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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