The term “Exposure Triangle” was coined by Bryan Peterson in his excellent book, Understanding Exposure. Understanding the exposure triangle is the first step you take after choosing to exit your camera’s auto mode, and venturing into the exciting world of manual exposure.
The exposure triangle is composed of three elements:
- Aperture – The size of the lens opening when the shutter release button is pressed
- Shutter Speed – The length of time the shutter stays open
- ISO – The camera’s sensitivity to available light
Changes to any point on the exposure triangle will impact the other two points. Make the shutter speed faster, and you will be able to capture fast moving objects in sharp focus. However less light enters the camera because the shutter closes quickly. Therefor either aperture or ISO must be adjusted accordingly to obtain a correct exposure.
By the same token, choosing a small lens opening (high f-stop) by manipulating the aperture will allow you to keep a larger than normal portion of your scene in focus. However the decrease in available light through the small lens opening will require shutter speed to be slower, or ISO to be raised, to obtain a correct exposure.
Strategic Approach to The Exposure Triangle
Once you understand what aperture, shutter speed, and ISO do, you can develop a strategy for how to use the exposure triangle in any given photo.
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California Spring Rolls, photographed with a macro lens and a low depth of field. The large lens opening required that I increase the shutter speed to get a proper exposure.
Approach each photo opportunity by asking yourself whether aperture or shutter speed is most important for the scene you wish to capture. If depth of field is of the utmost importance, then aperture is your number one priority. If you wish to capture a fast moving subject in sharp focus, or render an artistic blur of movement, then shutter speed is the most important component to consider.
Once you decide whether aperture or shutter speed is most important to you, set it exactly as you desire. Then adjust the other component until your viewfinder indicates a correct exposure. Don’t increase ISO unless it is a last resort.
Example – Large Depth of Field
In a case where you wish to have a large depth of field, you would set your aperture to a small lens opening such as f/16 or higher. Doing so would probably reduce the amount of available light to the point that your image would be underexposed.
The next step would be to compensate by lowering shutter speed to allow more light to enter the camera. A slow shutter speed might require placing your camera on a tripod to negate blur from camera shake. If no tripod was available, the third and final choice would be to increase your ISO from 100 to a higher number. Every step up in ISO will introduce additional noise into your image, which is why I consider ISO increases to be a last resort.
Example – Fast Moving Sports
In this case we wish to photograph fast moving sports and capture the subject in sharp focus. Our number one priority becomes having a fast shutter speed (1/500s or faster). The downside of the fast shutter speed is that there is less time for light to enter the camera, and so we must compensate by going with a larger lens opening (aperture of f/5.6 or lower).
If lowering the aperture does not yield enough light, or we want more depth of field, then we will have to raise ISO to obtain enough light for our exposure.
Example – Motion Blur
Here we wish to use a slow shutter speed to create a motion blur of objects moving through the scene. This artistic effect is quite interesting, and usually necessitates unusually slow shutter speeds and the use of a tripod to keep the camera steady.
The increased length of time the shutter is open means a lot of light is getting into the camera. Probably too much. This requires that we raise the aperture to a smaller lens opening, thus decreasing the available light.
We cannot lower ISO below the minimum, so if there is still too much light entering the camera, we will need to use a filter to decrease the light in the scene.
Wrapping it up
Employing use of the exposure triangle is as simple as understanding which point on the triangle is most important artistically to our scene, and then compensating with the other two points until a correct exposure is obtained.
Keep shooting!
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