Now that you (hopefully) understand the concepts behind aperture, shutter speed, and ISO, it’s time to review how they interplay with one another, and show how you can manipulate them to improve your photography.
For the most part, manipulating exposure is a matter of simple math, with a good understanding of the settings themselves, and their impact on the image.
Example 1: You have your camera set at ISO 200 (as stated in the previous post, you should set your camera to the native ISO of your camera’s sensor whenever possible, so adjust as necessary for your particular camera), and your camera’s meter suggests an exposure of f/5.6 @ 1/250 for the scene.
You make the artistic decision that you prefer to use a slower shutter speed in order to blur motion, and you set the shutter speed for 1/60. However, you now know that, by doing so, you’ve increased the exposure by about 2 stops.
1/250 –> 1/125 (1 stop) –> 1/60 (2 stops)
If you left all other settings alone, you’d end up with an exposure that is overexposed by 2 stops. To fix this, you need to decrease your exposure by 2 stops. Since your ISO is already at it’s lowest setting, it cannot be decreased any further. Your only remaining option is to reduce your aperture by 2 stops, from f/5.6 to f/11.
f/5.6 –> f/8 (1 stop) –> f/11 (2 stops)
Example 2: You have your camera set at ISO 200, and your camera’s meter suggests an exposure of f/8 @ 1/50.
You again make an artistic decision, now that you prefer a shallower depth of field in order to isolate your subject against an out-of-focus background, so you reduce your lens’ aperture to f/4. Again, you’ve increased the exposure by 2 stops, so you need to compensate by adjusting another exposure setting or, again, you’d end up with an overexposed image.
Once again, it’s recommended that you leave your ISO at it’s native setting, so the only remaining option to adjust is shutter speed. Increase the shutter speed by 2 stops from 1/50 to 1/200, and you once again have a correct exposure with the artistic result you wanted.
For the most part, these adjustments can be done in your head, since a stop is effectively doubling or halving the particular setting. Most current SLRs also allow you to set them for adjustments in 1/3 or 1/2 stop intervals. Again, it’s a simple matter of mathematical compensation: if you increase one setting by 1/3, you must decrease another by 1/3.
Now that you (hopefully) understand the basics behind exposure and it’s component functions of aperture, shutter speed, and ISO, there is an excellent resource that I strongly recommend to every photographer who’s looking to improve their images. Understanding Exposure, 3rd Edition: How to Shoot Great Photographs with Any Camera by Bryan Peterson is a fantastic book that thoroughly covers the topic of exposure with much more detail than can be presented in any blog. Since buying my first copy of the book, I’ve purchased the following 2 editions, and consider it a must-have in any photographic library.