Tuesday, July 26, 2011

Elements of Exposure - The Shutter - Part IV

This is part four of my elements of exposure series.

The shutter is the curtain in front of the recording medium. The shutter speed is the amount of time the curtain in front of the recording medium is left open to the light. The shutter speed is expressed in seconds or fractions of a second. The typical DSLR has many shutter speeds and will typically include those diagramed below.


<------------- Fast ---------------------- Moderate -------------------- Slow -------------->

In addition to these standard settings there is typically a Bulb setting that allows for an infinite amount of time for an exposure as decided by the photographer. In the standard shutter speeds illustrated above as we move from left to right we are doubling the amount of light for each successive speed or we are increasing exposure by one stop. Conversely, as we move from right to left we are cutting in half the amount of light or reducing exposure by one stop for each successive move in that direction. Shutter speed controls our ability to freeze action or reduce motion blur associated with a moving subject or blur as a result of camera movement or camera shake. A photographer would employ a fast shutter speed if he/she wishes to freeze action and/or reduce motion blur associated with camera shake. On the other hand, if a photographer wants to accentuate motion blur he/she would employ a slow shutter speed. The ability of the photographer to take images without noticeable blurring by camera movement is an important parameter in the choice of slowest possible shutter speed for a handheld camera. The rough guide used by most 35 mm photographers is that the slowest shutter speed that can be used easily without much blur due to camera shake is the shutter speed numerically closest to the lens focal length. For example, for handheld use of a 35 mm camera with a 50 mm normal lens, the closest shutter speed is 1/60 second. For a wider angle lens of 28 mm the shutter speed would be 1/30 second and for a telephoto lens of 200 mm the shutter speed would be 1/200 second. This rule can be augmented with knowledge of the intended application for the photograph, an image intended for significant enlargement and close-up viewing would require faster shutter speeds to avoid obvious blur. Through practice and special techniques such as bracing the camera, arms, or body to minimize camera movement longer shutter speeds can be used without blur. If a shutter speed is too slow for hand holding, a camera support, usually a tripod must be used. Image stabilization can often permit the use of shutter speeds 3–4 stops slower (exposures 8–16 times longer). The shutter speed compromise is deciding how much we wish to reduce motion blur of a subject or blur associated with camera shake versus our intention to artistically blur the action or increase camera blur to accentuate the feeling of motion in a scene. But, slower shutter speed, those typically longer than a couple of second, can introduce noise as well. Generally this is not a big issue due to most modern digital cameras ability to employ a noise reduction algorithm to counteract long exposure noise. However, longer exposures will have more noise than moderate or fast exposures.

Next we will take a look at dynamic range and what we can do when the dynamic range of a scene exceeds the camera's ability to capture it.

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