Understanding your camera – ISO. The ISO setting on a digital camera adjusts the sensitivity of the image sensor to light.
A digital camera “base” ISO, of 100, or 200 will give the best quality
images. Taking any photograph, entails making choices between shutter
speed, aperture settings, and the ISO. As ambient light levels fall, on
perhaps a cloudy day, the ISO can be set to 400, to make the image
sensor more sensitive to light, indoors the ISO could be set to 800. The
potential problem with increasing the
ISO too much is that grain, or noise is introduced into the final image,
reducing the quality of the photograph.
Why is it important to
understand this? Well, if you intend to use your digital camera in
aperture priority, (A or Av on the mode dial) or shutter priority (S on
the mode dial) mode, it is one of the adjustments that you can make to
get the best quality images.
Practical application; find a subject
with a lot of detail, a flower, or a wine bottle, set the camera to
“aperture priority” mode, and choose f4, now set the ISO to the “base”
ISO of 100 (or 200, dependant on camera make / model), position the
camera on a tripod, or place on a firm surface, set the camera
self-timer to a short time interval, and take the shot, make a note of
the shutter speed, increase the ISO to 200 (400), and notice that
because the aperture is fixed to f4, the camera will double the shutter
speed to give the correct exposure (doubling the ISO will double the
shutter speed in aperture priority mode). Take the same shot at 1600,
3200, 6400 ISO settings, when you examine the images you will be able to
see how much grain / noise is introduced.
To sum up always use the
lowest ISO setting for the highest quality images and having undertaken
this exercise you will have an understanding of the maximum ISO setting
for your camera make / model. Many photographers use auto ISO on their
cameras, and that is not necessarily a wrong choice especially with
continually changing light levels, wedding photographers often make this
choice, setting the upper limit of ISO to maybe 1600 / 3200 depending
on the camera make / model.
I've posted a couple of images to illustrate the grain / noise effect of high ISO settings.