This is the 3rd element in getting the right exposure for your photo–the other two being: shutter speed and aperture. For the film-buffs out there, it’s the ASA rating (US standard, DIN is the European equivalent) or film sensitivity value. The rule of thumb was that if you’re shooting in birght sunlight, you used ASA 100 or 200. If it’s indoors, you used ASA 400. If you’re shooting a sports event, you went with ASA 800 or 1600. The image below (left) shows a shutter speed – ASA selector knob that is common in film-based SLRs. The ASA value is the one with the black numerals with white background (e.g. the image below, left shows the ASA selector is set at 100).
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For digital cameras, the ISO value reflects the image sensor sensitivity. The general rule is that lower ASA/ISO value means less sensitivity and more exposure time (longer shutter speed); and consequently, the higher the ASA/ISO value, the more sensitive the film/image sensor is to light, and needs less exposure time (shorter shutter speeds). In the illustration above (right), you can see the back panel of a Pentax K-x currently in ISO selection mode. For the K-x, there are two methods: (1) select a range for auto shooting mode, and (2) fixed ISO (e.g. 200) for manual shooting mode. In auto shooting mode (e.g. 200 – 1600), the camera decides the correct ISO value based on the exposure parameters set (a useful alternative to bracketing when unsure of the ISO value to use).
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The three pictures above were all shot with the same shutter speed (1/60 sec) and aperture (F/4.0). The first one was shot using ISO 800. The next one was shot with ISO 1600. And the last one was shot using ISO 6400. Notice that the image gets brighter as the ISO value inccreases.
Aside from sensitivity to light, lower ASA/ISO value results in finer details; higher ASA/ISO value results in “grainy” look in images captured. The next pair of pictures will illustrate this for you:
 Finer image (esp. blue cast on the face) |
 Grainy shot (esp. blue cast on the face) |
As it was night time outdoors (with just candlelight and and the light from the screen of the PSP to illuminate the area around my son), I used ISO 1600 in my 1st shot (left, above). Notice the fine quality of the blue light (from the PSP’s LCD screen) illuminating the face of my son. At ISO 6400 (see above, right), there is a grainy look on the same blue light on my son’s face. Please note that in order to maintain a similar exposure level of the photo, I had to adjust the shutter speed and aperture based on the new ISO value. The higher ISO (naturally) resulted in a brighter shot (i.e. more of the surrounding was illuminated).
So which is better? It depends on several factors:
- Subject availability – if you need to get a quick shot, you go with the highest possible ISO to lessen exposure time (faster shutter speed). You can couple this wider aperture (if depth of field is not a consideration) to get the quickest shot. This is applicable when the subjects are not in your control and could move at any moment (e.g. candid shots of people, astrophotography).
- Image quality – if this is your primary consideration, you need to use a lower ISO value (say 800 or 1600 or even 400). This will require a longer exposure but will result in finer image. In low-light conditions, you should use a tripod to keep the camera still.
- Level of detail – in low-light conditions, this means a higher ISO value to capture as much of the ambient light as possible without risking blurred images due to camera shake. In the preceding example (above, right), notice that the area around the light sources are better lighted thus revealing more detail.
In summary, the ISO value is inversely proportional to the available light: the brighter the available light, the less ISO sensitivity you need; inversely, the less available light, the greater the ISO sensitivity need (i.e. higher ISO).
Noel Ombrog's Blog 