In the old days our ISO choices (ASA back then) were only incrementally variable. Our films had a "fixed" ISO for best results. Sure there were some notable exceptions like Tri-X and later T-Max Black and White films that we could "push" one or two stops higher and get great, grainy, results. I used to push my 320 ASA Ektachrome slide film to 1000 ASA with nice results when photographing rock concerts. Having the freedom to move my ISO so I can use the shutter speed and f-stop I want is the best thing about digital cameras! This is especially true for action/sports photography.
|f5.0 @ 1/2500 sec., 800 ISO, lens @ 200mm|
Since this rider was racing towards me at a full gallop I needed a high shutter speed--especially with a relatively wide f-stop--so, 800 ISO was the necessary ISO to use.
|f50 @ 1/30 sec., 1600 ISO|
This antique pump at the State Fair was in a shaded area and to make the spinning rotor transparent I needed a slow shutter speed. At f5.0 for adequate depth of field I needed 1600 ISO.
|f5.0 @ 1/2000 sec., 125 ISO|
This ice encrusted leaf was in a lot of back light and with a slight breeze I needed a high shutter speed to freeze it. I also wanted a shallow depth of field. So, with that shutter speed I had to drop the ISO down a lot.
As you can see in each case I don't pre choose my ISO --as some photographers teach on the internet to always use your camera's native (lowest) ISO--NO, all you do then is limit your creative possibilities.
Our most creative tools in photography, besides lens choice, are f-stop and shutter speeds. You can't use their full range of possibilities without a really variable range of ISO's.
As usual, should you have questions please don't hesitate to ask...
Author: Jerry W. Venz, PPA Master Photog. Certified
Training site: http://www.LightAtTheEdge.com