ISO of course is the camera's sensitivity to light. Generally speaking, the higher the number the more noise (grain) is added to an image. It is part of the triad of a photographers exposure (aperture and shutter speed being the other elements). Without getting too deep into this triad, the question to ask yourself is a high ISO Always bad?
- What am I shooting for in terms of output?
- Is the shoot for the web; possibly a candid or even a professional shot that will only be published on the web?
- Do you plan to print? what size?
- If you are shooting for the web you can probably go to ISO 800 or 1200 on most modern cameras with only a minor level of noise correction in the editing software.
- If you plan on printing larger than 8x10", Generally it is a good idea as low as possible under prevailing lighting conditions.
- Where am I shooting?
- Indoors with a studio strobe, flash, constant light or strong natural light
- Outdoors on a clear day for a few hours?
- If you have a good knowledge of your light sources a low ISO (less than 800) is almost always possible
- Shooting for an hour or two under fairly constant light outdoors a low ISO is typically possible. The tricky part times in when clouds roll in, you are moving to shaded areas, or the sun start to set later in the day.
- How does your camera handle noise?
- Some cameras have a pretty large tolerance for noise even at high ISO levels. Other cameras will show visible noise even at ISO 400. The smaller the sensor in your camera, 'generally' the more likely high ISO's will result in visible noise. The only way to tell for sure is to take test shots and examine the image.
- Is your project a beauty shot?
- If you want the skin to look its best, an ISO of 100 to 200 is preferred.
- How do you plan to edit?
- At high ISO's (generally above 800) some noise reduction may be necessary. Of course, some photographers prefer noise, especially in black and white photography. Most major editing programs have a noise reduction feature including the Nik software recently released by Google.
If you are not planning to print, and your final product is not a high end beauty shot, you may elect to leave your camera on auto ISO in certain cases. Particularly when shooting outdoors, this gives you more latitude with your aperture and shutter speed. By the way, the image at the top was shot with an ISO of 1600 outdoors.