Photography: Camera Shutter Speed
Aperture, Correct Exposure and Managing Movement and Motion
Shutter speed not only influences the exposure it also affects the sharpness of a photograph, and is a critical factor in portraying motion in an image.
For correct exposure the camera’s shutter must open for the precise amount of time to allow the correct amount of light to reach the light sensitive recording medium, be it digital or film.
In bright light, the shutter only needs to open for a short period for correct exposure; this is a fast shutter speed. Conversely, on dull days, or in the evening, the shutter must remain open for longer to allow the same amount of light to pass, or a slow shutter speed.
Nearly all lenses have a variable aperture that can open to allow more light to pass, or close in measured steps (f stops) to reduce the amount of light passing through in brighter conditions.
These two camera settings do more than ensure correct exposure. One step on the adjustment scales for aperture and shutter speed produces the same change in the amount of light for the image. Starting from a correct exposure setting, closing the aperture by one f stop reduces light coming through the lens, and then selecting the next slowest shutter speed allowing more light to pass, you will have exactly the same total amount of light entering the camera as before. This seemingly self-defeating exercise is important because it shows that there is more than one combination of shutter speed and aperture to provide correct exposure. Aperture affects the depth of field, while varying the shutter speed helps with movement of the camera and the subject.
Camera Shake
To get the clearest photos the camera needs to be held stationary when the shutter is open. It can be difficult to hold a camera absolutely still when taking a photograph without some form of assistance, such as tripod. The longer the focal length of the lens, the more apparent any slight camera movement is.
However, there are occasions when a bulky tripod is not available. A faster shutter speed minimizes the effect of any camera movement while the shutter is open. A starting point for selecting a shutter speed is that the preferred shutter speed is equal to 1/focal length. Therefore a lens with an effective focal length of 350 mm requires a shutter speed of at least 1/350 of a second, so that camera shake is not too obvious.
Capturing Motion.
Whenever the subject is moving the choice of shutter becomes an important consideration in creating the impression of movement in a still image.
For sports situations, a very fast shutter speed can capture a moment that is too fast to for the human eye. For example, the flexing of a golf club’s shaft as it strikes the ball, or a tennis ball stretching the strings of a racquet on a backhand. The distortion of the shape of familiar objects conveys the impression of motion and power. For digital camera users altering the ISO setting can help obtain faster shutter speeds.
On other occasions, a slow shutter speed may better portray the sense of movement. Landscape photographer often use slow shutter speeds to give a blurred sense of moving water, such as a waterfall or waves around rocks. In motor racing photographers use a shutter speed slow enough to purposely blur the wheels, indicating they are part of a fast moving object, while panning with the subject.
Camera Mode Selection
Many digital cameras offer shutter priority, or Tv mode, for conditions when the shutter speed becomes the prime consideration in the exposure setting. Using the Tv mode the photographer manually selects their choice of appropriate shutter speed and the camera automatically varies the aperture for correct exposure.
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