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ISO

Once your aperture has allowed your chosen amount of light into the lens, and the shutter speed has controlled the length of time the light has passed through the lens, the ISO controls how much of that light is absorbed.

The ISO is the camera’s measurement of light sensitivity. In analog photography, a higher ISO film absorbs the light faster, and film with a lower ISO number absorbs light more slowly. In digital photography it works in the same way, just with an electronic sensor in place of the film. A low number equals slower absorption.

Like both shutter speed and aperture, the ISO increases in stops that double or halve the light absorbed.

Standard digital ISO range

Some newer dSLRs have a higher range than this, but the principle is still the same. These stops correlate with the shutter speed and aperture stops. One ISO stop up or down equals one f/stop up or down, or one shutter speed stop.

Okay, that’s interesting, right? But how is it useful?

This is the first part of a four-part series on exposure.

Click here to read more about shutter speed, aperture, and ISO.

Exposure – an overview

No matter how manual or high-tech your camera may be, or what kind of photos you like to take, what photography really boils down to is exposure. It’s all about making sure the right amount of light reaches your film or electronic sensor to create the desired effect.

There are three things that affect the final exposure: the shutter speed, the aperture, and the ISO. Each one of these aspects works with the other two to create a perfect image.

It’s impossible to go into any kind of depth on such a huge subject with just this one post, so for today I’ll give a quick overview of all three from both a technical point of view, and a creative one. In the following weeks I’ll write about each of them in more detail.

1: Shutter Speed

The shutter speed is probably the most commonly understood aspect of exposure. This is the length of time the shutter is open for, thus allowing light onto the film or sensor. It’s a very simple idea; a fast shutter speed is open for a shorter time and lets in less light, a slow shutter speed is open longer and lets in more light.

The shutter speed is measured in seconds, and fractions of a second. For instance, 1/250 is a two hundred and fiftieth of a second, 1/1000 is one one-thousandth, and so on, and if you take the speed down you get to full seconds, such as 2”, which is two whole seconds.

Creative effect

Apart from the way they control the light, each one of the aspects of exposure has its own special influence on the image’s style. The effect of the shutter speed is the way it captures motion in an image.

A fast shutter speed will freeze action, which is great when taking photos of children, animals, drops of water, or anything else that doesn’t stay still for long.

I dragged my brother out into the cold garden this morning to juggle so I could demonstrate this for you properly. These photos aren’t works of art, but they get the idea across. The first photo was taken with a fast shutter speed; you can see the juggling balls frozen in midair.