March/April 2007 HelpLine

In Search Of ISO

     • ISO Settings And Exposure Compensation

     •RAW Delivered

ISO Settings And Exposure Compensation

Q)  My SLR offers ISO settings in 1⁄2-stop increments. Is the range of light control insufficient or as good using exposure compensation?

Frank Moore
Via the Internet

A)  This is a case of the camera designer giving you more options than before. A camera’s ISO setting controls the degree of apparent image sensor sensitivity—apparent because the sensitivity of the sensor really doesn’t change. You’re adjusting the amplification of the signal coming off the sensor. One thing to keep in mind is that when you amplify a signal, you also amplify the noise that’s in the signal. So, as you increase the ISO, you increase the amount of noise in your image.

Your digital camera’s ISO values are the same as those used for film sensitivity and typically are given in “one-stop” increments: 100, 200, 400, 800, etc. A change from ISO 100 to 200 represents a doubling of sensitivity—the camera becomes twice as sensitive to light. This is different from exposure compensation, which is about how much light is let into the camera.

To give you even more control over ISO, the camera designer has given you steps between the steps—hence your comment about 1⁄2-stop increments. Instead of the ISO going from 100 to 200, it would go to 140 (I know you’d think it should be 150, but the math involves logarithms). If the camera offered 1⁄3-stop increments, the ISO would go 100, 125, 160, 200.