Exposure in digital photography refers to the amount of light being absorbed by the image sensor. Proper exposure means you’ve used the correct amount of light, through a combination of shutter speed, ISO, and aperture settings. Most of the time, the camera’s automatic exposure settings will result in proper exposure. However, in some lighting situations, you need to set the exposure manually for the perfect photograph.
With most DSLR cameras, the mode in which you’re shooting will determine which settings are selected by the camera and which settings you must select. Programmed (P), Shutter-priority (S), Aperture-priority (A), and Manual (M) are the most common modes where you can set at least some of the exposure settings manually. In other modes, the DSLR camera probably will set the exposure automatically.
The DSLR camera will use a few different metering modes to determine the proper exposure automatically. You can pick the metering mode, though, which can help you end up with the best results.
- Matrix metering. Try matrix metering for the best overall results; consider matrix metering as your “fall back” option for determining exposure. Matrix metering is good for photos that don’t have a precise subject.
- Center-weighted metering. This type of metering assigns the greatest emphasis for determining exposure on the center of the frame.
- Spot metering. Spot metering is used when the subject is off center. You tell the camera which specific area to use for determining exposure.
As with manual focus, you also can “lock” the exposure on a particular area in the scene. The camera will then automatically apply the exposure it selects for that particular area on the entire scene.
You might want to lock the exposure when shooting a moving subject, for example. Depending on your model of camera, you’ll probably need to be using center-weighted or spot metering, rather than matrix metering, to activate auto-exposure lock.
The method for using auto-exposure lock differs from camera to camera, but most models require you to press the shutter button halfway, while also pressing an AE or AE-L button. You might need to continue pressing the AE button as you recompose the photo and shoot it.
As you turn to using manual settings to control exposure with your camera, and you start locking at what's available, you may wonder which are the most common settings in ISO, shutter speed, and aperture that can help you achieve the proper exposure.
- ISO. With the ISO setting, a higher number will allow the image sensor to perform better in low light, but it also will create more stray pixels, called noise. A low ISO setting will have little to no noise, but it is best used in excellent lighting. A typical ISO setting is 400.
- Shutter speed. Shutter speed numbers can be a little confusing. On some DSLR cameras, the shutter speed may be listed as 500, which is equal to 1/500th of a second, which is a pretty fast shutter speed. Fast shutter speeds will allow less light to strike the camera image sensor versus a slower shutter speed, say 1/30th of a second. When shooting with a slow shutter speed, you can shoot in low light conditions. A typical shutter speed is about 1/125th of a second.
- Aperture. The aperture is marked with the F setting on the DSLR camera. You typically set the aperture through a manual dial on the lens or through on-screen menus. A higher aperture number means that less light is allowed through the lens but more of the scene is in focus. A lower aperture number means more light enters, but a smaller “slice” of the scene, called the depth of field, is in focus. A typical aperture number is F5.6.
Each of these settings affects the exposure of the scene, and they work together. For example, if you’re shooting at the typical settings listed above and you want to increase the shutter speed to 1/250th of a second – which means less light will strike the image sensor – you’re going to either have to increase the ISO or use a lower aperture number to maintain the same exposure level, as adjusting one of those settings will allow more light to reach the image sensor.
As you make adjustments to each of these settings, always keep the exposure in mind!