Digital Basics - Shutter Speed - Aperture - ISO

Here is an explanation of the relationship between shutter speed, aperture and ISO (alternatively called ASA). We will start first with ISO as this directly effects the other two, and the shutter speed and aperture effect each other.

ISO - ASA

ASA - American Standards Association - Original Kodak system introduced in 1940
ISO - International Organization for Standardization - Web Site

Now digital cameras have an ISO setting, this has the same (well similar) effect as changing the film in a camera. The higher the ISO setting the more sensitive to light the camera sensor is (CCD/CMOS sensor) this is the same as changing for an ISO 100 film to ISO 400 film.

Shutter Speed & Aperture

These work together to give you your correct exposure. The shutter speed is the time the CCD (or film) is exposed to light, and the aperture is the size of the hole allowing light to enter the camera, therefore to give a "correct" exposure you need to balance both of these settings.

Example - Lets say a light reading was taken and the result was 1/125th sec @ f8 (ISO 100) then all the exposures below would be correct :-

All settings allow the same amount of light onto the CCD (or film)
1/30th Sec	1/60th Sec	1/125th Sec	1/250th Sec	1/500th Sec	1/1000 Sec
f22	f11	f8	f5.6	f4	f2.8

But that not all, as different shutter speeds and apertures produce different results you then need to select the best for your subject here are some general examples all assume the same light reading as above (please do not e-mail me saying some are not 100% accurate, I know, it is a guide only!):-

Portrait (Blurred background)
You will need a large aperture (small number) to achieve a shallow depth of field say 1/250th @ f5.6 or 1/250th @ f8. Other factors such as lens focal length also effect depth of field, a 28mm lens @ f8 has a much larger depth of field as a 800mm lens @ f8.

Motor Sport Photography (Freeze a Car)
You would require a fast shutter speed to freeze a car say 1/1000 sec and your aperture would be f2.8. Now f2.8 will give you a very shallow depth of field and focusing will be critical. You have another option to increase the CCD ISO setting (or film ISO), lets say you change the setting or film from 100 to 800 (3stops 100-200-400-800) now as all the shutter speeds OR apertures move along 3 places you could then shoot at 1/1000th @ f8 or 1/2000th @ f5.6 is the same.

Question?

If the lighting does not change which one of these would also give a correct exposure?

Correct	Alternative 1	Alternative 2	Alternative 3	Alternative 4	Alternative 5
1/125th sec	1/250th sec	1/30th sec	1/8th sec	1/125th sec	1/125th sec
f8	f5.6	f16	f16	f8	f22
100 ISO	100 ISO	100 ISO	25 ISO	800 ISO	100 ISO

Alternative 1,2 and 3 would all give a correct exposure (same light allowed into the camera for ISO setting as the "correct" one). Alternative 4 would be over exposed by 3 stops and Alternative 5 would be under exposed by 3 stops.