Adjusting Exposure

Let’s do a quick review of what you should know about adjusting exposure (the amount of light reaching the camera’s sensor). There are four camera settings that affect exposure. For each of these settings a change of 1-stop halves or doubles the amount of light (this halving/doubling is a fundamental part of the definition of an f/stop). The four settings are –

  1. Aperture (higher f/stop = smaller opening = less light, and vice-versa. One stop increments would include 2, 2.8, 4, 5.6, 8, 11, 16, 22
  2. Shutter speed (faster = shorter aperture opening time = less light, and vice-versa) One stop increments are 1, 1/2, 1/4, 1/8, …. 1/500, 1/1000….
  3. ISO setting (lower ISO = sensor is less sensitive to light = less light, and vice-versa) One stop increments are 100, 200, 400, 800, 1600….
  4. Exposure Compensation setting (not available in Manual mode). Exposure compensation is adjusted in f/stop increments ranging from -5 to +5  (Nikon)

Depending on your camera all of the above adjustments can be made in either 1/3 or 1/2 stop increments (the choice between the two is usually a camera menu option).
Some observations –

A one-stop change in shutter or ISO setting is a doubling/halving of the corresponding setting value (as well as in the amount of light reaching the sensor). This is not true for aperture where a doubling of the value (f/4 to f/8 for example) is a two-stop change (quadruples the light), not one-stop (4 to 5.6 is one-stop). Why? For the the curious – think of a circle where doubling the radius quadruples the area – for the rest you’ll just have to accept it. See the examples in 1-4 above.
Offsetting changes in two of the camera’s settings result in the same exposure (same amount of light reaches the sensor). For example –

  • An aperture change from f/16 to f/11 is offset by shutter change from 1/500 to 1/1000 (otherwise changing just the aperture will result in the image being overexposed by one-stop compared to the original settings).
  • The above aperture change could also be offset by reducing the ISO by a stop (say 400 > 200) or reducing Exposure Compensation by a stop (say from 0 to -1) or any combination of these settings that adds up to a one stop reduction.
  • It’s up to the photographer to recognize the trade-offs involved. There is no free lunch; each of the first three settings serve a purpose (in order – DOF, motion blur, noise) and you have to know what you’re gaining & losing.
  • Note – When you’re using aperture priority, for example, changing shutter speed in response to a manual aperture change is done automatically by the camera. That’s the whole idea of aperture (and shutter) priority – the camera figures out how to compensate for whatever change you make in order to keep exposure the same – but it may not always be possible for the camera to adjust (can you see why? when?)

In the next lesson we’ll use the 4th setting, Exposure Compensation, in combination with the Histogram to get the exposure we want – possibly in spite of the camera.
Suggested exercises –

  1. Use your owner’s manual to determine how to change the above four settings. It may depend on your shooting mode (aperture vs shutter vs manual vs program).
  2. Using what you learned in #1, demonstrate to yourself that you can change two or more settings without changing the overall exposure (use the histogram to convince yourself)
  3. For extra credit, do #2 while changing all four settings. Is it possible? If not, what’s the most you can change at once?

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