Today's Photo Minute

The Basics of ISO – Part 1

The Concept:

Today’s Photo Minute deals with the very foundational concept of ISO. Brian Osborne discusses what ISO is, how to adjust it on your camera and why you might want to change it in different situations (including some sample photos in the actual post). This is part 1 of a 2 part series and tomorrow’s session will discuss higher ISO’s and how to know what your camera is capable of.

The Assignment:

Many people get confused about ISO. Some think that changing the ISO actually changes the resulting brightness of the photo but this is not the case if you are in let’s say, Program (P) or Aperture Priority (A of AV) mode. The phrase to remember is that the higher the ISO the higher the aperture and/or shutter speed you can get in the same lighting condition. Therefore, for this exercise, we suggest you put your camera in A mode and set the aperture at a given value (let’s say F11). Then at ISO 400, point your camera at a subject and take the photo. Note what your shutter speed is. Then raise the ISO to 1600 and take the same photo. What you should learn from this is that first of the all, the exposure of both images is the same. Secondly, the possible benefit of the higher ISO is a faster shutter speed. Like in the example below, while at a low ISO of 125, I was able to get F14, I was at a shutter speed that was a little too slow to be handholding the camera much less the flower blowing in the wind. Therefore, to keep the same aperture but to get a higher ISO, I needed to raise my ISO. The exposure did not change but I had an aperture and shutter speed combination that was more appropriate for what I was trying to accomplish. Try it for yourself!

Dogwood flowers
ISO 125 resulted in F14 (the aperture I wanted) but in a shutter speed of 1/30th which was too slow (see the motion blur)
Dogwood tree
ISO 400 resulted in the same exposure and the same F14 aperture but the shutter speed was now 1/100th. This was better but still had a little motion blur.
Dogwood tree
ISO 1600 (higher than we would generally think we would need outside in good light) resulted in the same exposure and F14 aperture but now the shutter speed was 1/320th which was about right for my movement and the movement of the flowers.

This is why I needed a fast shutter speed not only because the flowers were blowing in the wind but I was up on the ladder (pictured below) to get these shots!

ladder under a tree