fortunately, there is a better way to do it.

Fig 1.2.1 A light shone into a glass
tank of water bends. Source.
Fig 1.2.2 As light passes into a more
refractive material, it slows and bends.

As we touched on briefly in Lesson 1, light is a form of energy that can be bent. Bending light is called refraction. What happens when light is refracted is that it actually slows down. It’s a common misconception that light always travels at the same speed. In fact, the speed of the light depends on the type of material that it is travelling through. The really useful thing about refraction is that it can bend the path of light.
I don’t want to get into the mysterious “dual nature of light”, but remember that light can be seen as a series of waves. Line after line of these waves make up light, similar to waves hitting a beach.

Imagine we have a fishtank of water and a torch. For the sake of simplicity lets also imagine that we can see the beam clearly in the air and water. When you shine the torch at the surface of the water at an angle, from the side of the tank, you can see that the beam has been bent, see Fig 1.2.1. The many wavefronts of the light are aligned perpendicular with its direction of travel. When the wavefronts encounter the water, one part of the front hits it before the rest. The part that has entered the water and slows down, while the rest of the wave is still travelling at the same speed. The effect of this is to bend the beam. See Fig 1.2.2.
Okay that’s enough physics for now. Lets talk optics.

Lenses

This bending of light can be very useful! Lets say we wanted to concentrate all the light from a wide beam onto a narrow point. If we can direct each beam of light by bending it slightly – a little right for the light in the left side of the beam, a little left for the light in the right side of the beam – then we should be able to focus the light. This is exactly what a lens does.

There are two main factors that determine how much a lens bends the light. The refractive index of the material, which is how much it slows down the beam, and the angle of incidence. The angle of incidence (or incident angle) is how far from perpendicular the light beam is when it passes through the surface. The greater the angle, the more the bending. This is why wide-angle lenses, that need to bend the light a long way, have such a bulging appearence.

Fig 1.2.3 How much the light beam is bent depends on the angle at which it hits the lens (all other things being equal). Light passing through the very centre of the lens is unaffected, while those at the edge are bent the most. This is why lenses are curved.

Fig 1.2.4 Different shaped lenses focus the light at different distances. This is the focal length of that lens.

A simple experiment

Click for larger version

Fig 1.2.5 An everyday magnifying glass can create an image. In a darkened room, set up a candle, a magnifying glass and a sheet of paper as a screen. With the magnifying glass squared up with the cangle and the screen, slide the glass and screen backwards and forwards until you bring an image of the candle into focus. Just as with the pinhole camera, the image projected by the lens us upside down. Notice that the shadow of the glass is dark except for the candle, even though the magnifying glass is see-through. This is because all of the light that passed through the glass has been focused into the image.

Fig 1.2.6 Click for larger version

Fig 1.2.7 Click for larger version

Not all lenses are equal
It’s not always the case that focal length equals lens length, as the complex optics in modern lenses can give a “virtual” focal length while keeping the actual lens size small. As a rule of thumb, the focal length isusually pretty close to the actual length of the light path through the lens.

Focusing

So far, we’ve been imagining a perfect beam of light hitting a refractive surface. In this beam all the light is parallel. Parallel light passed through a lens will always converge on the same point. The distance from the surface of the lens to the focus point is called the focal length and is measured in milimeters. Most lenses are described by their focal length. Zoom lenses have a range of focal lengths, a feat which is accomplished by using a complex series of lenses which can be moved relative to each other. The mm number translates into a real distance, from the front of your lens to the chip of your camera. In that way you can tell that a 400mm telephoto lens will be much longer than a 24mm wide-angle, without even looking at the lens.
If an object is close to a lens, even several hundred meters away, its reflected light entering the lens isn’t perfectly parallel. The closer the object to the lens, the less parallel, and the more the lens must be moved in order to keep focused. This change is much more noticable when objects are very close to the camera, and is one of the reasons why the depth of field in macro photos is so small – a point we will return to in a future lesson.

Fig 1.2.6 The closer an object is to a lens, the more its focus point moves, and so the more the lens must be moved to compensate.

In order to keep the image of a close object sharp, the lens must be moved relative to the screen (or camera sensor). This process is called focusing. When you are focused on an object at a certain distance, then objects which are closer or more distant than that will not be in focus. The situation can be helped somewhat, by reducing the size of the lens, just like we did with the pinhole camera, to restrict the variety of angles of light entering the lens. But we again are faced with the loss of brightness as a result.
We’ve hinted at the main reasons to use a lens: to make an image brighter and to make it bigger (or smaller!). Next week we’ll take what we have learned about lenses and see how we can use that to understand the concepts of focal length and f-ratios, and how they translate into maginification and image brightness.

Homework

I was disapointed at how few of you submitted homework for last weeks lesson. In fact, nobody did! Peter Emmett deserves some extra credit however, for his DSLR body cap pinhole camera photo taken coincidentally the weekend before the first lesson. This week’s lesson is challenging for setting homework, so I’d like to encourage you to experiment and think of how you can apply what you have learned here. Here’s some suggestions:

• Project an image with a magnifying glass or a lens from your camera gear and take a photo of it. If you want to get really creative about it, be inspired by this spectacular example seen recently on Strobist.
• Find and photograph examples of light refracting in everyday objects. The clearer the example the better. For example the classic pencil in a glass of water, or maybe play with some large crystals from a jewelery box.
• Shoot some natural lenses. Drops of water can be creatively used as little magnifying glasses to show an inverted image of the scene beyond them. This would be a good exercise for lovers of macro photography.

Resources

• Lenses (optics) on Wikipedia
• Refraction – Ch4 of Optics by Benjamin Crowell.
• Refraction group on Flickr

Next Week

Photography 101 – Lenses, Light and Magnification.
In addition to posting his Project 365 photos to his blog, Neil also runs a monthly photography project. This month’s topic is Iron Chef Photography – The Fork.

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Photography 101 – Lenses, Light and Magnification

By: Neil Creek

Welcome to the third lesson in Photography 101 – A Basic Course on the Camera. In this series, we cover all the basics of camera design and use.
We talk about the ‘exposure triangle’: shutter speed, aperture and ISO. We talk about focus, depth of field and sharpness, as well as how lenses work, what focal lengths mean and how they put light on the sensor.
We also look at the camera itself, how it works, what all the options mean and how they affect your photos.
This week’s lesson is Lenses, Light and Magnification
Last week we looked at the basics of the lens. We saw how lenses bend light by slowing it down, how the angle the light enters the lens affects how much it is bent, and how we can use this property to bring light that enters a lens into focus and create a bright, clear image.

Reading Optics Diagrams
Throughout this series, I will be using optics diagrams to illustrate various concepts. To help you get up to speed, I’ve written a short introduction on how to read these diagrams. I recommend you pausing the lesson for a moment to learn how to read and understand these diagrams.

The power of lenses

The advantage that lenses gives us over pinhole cameras is twofold: brightness and magnification.

Brightness and f-ratios

We saw in lesson two, with our experiment with the candle and the magnifying glass (Fig 1.2.3), that all the light that entered the lens from the candle was focused into the image. If we substituted a larger lens with the same focal length, then more light would be focused, and the resulting image would appear brighter, but no bigger.
It seems logical that if you double the diameter of a lens, you’ll double the size of the image it makes, but as you can see in Fig 1.3.1 below, that’s not true.

Fig 1.3.1 Doubling the diameter of the lens halves the f-ratio (see below) and collects more light but does not change the size of the image, which is a function of focal length (also see below). Doubling the diameter actually more than doubles the brightness of the image, as the light collecting sufrace of the lens increases rapidly as the radius increases (per the formula Πr² – pi times the radius squared).

Fig 1.3.2 The f-ratio indicated on a 50mm f1.8 lens.Fig 1.3.3 The f-ratio indicated on an 80-400mm f4.5-5.6 zoom lens.

F-ratio

In photography there’s a handy number used to describe the relationship between lens diameter and focal length: the “f-ratio”. Simply put the f-ratio is the focal length divided by the diameter. In Fig 1.3.1 above we have a lens with a focal length of 50mm and a diameter of 10mm. 50/10=5 which gives us an f-ratio of 1/5 or f5. If the lens was still 50mm focal length with a 20mm diameter, it would be f2.5.
The f-ratio for an SLR lens should always be written on the lens somwhere. Most compact cameras also describe the f-ratio somewhere on the body. The “shorter” the f-ratio, that is the closer it is to 1, the brighter the image the lens will produce. The term “speed” is also used to describe a lens. The word speed in this case refers to how fast the lens will allow the camera to capture an image, given the amount of light available. If the lens produces a bright image, then the shutter can be open for a shorter time to capture enough light to make an image. Thus a short f-ratio lens like f1.8 is considered a very “fast” lens, while a longer lens such as an f8 or f11 is a “slow” lens.
Recalling lesson 1, we learned that a large hole for the light to pass through makes for a brighter but less sharp image. Now that we know about f-ratios, we can connect these two facts together and understand why faster lenses have a narrower “depth of field” – the area which is in focus. We’ll talk more about this in the next lesson, but it’s helpful to connect the dots and see how all these various principles fit together.
Modern cameras allow a photographer to have some level of control over a lens’ speed by adjusting the aperture, we’ll also cover that in more detail in the next lesson.

Magnification and Field of View

Anyone who has played with a magnifying glass knows that, as the name suggests, lenses magnify. The amount of magnification depends on the focal length. The “longer” the lens, the more it magnifies the image. Short focal lengths have the opposite effect, reducing the size of the image.

Fig 1.3.4 All other things being equal, as the focal length of the lens increases, the relative size of the image also increases.
We saw above that f-ratio affects the image brightness. The two factors in the ratio are lens diameter and focal length. So far we have only talked about changing the lens diameter, but with greater magnification you increase the focal length, so you also increase the f-ratio. This means that the more you magnify the image, the dimmer it becomes. Most telephoto (long focal length) lenses have large f-ratios, and are therefore slow lenses. The exception of course are the hugely expensive and very heavy, giant lenses used by sports photographers. These use long focal lengths, and big diameter lenses. These telephotos are not for the casual photogrpaher!

Fig 1.3.5 A macro zoom lens showing magnification factors on the barrel.
Photo: Martini Captures used under CC license

We’ve talked about how lenses make the image bigger, and that’s certainly how it appears when you look through the viewfinder, or at the print from a telephoto lens. In reality, because most objects are distant, and the sensor is small, the vast majority of lenses produce an image which is smaller than the object itself. There are some specialist lenses, however, which do make an image larger than the subject. For this to be possible, the focal length needs to be long and the subject close. These are, of course, macro lenses.
Macro lenses will often be described by their “magnification factor”. A lens with a 1:1 magnification factor produces a projected image on the sensor which is the same as the subject. So the image of a 20mm diameter coin will span 20mm of the physical sensor, resulting in an image which will nearly fill the entire frame of a typical DSLR. A 1:1 magnification factor is usually considered the minimum for a lens to be described as a “macro” lens. Specialist macro lenses are often 1:3 or even 1:10 magnification factors, meaning that 1mm across the subject becomes 3mm or 10mm when projected onto the sensor, thus 3 or 10 times magnification.
Field of View
The final variable in this initially confusing balancing act of optics is the field of view. This refers to how much of the world the camera can see. A lens’ field of view depends on the focal length of the lens and the size of whatever the image is projected onto. In the case of digital cameras this is the sensor chip.

Fig 1.3.6 As the focal length increases, the field of view narrows and the image enlarges.

Fig 1.3.7 An example photo taken with an 8mm fisheye on a 1.6x cropped sensor. Fig 1.3.8 The comparative differences in frame size from compact camers through film and DSLR to medium format.

Fig 1.3.6 makes it obvious that at the wide-angle end, a slight difference in focal length translates to a large difference in field of view. The difference in field of view between a 10mm and 20mm lens is far greater than the difference between 210mm and 220mm. Some lenses can have exceptionally short focal lengths and wide fields of view, such as 12 or 8mm. These are fisheye lenses, so-called because the front of the lens bulges so much it looks like a fish’s eye. These lenses can have a 180 degree field of view, or even greater.
The size of the sensor also contributes to the field of view of a particular lens. In Fig 1.3.6 a particular sensor is shown at different focal lengths. Obviously if the sensor is smaller, it can see less of the image presented by the lens, thus the field of view is reduced and magnification is increased. This is the case for “cropped sensor” DSLRs, and compact cameras.
The “standard” frame size is 35mm, the size of a single picture on a roll of film. Cameras with this sized sensor are known as a “full frame” cameras. Large format film cameras exist with much larger film sizes, such as 150mm x 100mm. Less expensive, or earlier model DSLRs use sensors smaller than a 35mm film frame, and are referred to as cropped sensors. A typical cropped sensor may be described as a 1.6x, meaning that the apparent focal length of a particular lens is 1.6 times longer. Compact cameras use the smallest frame sizes of all, and as such require very short focal length lenses to get wide angle views.

Next Week

Photography 101 – Aperture and stops.
Now that we’ve pulled together the main theory behind the lens and creating an image, we’ll next turn our attention to exposure and how we control the capture of an image. Next week will see the introduction of the exposure triangle, an explanation of “stops” and brightness levels, and a look at the first point on the triangle: aperture.

Homework

• Find out your lenses’ field of view. Using whatever method works best for you (eg a tape measure on a wall), work out the field of view of your camera at the widest and longest settings. Measure it in degrees from side to side. Present photos of your findings.
• Shoot your entire focal range. Find an appropriate subject (eg an urban street or distant tree) and take a series of photos starting at your widest angle and zooming in at say 20mm intervals to your longest zoom. Compile them into a single image and post.
• Exploit magnification and field of view artistically. Take a photo at each extreme of your camera’s focal length range, carefully selecting the subject to take advantage of the magnification and field of view. Share the results here.
• Get up close and personal. This is ideal for users of compact cameras, which due to the optics of a small camera system are capable of focusing very close. Experiment with macro photography and show us photos of the world of the very small. Use macro mode if you have it (usually identified with a tulip symbol). DSLR users with macro gear may also participate.
• If all this is new to you, find an online camera store (for example the Canon or Nikon sites) and browse the lens catalogue. Pay close attention to the lens specifications we have discussed, and see how the shape and form of the lens matches up with these numbers. Look at how long telephoto lenses are, how wide fast lenses are, and how much ultra wide angle lenses bulge out the front.

Resources

• Canon lenses
• Nikon lenses
• Sigma lenses
• Tamron lenses
• Angle of view on Wikipedia
• Interactive focal length comparison on Canon
• Field of view calculator

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Photography 101.4 – Exposure and Stops

By: Neil Creek

Welcome to the fourth lesson in Photography 101 – A Basic Course on the Camera. In this series, we cover all the basics of camera design and use. We talk about the ‘exposure triangle’: shutter speed, aperture and ISO. We talk about focus, depth of field and sharpness, as well as how lenses work, what focal lengths mean and how they put light on the sensor. We also look at the camera itself, how it works, what all the options mean and how they affect your photos.
This week’s lesson is Exposure and Stops
So far we have covered the basics of turning light into an image, starting with the concept of the pinhole camera, then introducing lenses and how they focus light, and last time about how the qualities of the lens affect the size and brightness of the projected image. So far this has all been pretty theoretical, but it’s important groundwork to helping you internalise how the camera works. When you know these basics, you can get to the solutions of photographic problems that much faster.

Some Housekeeping Lesson naming
I’ve updated the naming scheme for this series to include the lesson number as a “point” value after 101. This will help you keep track of where you are up to in the series.
What happened to aperture?
Last time I said that this lesson would cover aperture and stops. I think introducing exposure and stops as a concept is important enough to dedicate a whole lesson to. Aperture will be now be discussed in Lesson 5.

This lesson we’re finally going to start getting a bit more practical. You will learn about the brightness of light, and how it is controlled. Of all the fundamentals of photography, this is probably the most important to understand, and can be the most intimidating because of the terminology used. But fear not! The mysteries of exposure and stops are about to be revealed!

What is “exposure”.

In the simplest terms, exposure is: “is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph” (Wikipedia).
Whether it is a digital sensor chip or grains of chemically dosed silver on a film, it is the same thing. The greater the amount of light that falls onto a particular region of the photographic medium, the brighter that part of the recorded image will be when reproduced, whether on screen, print or slide.

What We’ve Covered Introduction
Lesson 1
Light and the Pinhole Camera
Lesson 2
Lenses and Focus
Lesson 3
Lenses, Light and Magnification

The variation of brightness in the real world is absolutely huge, much more than you might think from your subjective experience of it. A subject lit by the midday sun on a beach looks over four thousand times brighter to your camera than the same subject lit by the quarter moon! (fredparker.com)
Brightness is measured in “Exposure Value” or EV.
You might recognize this acronym from your camera’s settings or manual. An EV of 0 is defined an image exposed for 1 second at f1. Steps of one up or down from zero are a change in the light by a factor of two. So an EV of 1 is twice as bright, EV 3 is eight times as bright, and EV -2 is one quarter as bright.

Fig 1.4.1: From a base exposure, the exposure is increased and decreased in one stop steps to +/- 4 stops.

The “Stop”

A step up (doubling) or down (halving) by one EV is called a “stop”.
If you only come away from this lesson having learned one thing, it is this. Photographers talk about light and exposure settings in terms of stops. In photography a stop can refer to different settings in any of the three points of the exposure triangle (more below). One of the most important and useful things you can learn as a photographer is to get an intuitive feel for light levels.
I’m not suggesting that you should be able to walk onto a location and immediately be able to assess the EV of the light and determine the correct exposure settings (although some very experienced photographers can do just that!) – that’s what your exposure meter is for. However, if you can learn to look at a photo you have taken on the back of your camera, and see that the exposure needs to be increased by say 2/3 of a stop, then you will become a much more efficient and successful photographer.

Controlling Exposure

To accommodate the huge variety of brightness levels we see in the real world, we need to be able to control how much light gets to the camera’s sensor. We do this by adjusting one or more of the three points of the “exposure triangle”. These three points are ISO, Shutter and Aperture.
The aperture is an adjustable iris or opening that can be made wider to let in more light, or narrower to let in less. The shutter is the “gate” that allows light onto the sensor, and it can be left open for different lengths of time, to let the sensor collect more or less light. Finally, the ISO once referred to the sensitive to light of the film in the camera. In digital cameras it refers to the “gain”, or amplification of the information collected by the sensor. In film days, changing ISO meant changing films. Today the ISO can be easily adjusted with a dial.

Fig 1.4.2: The exposure triangle.

Each of these points will be the subject of future lessons in Photography 101. For now, you need to know that they are there, and that they all work together to control the exposure. At the centre of the exposure triangle is your camera’s light meter. It is by reading this that you determine how to set each of the three points. We’ll cover that in a future lesson as well, probably in Photography 102 – A Basic Course in Taking Photos.

Trade-Offs

Each method of controlling exposure does so in a different way, and as such, has a different effect on the character of the resulting photo. Increasing the shutter speed reduces the light, and freezes motion. Decreasing it allow more light in, but blurs movement occurring while the shutter is open. Closing the aperture decreases the light, but increases the depth of field, meaning sharp focus over more of the image. Opening the aperture lets in more light, but decreases the depth of field, meaning a narrower window of sharp focus. Increasing the ISO amplifies the light collected, but also amplifies the random noise in the chip, which can become visible in photos at higher settings.
It’s important to note that all of these effects can be used for creative purposes in photography. Having a narrower depth of field for example can be an artistic effect in a portrait, a slower shutter speed can convey a feeling of movement. Taking a good photo is the result of the conscious choice of the three points on the exposure triangle in order to get a well exposed image which has a character pleasing to the photographer. Adjusting the settings is a balancing act that affords huge creative options to the photographer.

Homework

• Put your camera into manual mode, and find the controls to adjust each of the three exposure triangle points: ISO, Shutter and Aperture.
• Set your camera to full auto, find various scenes, and “half press” the shutter and see what exposure settings it recommends. Now go to manual, make the settings using the manual control and take the photo. Repeat until you feel comfortable adjusting the manual settings.
• Using the technique above, see what the camera recommends for various scenes, then adjust the photo up or down one stop of exposure with each of the exposure controls. Note the difference in the appearance of the resulting photos.
• For those already familiar with manual control, find difficult, high contrast scenes (eg: a wall with a window outside, or under a shady tree on a sunny day). Shoot the scene on auto, then looking at the result, try to guess how much you need to adjust the exposure manually to see detail in the over or under exposed areas. Practice to see how close you can get just by estimating.
• Apply these lessons artistically. Experiment with deliberately over or under exposing photos (high and low-key photos) or tweaking the auto exposed settings to emphasise otherwise under or over exposed areas.
• If you wish to share your homework photos, upload them somewhere (eg: your Flickr account) and post a link to them in the comments on this post below.

Resources

• Exposure (photography) – Wikipedia
• The Ultimate Exposure Computer – fredparker.com
• Master the Manual Mode – PhotoAficianado.com
• Bright Ideas For Shooting In Midday Sun – DPS
• BOOK – Understanding Exposure – Bryan Peterson
• BOOK – Nature Photography Field Guide – John Shaw

Next Lesson

Photography 101.5 – Aperture
An overview of the first point on the triangle: aperture, including creative applications.

Any Questions?

I want to help my students learn, so I am always happy to answer any questions you may have. I’d also love to make friends with you, and perhaps talk about better ways to teach what I know, and learn more myself. So far I have been answering questions posted in the comments on each lesson. This time I’d like to experiment with using Twitter to take your questions and suggestions. So if you’d like to tweet me, then please feel free to add me to your twitter ‘following’ list:
Follow me on Twitter, username: neilcreek
I also strongly encourage you to participate in some extra curricular activities to develop your photography skills, and the monthly photography projects I run at my blog are ideal for this! While you’re there, please feel free to check out some of my other photography posts, and if you like it, please subscribe to my feed! I also am part of the Fine Art PhotoBlog, where I sell my photography as high quality fine art prints along with six incredibly talented photographers.
See you next lesson and good luck with your homework!

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Photography 101.5 – Aperture

By: Neil Creek

Photo: Rainer Ebert used under CC license

The following post is from Australian photographer Neil Creek who is part of the Fine Art Photoblog, and is developing his blog as a resource for the passionate photographer.
Welcome to the fifth lesson in Photography 101 – A Basic Course on the Camera. In this series, we cover all the basics of camera design and use. We talk about the ‘exposure triangle’: shutter speed, aperture and ISO. We talk about focus, depth of field and sharpness, as well as how lenses work, what focal lengths mean and how they put light on the sensor. We also look at the camera itself, how it works, what all the options mean and how they affect your photos.
This week’s lesson is Aperture.
Here’s What We’ve Covered Previously in this series:
Lesson 1: Light and the Pinhole Camera
Lesson 2: Lenses and Focus
Lesson 3: Lenses, Light and Magnification
Lesson 4: Exposure and Stops
In previous lessons we have talked about the basic theory of how a camera works, including some basic optics, and introduced the idea of exposure and how we control it with the exposure triangle. In this lesson we will be drawing upon what we have learned to understand the first point on the exposure triangle – aperture – and how it works to create your photo.

Aperture

Fig 1.5.1 The iris opens and closes to change the aperture.
Based on (source-http://www.camerarepairer.co.uk/Glossary.htm)

The word aperture simply means “an opening” (reference.com). In the case of photography, the aperture is created by an adjustable iris that can be opened or closed to control how much light enters the camera. This iris is made of a series of thin metal blades that move together to create a roughly circular opening of variable size. In most DSLR cameras, the iris is built into the lens itself. It is the opening in this iris that is actually the aperture.
When adjusting the size of the aperture, we describe “opening” the aperture up and “stopping” it down. That simply refers to making the hole wider or narrower. A photographer may say they are “shooting wide”, meaning they have opened the aperture a lot, or they may say they are “stopped way down”, meaning the aperture has been closed a lot.

f ratio revisited

In Lesson 3, we discussed the f ratio, and described that as the focal length of the lens divided by its diameter. This is the focal ratio. For a single lens, the f ratio is always the same. However, with our adjustable aperture, we can do a very neat trick. The aperture acts on the lens as if it is cutting away the part of the lens which is covered. So as we stop down the aperture, we effectively make the lens smaller, and thus change the f ratio of the lens. As such, the size of the aperture is described by the f ratio that it creates. A wide aperture may be f2.8, a narrow aperture may be f22.
As we discussed in lesson four, we measure the change in exposure with stops – a doubling or halving of the light, and fractions of stops. As such, the same measure applies to aperture. To double the light getting through a lens, we need to double the area of that lens which is uncovered. The area of a circle is determined by the formula πr2, so a doubling of the area increases the diameter by approximately 1.41. From this figure we get the sequence of f ratios:

The f ratio sequence in stops.
1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22 – 32

Depth of Field

Fig 1.5.2 A narrower aperture has a much greater depth of field.

If we look again at the exposure triangle diagram in lesson 4, you’ll see that the aperture influences the depth of field. The depth of field is the region of the photo which is in focus when the image is captured. It is a range of distance from the camera where objects look to be in focus. Aperture influences the depth of field by widening or narrowing this range, thus bringing more or less of the photo into focus, based on its distance from the camera.
Recalling what we learned back in lesson 1, the larger the hole that the light passes through a pinhole camera, the blurrier the image will be. When a lens is added to the camera, the image can be brought into focus, no matter the size of the hole, however, objects just either side of the focus point will be affected. If the hole, or aperture, is large, only objects very close to the focus point will be in focus. If the aperture is small, then the depth of the focus field is much wider, and more remote objects will be in focus.
Depth of Field can be pretty complex when you look deeply into it (such as: hyperfocal distance, airy discs and diffraction), but for an introductory course, the most important thing to remember is that a wide aperture (low value) means a shallow depth of field with less in focus and a narrow aperture (large value) means a broad depth of field with more in focus. We may return to discuss the more complicated issues in a future course.

Examples

Creative use of DOF isolates subject from background. Photo: annia316 used under CC license

The highlights on the strings clearly shows the changing DOF. Photo: Paul J. S. used under CC license

A narrow aperture gives a very wide depth of field. Photo: wili_hybrid used under CC license

These images show focus from the closest objects to the horizon. Photo: Neil creek all rights reserved.

Homework

• Find a scene with interesting stuff at many distances from you. Photograph the scene with different apertures (keeping the photo exposed correctly by changing shutter speed), and see the difference in the depth of field.
• Find a small subject that stands against a background a few meters away, like a flower. Try adjusting the aperture to isolate the subject from the background with depth of field.
• Find a view. Try and photograph a vista with something in the foreground, such as a rock at a lookout. Try and adjust the aperture to keep the whole photo in focus.

Resources

• Aperture – At wikipedia.com
• Understanding Depth of Field – At Cambridge in Colour
• Depth of Field Calculator – At dofmaster.com
• Introduction to Aperture – At digital-photography-school.com
• DoF group – At flickr.com

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Photography 101.7 – ISO

By: Neil Creek 

Photo: Rainer Ebert - CC license

The following post is from Australian photographer Neil Creek who just launched a free background image site featuring his photography, and is developing his blog as a resource for the passionate photographer.
Welcome to the seventh lesson in Photography 101 – A Basic Course on the Camera. In this series, we cover all the basics of camera design and use. We talk about the ‘exposure triangle’: shutter speed, aperture and ISO. We talk about focus, depth of field and sharpness, as well as how lenses work, what focal lengths mean and how they put light on the sensor. We also look at the camera itself, how it works, what all the options mean and how they affect your photos.
This week’s lesson is ISO.
Here’s What We’ve Covered Previously in this Series:
Lesson 1: Light and the Pinhole Camera
Lesson 2: Lenses and Focus
Lesson 3: Lenses, Light and Magnification
Lesson 4: Exposure and Stops
Lesson 5: Aperture
Lesson 6: Shutter
In previous lessons we have talked about the basic theory of how a camera works, including some basic optics, and introduced the idea of exposure and how we control it with the exposure triangle. In this lesson we will be drawing upon what we have learned to understand the third point on the exposure triangle – ISO – and how it works to create your photo.
ISO is probably the most mysterious and complicated aspects of modern photography. ISO simply stands for International Organisation for Standards, and refers to – in simplest terms – the sensitivity of the camera’s sensor. Confusingly, there are several different standards, some which measure different things, and only a few completely agree with each other. To understand how to use ISO in your photography, you don’t need to know anything about that. You should understand how the camera’s sensor chip works, however.

The Sensor Chip

Fig 1.7.0 A typical digital camera sensor and mount.

The sensor inside your digital camera actually works on the same principle as a solar cell. When light hits the chip, a tiny electric current is generated: the brighter the light, the stronger the current. Instead of there being one giant solar panel generating lots of electricity, there are millions of extremely tiny solar cells collecting very small and precisely measured amounts of light. These photosites will eventually create the pixels in the image you capture.
It should be noted that a photosite is not the same as a pixel. Several photosites will add their captured light, which is filtered and processed and eventually combined to make a pixel. That’s a complex topic for discussion in another lesson.
All of this carefully measured electrical current, which reflects light intensity, is measured and stored by the camera’s circuitry. This data is called the signal. The signal, however, must compete with the noise inherent in all electrical equipment.

Signal vs Noise

Fig 1.7.1 Noise is at a contstant background level to the signal.

An unavoidable fact of electronics is noise. While the sensor is measuring the tiny electronic currents generated by the light, there is also a tiny electric current that comes from other places. This unwanted current is called noise, and it mostly comes from the ambient heat of the sensor. The difference between the value of the signal and the value of the noise is called the Signal to Noise Ratio. When the ratio becomes smaller, the noise is more apparent, and the signal may get lost in it.
There are two ways that the signal to noise ratio can become low: by a drop in the signal, or an increase in the noise. In modern cameras, the noise is mostly a constant value, so we only need to worry when the signal drops – that is, when we’re photographing a dark subject. The graph to the right may help to visualise the situation.

The ISO’s Effect on your Photos

Sometimes we aren’t fortunate enough to be shooting with enough light to be able to ignore the noise. When we need to keep a fast shutter speed, or there’s not enough light even with the aperture wide open, we can increase the ISO setting on the camera. When we do so, we are increasing the signal gain. Essentially this is like turning up the volume. All of the values of the measured current (whether from light or noise) are increased. Each doubling of the ISO value, is a doubling of the gain: a doubling of the measured current in the chip.

Fig 1.7.2 - As the ISO setting increases so does the noise, until it overwhelms the signal.

Doubling the light in your photo is a pretty easy way to make an otherwise under exposed photo bright enough, but it comes at a cost: you lower the signal to noise ratio, and the noise becomes more apparent. Imagine you’re in a candle-lit room, and to take your photo, you have to set your camera to ISO1600 to get a correct exposure. You have now increased the noise value – which at 100ISO would be invisible – sixteen times, resulting in a noisy, grainy mess.
Shooting at high ISO settings is one of the most challenging technical issues in photography. Noise can look ugly and obscure detail in your photos, but sometimes the light is so poor that you have to accept the noise or get no photo at all. The newest breed of cameras are able to get incredibly high signal to noise ratios, and let photographers get clearer images than ever before in very dark conditions. Compact cameras, with their tiny sensors are always going to perform relatively poorly at high ISO.
Much can also be done about noise in your photos in the processing stage, but that’s a topic for a later lesson. As always, one should struggle to do the best one can in the camera, before resorting to post-processing to fix problems.

This is All Too Confusing

I warned you! ISO and noise are difficult concepts, but the good news is that there’s a simple take-away lesson from all this:

Noise is ugly. Avoid noise by shooting at low ISO settings. Only increase your ISO if there is no other way to get enough light for a good exposure.

The good news is that most DSLRs are very good at handling noise at low ISO settings (100-400) so you don’t need to worry about them too much. When you start to get into the medium (800-1600) to high (1600+) ISO settings, does noise begin to become obnoxious. If the alternative is missing a great shot though, don’t be afraid to crank up the ISO.
The forward march of technology is very exciting, as new technologies and techniques ever improve the sensitivity of camera sensors. Even though ISO is a bear the photographer must wrestle, it’s getting friendlier and cuddlier every year.

Examples

Fig 1.7.3 A series of images taken with the Canon 5D MkII, changing the ISO and other setting to keep a constant exposure. Noise increases dramatically at higher ISO settings.

High ISO was required to balance low ambient light with flash, to fill in shadows. © Neil Creek

In order to get a bright photo of the stars without trailing, high ISO was necessary. © Neil Creek

Without a tripod, this shot in very low light was only possible with high ISO. © Neil Creek

Homework

Homework for this lesson is fairly simple. ISO is simply a matter of “turning up the brightness” on your photos, so it’s not really complicated in practice. What you should do, however, is experiment with your camera on various ISO settings and get a feel for how images look. If you know that ISO 1600 looks terrible on your camera, then you’ll be more likely to try to find other ways to get more light on the subject than just be lazy and increase the ISO. On the other hand, you’ll also know when it’s worth pushing it all the way just to catch the photo that can’t be missed.

Resources

• Film Speed – On Wikipedia
• Digital Camera Image Noise – On Cambridge in Colour
• Image Noise and Noise Reduction Review – On Digital Camera Reviews
• What Are CCD Image Sensors? – On Ezine Articles

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