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The Exposure Triangle

In any camera, obtaining a properly exposed photograph depends on three factors; ISO, Shutter Speed, and Aperture. As they are interrelated, these three items form the dynamics of the exposure. All cameras, from the least expensive Compact Camera (Point & Shoot) to the most expensive DSLR (Digital SLR), adjust these three items to ensure an exposure that is not too light and not too dark.

Most Compact Cameras do this automatically without any intervention from the photographer. DSLRs can also do this automatically, but they also allow the photographer to go into manual modes of exposure. Fact is, the camera is not always correct, so the ability to go into a manual mode overrides the camera's automatic system. This is a chief advantage of DSLRs over Compact Cameras. However, there are a few Compact Cameras on the high-end that have some manual control capability, however limited that might be.

Fortunately there is a tool called the Exposure Triangle that can assist in conceptualizing how ISO, Shutter Speed, and Aperture inter-relate. However, before you can understand the Exposure Triangle, you have to understand these three factors.

  • ISO - adjusts how sensitive the digital sensor is to light. A higher ISO value means it becomes more sensitive to light, and a lower value means it is less sensitive.

  • Shutter Speed - whether an electronic shutter in the case of a compact camera, or an electromechanical shutter in a dSLR, the shutter is adjusted to open for a specific amount of time. Faster shutters let in less light, and slower shutters let in more light.

  • Aperture - every lens has an aperture diaphragm, which is somewhat like the eye's iris. When it "constricts", it lets in less light, and when it "dilates" it lets in more light. However, some low quality lenses may have a "fixed aperture" that is preset to some value and cannot be adjusted.

The classic Exposure Triangle shown below is often used to represent the relationship between the three factors, along with their side effects. The side effects change as each factor is changed; which is sometimes advantageous, and sometimes not. The idea here is that the three factors must be in balance. If one is changed, either or both of the other two have to be changed. If you don't maintain the balance, the exposure will be incorrect.

Side effects: Noise is a side-effect of ISO, and due to sensor construction, as the ISO increases, noise also increases, which shows up as randomly colored pixels on the photo - especially noticeable in the darker areas. Therefore, lower ISO is always preferable, and will result in a better photograph. However, a low ISO may be too low for anything but an outside scene in the bright daylight.

Blur is a side-effect of Shutter Speed, and the slower the speed the more blur a moving object will have. A shutter speed that is set too low can be detrimental to a fast moving object, like a race car, but can aid in a photograph of a waterfall, as only the water will be blurred, creating the sensation of motion. So blur can be creatively used to improve the photo.

Depth-of-Field is a side effect of the Aperture, and like Shutter Speed, it can be either a disadvantage or advantage. Depth-of-Field is the measurement of how much of the scene is in focus. A small depth of field will result in only a small area to be in focus. This is desirable in portraiture where only the subject is in focus, and the background is blurry. A blurry background focuses your attention to your subject.

In situations such as photographing a mountain in the distance, it is desirable to have as much of the photo in focus as possible. Using small aperture openings results in long depths-of-field, while using large aperture openings results in short depths-of-field.

The relationship of ISO, Shutter Speed and Aperture are as follows:

  • Increasing the Aperture (larger opening) requires increasing the Shutter Speed or reducing the ISO.
  • Increasing the ISO requires increasing the Shutter Speed or using a smaller Aperture.
  • Increasing the Shutter Speed requires Increasing the ISO or Increasing the Aperture opening.

Unfortunately the Exposure Triangle does not show this relationship very well. I have come up with a method that can show this relationship, which I call the Exposure Pie.

You use the Exposure Pie by covering up the item you want to change. The result is the proportional relationship between the other two. It's best visualized with a little psuedo math. For example, you want to know what to do when you change the Shutter Speed, simply cover the desired setting with your thumb, and you will have your answer:

If you cover Shutter Speed, then: ISO x Aperture
If you cover ISO, then: Shutter Speed / Aperture
If you cover Aperture, then: Shutter Speed / ISO

So how does this solve anything?

In the first example (S = I x A), both ISO and Aperture are directly proportional to Shutter Speed. Directly proportional means you go the same direction. So if you increase Shutter Speed, you must increase either the ISO (make it more sensitive), or increase the Aperture (make it larger). And the reverse works; if you reduce Shutter Speed, you must decrease either ISO or Aperture.

For the second example, ISO is directly proportional to Shutter Speed, but inversely proportional to Aperture. Inversely proportional means you go in the opposite direction. In other words, if you increase ISO, you must either increase Shutter Speed (shorter shutter open time), or decrease Aperture (make it smaller). The third example is similar to the second.

OK that is pretty cool, but how do you remember what goes where in the "pie"; simple. Just remember Shutter Speed goes on top, and ISO and Aperture can go in either of the bottom positions. Or simply remember "S = I x A"

Now that we understand the relationship between ISO, Shutter Speed, and Aperture - how do we know how much to adjust each one. That is covered in my Reciprocity Section. If you got through this topic, sigh a breath of relief, and read on Reciprocity after you have digested this information.