Hopefully now we understand the relationship between shutter speed and aperture and how they combine to give a correct Exposure. We have finally come to the last of the three variables.

3. ISO. (Pronounced Eye-So) ISO is how sensitive your camera is to light, the higher the number the greater the sensitivity. The ISO scale is the easiest of all to understand, but it still maintains the twice as much or half as much relationships that f numbers and shutter speed have.

Back in the days of film you had to change the film to alter ISO, with digital it’s a lot simpler, even the cheapest digital camera will offer two or three settings.

The scale is usually 100, 200, 400, 800, 1600, 3200, 6400 and even 51200 on some high end DSLR’s! In other words they are at 1 stop intervals. So why not just set it to it’s highest setting, your shutter speeds would then be fast enough to freeze any action, you would not have to worry about camera shake or carry a tripod? Well there’s a huge sting in the tail. The more you increase ISO the more noise you get in your image.

Noise shows itself as speckles mainly in the darker areas of the picture at first, but as noise increases it starts to effect more tones and get stronger, rapidly reaching the point where the picture is virtually useless. It gets worse, I said that noise starts to effect the darker parts of your image first, those nearer to the ever present noise threshold, so it’s in dim lighting conditions where noise is at it’s greatest, just the conditions you want to use a high ISO in.

By and large it’s better to use the lowest ISO you can, you will have less noise in your picture even in dim conditions if you put your camera on a tripod use low ISO and have a long shutter speed, than using a high ISO and a shorter shutter speed.

As sensors improve you can use higher ISO with little noise penalty, these days most cameras will handle an ISO of 400 or even 800 acceptably, especially if the Ambient light is bright, but if you print big or crop noise will be evident even here.

In bright conditions you can use a higher ISO to help with camera shake particularly with telephoto lenses that magnify any camera movement, or to freeze fast moving action, you will be increasing noise, but in bright conditions it will probably be acceptable.

EXPOSURE 3 - ISO

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