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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