Exposure is the amount of light allowed through the lens onto the sensor of a camera. The primary controls for exposure are aperture (amount of light) and shutter speed (length of exposure). The ISO setting also effects exposure.
A picture can be underexposed, overexposed or properly exposed. See examples below.
An over exposed image will suffer a loss of detail in the brighter areas and appear bright white and somewhat washed out.
An under exposed image will appear too dark and will have a loss of detail in the darker areas of the image.
A proper exposure will have good balance of brightness across all areas of the photo. Details will appear clear in both dark and bright areas.
The cameras internal light meter will judge the amount of light entering the camera and give a recommended exposure. In most cameras the meter display is located at the bottom of the viewfinder. At its most basic, achieving the proper exposure is a matter of aligning the marker on the meter to the center of the display by adjusting shutter speed and aperture.
Under Exposed (Green marker is on the left of center)
Over Exposed (Marker is on the right of center)
Proper Exposure (Marker is in the center of the meter)
With partial, center weighted, or spot meter settings, a good way to properly expose your subject, is to aim the selected auto focus point at the subject and depress the shutter button half way.The meter will use the auto-focus point as the area to measure brightness. Continue to hold the button down then recompose the shot. This will ensure that you are metering for your subject and not the brighter or darker background. The amount of light available will largely dictate your shutter and aperture (fStop) settings. Low light will require slower shutter speeds and lower (wider) aperture values.
When selecting a shutter speed its important to generally keep the speed faster that 1/60th of a second. Speeds slower than that can lead to blurry images due to camera shake or movement within the photo. With slow speeds, mount the camera on a tripod or brace it against something solid. For a more accurate rule on shutter speeds and sharp images, see the Reciprocal Rule in the Focus section.
fStops will influence focus. Depth of field is directly related to aperture. Low fStop values will give a shallow depth of field (only a small range of the photo in focus).
High fStop values will give a large depth of field (a large range of the photo in focus). For a better understanding of this, see Depth of Field in the Composition, Color and Light section.
Sharpness is also effected by aperture. All lenses have an fStop value that will produce the sharpest image possible for that lens. For more on this see the following section on Focus.
Which do you select first, shutter or aperture"> Anything you can do to stabilize a camera will help prevent camera shake. Use a tripod or brace the camera against something solid. Even touching the camera when shooting at slow shutter speeds can cause blur. Use of a remote shutter release or the cameras self timer will allow hands free use of the camera.
As a safety measure, bracketing can be used to ensure that your have captured the best exposure. For each proper exposure you take, take two more shots, one at 1 fStop over and one at 1 fStop under the proper exposure. Some cameras will offer an automatic means of doing this.
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