Balancing Exposure

Econ River WA
Trees at Sunrise
As we discussed in a previous post on exposure, you have proper exposure for your photograph when you have set your three exposure factors (shutter speed, aperture, and ISO) to give you the results you want--that is, you've let in just the right amount of light to give you the image you were hoping to capture.  But that's just the beginning of the creative process in photography.  You can change the balance of these factors to allow you to take control of the look of your photographs.  Let's review a little of what I mean by this:
  1. You can use a slow shutter speed to create a pleasing motion blur--such as the silky effect on a waterfall.  
  2. You can use a fast shutter speed to freeze motion, like a kayaker on rapids.  
  3. You can use a large aperture (small f/stop) to create photographs with a shallow depth of field, like a butterfly with a blurry background.
  4. You can use a small aperture (large f/stop) to to create photographs with both the foreground and background in focus, like a cascade with waterfall in the background. 
But to make any of these adjustments, you must compensate by adjusting one or both of the other exposure factors to keep your photograph properly exposed.  That is, you can change the balance of your exposure to give you the results you want.

Let's consider the above photograph.  For this photograph, I wanted a relatively fast shutters speed because I was handholding my camera and there was very little light. However, I also wanted to shoot at a low ISO to keep noise down on my point and shoot camera (I shot this with a Canon S100HS).  I didn't need a large f/stop, so I could let in light with a larger aperture (smaller f/stop) to keep my ISO low and my shutter speed reasonable.  So I set my ISO to 200 and my f/stop to f/3.2. The camera's light meter then calculated a proper exposure with a shutter speed of 1/60sec.

Here was my proper exposure:

SS  1/60sec     F/stop  f/3.2     ISO 200

If I wanted a faster shutter speed, I could have raised my ISO to 800 (a change in two stops).  This would allow me to make my shutter speed a faster 1/250sec.  So If I wanted to shoot this scene with a shutter speed of 1/250sec and an f/stop of f/3.2, I could raise my ISO to 800 and maintain the same properly balanced exposure.  The exposure is the same--the same amount of light is hitting the sensor--but there would be an affect on the image.  The most noticable difference would be an increase of noise from my point and shoot camera.