Exposure is affected by three elements:
These three elements in combination determine the amount of light that enters a camera to create an image. For a multiple exposure, where a single frame is exposed more than once, the total exposure is the sum of all the exposures for that frame.
There is no single correct exposure for a specific photograph. At ISO 100, a picture taken at f/22 at 1/4 seconds allows the same amount of light to hit the film or sensor as a picture taken at f/2.8 at 1/250 second. Of course, not many people can hand hold a camera at 1/4 of a second and get a sharp image and you don't have much depth of field at f/2.8; therefore, every exposure is a compromise. The photographer must decide how much depth of field they need, while also thinking about how much of the action they want to freeze (or blur), what focal length lens they are using, can they use some sort of support, etc.
To summarize these aspects of exposure:
- Camera support: the longer the exposure (the slower the shutter speed), the greater the chance of blurring due to camera shake. Unless the camera is attached to a sturdy tripod, a higher shutter speed should be used.
- The same is true if one is trying to photograph fast action.
- The same is true when using long lenses: the longer the lens, the higher the shutter speed should be.
- Depth of field: to increase depth of field (to get parts of the scene at greater distances in focus), a smaller aperture (higher f-stop number) should be used.
- To decrease depth of field (focus only on subject and let everything else go out of focus), a large aperture (lower f-stop number) should be used.
Exposure Bracketing & Compensation
- Exposure compensation is the practice of allowing more or less light into the camera than its light meter suggests is appropriate.
The light meter is most often tricked by strongly backlit scenes such as a bright sky behind the subject, or strong light reflecting off sand, water or snow. To compensate, many cameras allow for the exposure to be increased (or decreased) by increments of 1/3 of an f-stop, generally to a maximum of 2 full f-stops.
- Exposure bracketing is the technique of taking multiple photos at different f-stops to ensure at least one of them will be correctly exposed.
Generally, three shots are taken: One at the camera or light meter's suggested exposure, and one at perhaps one f-stop darker and then one f-stop lighter.
High Dynamic Range (HDR) photos often use three photos with bracketed exposures to create a composite photo with enhanced detail in areas of strong shadow or light that could not all be captured in any one single exposure. Rather than modify the aperture, it's preferable to shoot the three photos with different shutter speeds so that the depth of field remains constant.
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