According to my text, "Learning how to make good exposure is the first step in becoming a good photographer"(Fitzharris, 63). Getting the correct exposure depends on aperture size and shutter speed. If one of these two are off, the picture could come out looking way too dark, way too light, or not in focus the way you want it. The first step in determining your aperture and shutter speed settings is to first measure the light of your subject. This is done with the use of a digital histogram. It measures luminance values of your frame and tells you what parts of the frame are being recorded by the light sensor. The histogram should drop to zero before reaching the frame end. If the histogram is skewed to the left (humped to right), you must decrease exposure. If the histogram is skewed to the right, you must increase exposure.
Light metering is very important for helping to determine the camera setting needed to make a good exposure. For most normal circumstances, light meters work excellent. It is only the more complicated lighting situations where the photographer has to make a judgment on whether to fully trust the light meter. Most DSLR cameras today come with four or more light metering pattern. The first of these is evaluative/matrix metering. This type measures light from multiple locations around the frame and generates a "compromise" exposure. It is the general-purpose light metering mode that should be set as a default on your camera. Second, there is averaging metering. The author notes that this type of metering is barely ever used and unfortunately doesn't tell why it would be used. Third, there is spot metering. This type of metering is based off of a single spot in the frame. It is mostly used for more making sure the head region of wildlife is in focus. Fourth, there is center-weighted metering. This metering mode gives about 75% of the priority to the center of the frame.
There are three different exposure modes according to the text including aperture priority mode, shutter priority mode, and manual mode. Aperture priority mode allows you to set the desired aperture while the camera automatically sets the shutter speed. Shutter priority mode allows you to set the desired shutter speed while the camera automatically sets the aperture. Manual mode allows the photographer to set both shutter speed and aperture based on light meter readings in the viewfinder.
It is also important to compensate for extremely bright or dark frames by utilizing exposure compensation. If you were to just strictly follow averaging metering in bright or dark frames, the picture would come out looking gray. For frames that are extremely bright, like snow, you can increase exposure by a half to full stop by turning a wheel on top of the camera without taking your eye off the viewfinder. Likewise, if the frame is dark you can decrease the exposure by the same amount.
There are times when the frame has both bright and dark areas that the previously mentioned techniques don't always work to obtain good detail across the entire picture. In these cases you have to decide which parts of the picture are most important.
Backlit subjects can often be difficult to get sharp detail due to the contrasting brightness and darkness. This is because the digital light sensors have a hard time compensating for both at once. For these pictures, you have to decide which part of the frame is most important and give its exposure more priority. You can do this by taking a spot reading on the center of interest and decrease the exposure by about a full stop. Then just double check the the histogram hump is centered and falls to zero within the bounds of the frame.
White and light scenes can be difficult to shoot due to the fact that the camera will automatically want to turn the brightness into gray. The author suggests increasing exposure by one to two stops past the meter reading. If the main subject is small and bright while the rest of the scene is dark, the author suggests decreasing exposure of the evaluative readings by a half stop in order to prevent overexposure.
Citations:
Fitzharris, Tim. National Audubon Society Guide to Nature Photography Digital Edition. Buffalo: Firefly, 2008.
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