Canon® EOS 50D Digital Field Guide

Appendix A. The Fundamentals of Exposure

In This Appendix

The four elements of exposure

Equivalent exposures

Putting it all together

In my years of working as a photographer and in teaching photography courses, I've found many photographers have a good understanding of most aspects of photographic exposure while other aspects are vague or less well understood. And in other cases, I've worked with photographers who are returning to photography after a long absence and want a refresher on the fundamentals of exposure. If you fall into either camp, then this appendix is for you.

The Four Elements of Exposure

Photographic exposure is a precise combination of four elements: light, the medium's sensitivity to light, the intensity of light, and the length of time that light falls on the medium. The medium, of course, is the 50D's image sensor. And because all of the elements are interrelated, if one element changes, then the others must change proportionally.

• Light. The starting point of exposure is the amount of light that's available in the scene to make a picture. Every exposure depends on the amount of light the camera has to work with. Scene light is read using the onboard light meter. Then the camera bases exposure calculations on the meter reading.

• Sensitivity. Sensitivity refers to the amount of light that the camera's image sensor needs to make an exposure, or, in short, the sensor's sensitivity to light. Sensitivity is determined by the ISO setting.

• Intensity. Intensity is the strength ...