Do You Understand the Relationship Between Shutter and Aperture?

eHow Tech Blog

apertureNumerous people, myself included, have written about the exposure triangle to explain how you need to balance ISO, shutter speed, and aperture to get a good exposure. The confusing part is that the triangle misleads people into believing that a good exposure is an equal mix of ISO, shutter speed and aperture. Consequently, this isn’t a very clear way of looking at it. Instead of trying to use a triangle to explain exposure, let’s look at it like they used to do back in the days of film.

Whaaaat… film?

How can anything having to do with shooting film apply to an ultra-modern uber-pixel state-of-the-art modern and very non-film DSLR? After all, when shooting film, you would install whatever ISO speed film you had, and that is what you were stuck with. The only things you could adjust were aperture and shutter speed. With a current DSLR, you have the convenience of being able to dynamically adjust the ISO speed as well.

While ISO plays an important role in the overall exposure, people rarely adjust ISO simply to increase or decrease exposure. Instead, think of ISO as a way of getting you into a usable range where the aperture and shutter then work properly, just like changing the film in your camera.

The Exposure L

And that brings us to the Exposure L, which is a way of looking at the relationship between just shutter speed and aperture, without contending with ISO. Along the vertical axis are common shutter speeds in one stop increments. Along the horizontal axis are the different aperture settings, also in one stop increments. You need to use an ISO setting that will allow you to get a good exposure somewhere on the chart. Once you have a point on the chart that will deliver a good exposure, then you can follow a diagonal line to find other shutter and aperture combinations to get a correct exposure.

Exposure chartFor example, if you have a good exposure at f/16 and 1/60th of a second, any of the following will also give you the same exposure:

  • f/11 & 1/125th
  • f/8 & 1/250th
  • f/5.6 & 1/500th

By understanding what other aperture and shutter combinations give you the same exposure, you can make an artistic choice based on what kind of image style you are trying to create. To get a shallower depth of field, go down a stop, but compensate for moving up a stop in shutter speed.

If the scene is too bright or too dark to get an exposure somewhere on the chart, change the ISO setting.

Let’s look at some examples

You can see from the photos below that once you have a usable exposure, you can then slide diagonally up or down the chart to maintain that exposure.

Aperture ISO exampleBaseline Image settings f/8 1/60th Sec ISO 400

Refining aperture, ISO and shutter f/5.6 1/125th Sec ISO 400 (+1 stop aperture / -1 stop shutter)

Refining aperture f/4 1/250th Sec ISO 400(+2 stops aperture / -2 stops shutter)

controlling images with aperture f/2.8 1/500th Sec ISO 400 (+3 stops aperture / -3 stops shutter)

As you can see from the example, you slide diagonally down the scale so the exposure doesn’t change as long as you adjust the aperture opposite from shutter speed. As you adjust one up, the other has to go down at the same rate to maintain exposure. The effect is that depth of field gets shallower and shallower as you open up the aperture.

Here is another example using subjects that are further apart.

aperture control Baseline Image – f/2.8 1/640th Second ISO 400

aperture management f/4 1/320th Second ISO 400 -(-1 stop aperture / +1 stop shutter)

managing exposure and aperture f/5.6 1/160th Second ISO 400 (-2 stops aperture / +2 stops shutter)

iso, shutter and aperture management f/8 1/80th Second ISO 400 (-3 stops aperture / +3 stops shutter)

In the last sample set, the images started with a large aperture and as the aperture closed down to increase depth of field, the shutter speed was also slowed down accordingly to maintain correct exposure. The only time you need to adjust the ISO is if the scene is too bright or too dark to get a good exposure within the range of settings the camera can handle.

If you look back at the exposure chart, you’ll see that the ISO setting is not another axis like a triangle. Instead, it’s a third dimension that can add or subtract overall brightness to the scene.

Viewing it this way makes more sense than discussing an “Exposure Triangle,” because a good exposure requires a balance of shutter and aperture, and ISO is a entirely separate variable.

Photo credit: Kerry Garrison

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