Understanding “ISO”

Last modified on 2011-09-21 01:39:14 GMT. 14 comments. Top.

If all films and digital cameras had the same sensitivity to light the information above would become constant and easy to learn. But there is a third element for creative control of your photographic exposure. It is this third element that changes the shutter speed and aperture settings. It is called ISO

ISO is an acronym for International Organization for Standardization. In photography is designates the sensitivity of film to light. For digital users it designates the sensitivity setting of the image sensor to light.

When you buy film you should be concerned about its ISO rating. The most popular films are rated as ISO-100, ISO-200, ISO-400. Some film is rated at ISO-800. Why?

Number one if you are shooting in bright light you’ll want a low ISO rating i.e. ISO-100. If you are shooting in low light, perhaps indoors or an evening setting, you’ll want film with a high ISO rating i.e. ISO-400. Once you have inserted that roll of film into your camera you will have to use it up or take it out mid-roll before you can change to another ISO rated film.

Walla, the wonderful world of digital photography! You can change the ISO setting on your camera at anytime you want. Wow! What a difference that has made to my photography experience.

How do ISO ratings affect the exposure of your photograph?

Some digital cameras have ISO settings that range from 50 to 3200. That is a range of 6 stops in the exposure category. It works this way. Every time you double the ISO rating you essentially add one full stop of exposure. I.e. 50 to 100 is one full stop of exposure, 100 to 200 is one full stop, 200 to 400 is one full stop, 400 to 800 is one full stop, etc.

What does that mean in the practical sense? If you have your ISO setting at 100 and your light meter informs you that your camera should be set at f/4 and 1/60 second then you can take a well exposed picture at those settings. But what if your subject is moving or your lens only opens to f/5.6? 1/60 of a second will not stop the motion of a moving object and if your lens only opens to f/5.6 you are likely to have an under exposed and blurry photograph. Now is the time to step up the ISO setting. If you step up to ISO-200 then you can move one of the other elements one full stop. Your setting would now be f/5.6 and 1/60 or f/4 and 1/125. Only one setting can be increased by a full stop. If your camera allows for 1/2 stops you could increase both the aperture and the shutter speed by 1/2 to equal 1 full stop.

But in the case above we’ll probably need more than one stop to freeze the motion so we will set our ISO to at least ISO-400. This will give us the ability of setting our aperture at f/4 and the shutter speed at 1/250. This will really help out the end result of our photograph.

Limitations of ISO settings

There is one limitation to setting your ISO at the higher settings. In the film world we called it “grain” which sometimes was employed very creatively but usually was considered a degradation in the quality of a print, especially for a large blow up. In the digital world we call it “noise.” “Grain” and “noise” are, in a practical sense, the same thing. They each have a different quality about them but they both are essentially degradation in the quality of the photograph.

Higher ISO ratings tend to create more grain or noise in your image. I.e. ISO-100 will have much a smoother finish than ISO-800. So the rule of thumb is to use as low ISO rating as possible and still have enough light to obtain a proper exposure.

Below are variations from ISO 200 to ISO 2EV above 1600 taken from a Nikon D100.

ISO 200

ISO 800

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