The Exposure Equation

Damien Franco | Feb 09, 2009 | Comments Comments

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IMG_3370

In trying to grasp the technical aspects of photography exposure is the foundation. It is absolutely fundamental to understand the how’s and why’s of exposure in order to be able to truly capture and manipulate light. Light is after all what we are capturing in photography. More specifically; reflected light.

In Light & Exposure Harold Davis describes the exposure equation as:

“…another way of saying that the light reflected by a scene towards a camera is captured at a given dynamic intensity by shutter speed combined with aperture and sensitivity.”

The three factors of exposure are shutter speed, aperture, and sensitivity (ISO).

Shutter speed really needs to be looked at as a unit of time because that’s exactly what it is. Those precious seconds or fractions of seconds that we use in photography will directly affect the rest of the equation.

I love the way that Harold Davis uses the term sensitivity to describe ISO. It’s exactly correct to think of it that way and I believe that digital photographers that never worked with film have a tough time with this concept.

Aperture is of course the measurement of the opening in the lens that allows light to come into the camera. These are denoted as f-stops and the smaller the f-number the larger the opening.

All three of the above factors directly relate to each other and work in conjunction to allow for a photographer to capture images in stylistic and creative ways.

Fast moving objects require fast shutter speeds thereby prompting either larger apertures or higher sensitivity.

Captured Light = Shutter Speed • Aperture • Sensitivity

If you have a need to adjust one of the factors you’ll also have to compensate accordingly with the remaining two.

One of the best ways to learn how these factors co-relate is to go on a photo-walk and lock an element in place. Perhaps it’s a matter of only using one f-stop and having to adjust the ISO and shutter speed as you go. Or you could lock the ISO and adjust shutter speed and f-stops.

Once you’ve done this a few times I would consider locking two elements forcing you to adjust the remaining factor. Learning about exposure and how the three factors affect each other will help you get the most out of your camera.

Photograph IMG_3370 by gwgwgw

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