What I learned in my Digital Photography Class, Part 8
I'm on dangerous ground here, attempting to explain something that I can just barely understand myself. Just keep that in mind, and proceed with caution.
When the instructor in my photography class asked, "What is ISO?" I can't begin to tell you my thoughts, but I'd like to try. ISO? What do you mean, what is ISO? How the heck would I know? I thought you were the instructor here? Yes, my thoughts ran along those lines.
Things didn't get any clearer when one of my classmates answered rather triumphantly, "ISO is what ASA used to be!" I had to hold on to my chair to keep myself from fleeing.
Here is what I eventually took in during the class and worked out later, using a variety of resources:
ISO (according to my camera's manual) is a numeric indication of the sensitivity to light. The actual letters stand for International Organization for Standardization. A doubling of the sensitivity of a film requires a doubling of the numerical film speed value (from "Film Speed," Wikipedia). So, according to my calculations, an ISO of 200 means double the sensitivity of an ISO of 100. The numbers I can choose on my camera in manual mode range from 100 to 1600.
When shooting with an automatic setting, the camera will choose the ISO for you. My camera stays within 100 to 400, unless I mess with it, as I did on the fateful day when our assignment was to shoot photos at the Farmer's Market, using all manual settings.
All you really need to know is this: The brighter the day, the lower you should set the ISO number. On the other hand, if you are shooting in a low light situation, you will want to set the ISO at a higher number, which means that your camera will be more sensitive to light.
My problem was that I remembered this in reverse, thinking aha! the brighter the light, the higher the ISO number. You should forget that you ever read that sentence, or you will have some truly dreadful results, like this horrendously overexposed shot:
The good thing about this messed up assignment is that I will never forget: The higher the light, the lower the ISO.
For a much clearer and more knowledgeable explanation of ISO, please visit Pioneer Woman Photography: ISO Setting. Ree will set you straight in no time!