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Shifting Exposure

June 3, 2008 - Dave Hecei
Now that we have discussed apertures and shutter speeds, let’s take a look at how they relate to each other.

When the camera’s meter reads a scene, it measures the amount of light, the Exposure Value (EV), and decides what shutter speed and aperture to use. DSLR cameras have sophisticated computers that read the amount of light, the focusing distance, and the lens focal length to determine what shutter speed and aperture combination will work best. There may be times when this combination is not what is best for the shot.

If you are shooting a sporting event, football, baseball, soccer, etc., you know that there is plenty of fast moving action. To stop that action, you need to use a very faster shutter speed otherwise the photo will be blurry. When shooting in the P, or Program, mode, most DSLRs have the ability to ‘shift’ the settings while still maintaining proper exposure. Check your camera’s manual on how to shift the exposure. On most DSLRs this is done by turning the control wheel by the shutter release (on my Canon EOS it is a large control wheel on the back).

By turning the control wheel, the shutter speed will either increase or decrease in speed. If you look at the exposure settings while you do this, you will notice that as the shutter speeds go up, the aperture goes down. The opposite happens as you decrease the shutter speed the aperture goes up. As an example: the camera gives you a reading of 1/60 at f/8. You need to use a faster shutter speed so as you dial up to 1/125 the aperture goes down 1 stop to f/5.6. If you go to 1/250 sec, the aperture goes to f/4. Each of these combinations of shutter speed and aperture gives the same exposure for the scene. If you increase the shutter speed 1 stop you must decrease the aperture 1 stop to compensate.

This also applies if you are shooting a landscape, or even a portrait. We learned that the aperture controls the amount of light coming through the lens, but it also controls something called depth-of-field (DOF). DOF is the area in front of and behind the point of focus that will also be in focus. To decrease the DOF, use a larger aperture (remember that’s a lower number). This works great with a single portrait where you want the background to blur out. If you are shooting a wide landscape, you will want to use a smaller aperture (a larger f-number) giving you greater DOF. Just turn the control wheel to shift the exposure to get the aperture you need.

When shifting the exposure, just remember to keep the shutter speed high enough for shooting hand held. The rule was to use a shutter speed equal to or faster than 1 over the focal length of the lens. If you have a 200mm lens you need to use a shutter speed of 1/250 or faster. You can always shoot at slower shutter speeds if you have a tripod and cable release.


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