Based on comments and emails I have received there are still some people that are confused about how the three elements of exposure play together to determine how dark or bright an image is. Today I want you to think of the three elements of exposure as the three points of a triangle each having an equal effect on the final exposure of the image.

It all adds up

If the visual references isn’t quite enough, then another way to think about it is to think of a perfect exposure as the combination of the right proportion of ingredients made up of ISO, Shutter Speed, and Aperture setting. If a perfect exposure = 9, and you have equal parts of Shutter, ISO, and Aperture, then you have a good mix (3+3+3 = 9). If you need to adjust one of the ingredients, then you have to adjust one of the others to come up with the same final number (2+4+3=9), subract from one, you have to add to another to make up the difference. If you have too much of one, without substracting from another, you end up with an overexposed image. Conversly, if you take away from one but don’t add another, you end up with an underexposed image.

The Sunny 16 Example

In a previous article we looked at the sunny 16 rule, this states that on a bright sunny day, a perfect exposure should be f/16, ISO 200, 1/200th of a second. (ISO and shutter are the reciprocal of each other). If we wanted a shallower depth of field and wanted a larger aperture like f/2.8, then since f/2.8 is 5 stops brighter than f/16, we would then need to compensate with either a lower ISO or a faster shutter. Increasing the shutter is the easiest in this example then we need 5 stops of shutter giving us a speed of 1/6400th of a second.

Low Light Adjustments

In low light we have the opposite issue, trying to get enough light into the camera. We only go so slow before we run into blurring issues with slow shutter speeds, and the aperture limit is going to be based on the lens we are using. If we still don’t have a bright enough exposure then we have to compensate by increasing the ISO speed.

Bringing it all together

While each setting can have an effect on the overall image quality, the sum of the three effect the overall exposure. For a refresher on the other settings, be sure and go back through these previous articles:

• Beginner’s Guide to Aperture
• Controlling Exposure
• The Sunny 16 Rule in Photography
• Capturing movement with shutter speed
• Understanding Depth of Field

Author: Kerry Garrison

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