Tuesday, December 16, 2008
Sharp Is King
Strategies for waging the war between higher ISOs, sharpness and noise
Why ISO Matters
ISO (International Standards Organization) ratings represent the sensitivity, and thus the speed, of photographic films. Higher ISO films are more sensitive to light and record an image faster than lower ISO films, but the quality of the image is compromised by the resulting increased grain, loss of sharpness and degradation of color. In digital capture, ISO equivalents represent the sensitivity and recording speed of the digital sensor. As larger sensors packed with more megapixels are coupled with faster and more efficient processors, digital cameras can achieve clean capture in extremely low-light conditions, realizing at high-ISO equivalents results that are impossible to duplicate with silver-based film. Our tests show there’s little image degradation at ISOs as high as 1600, and acceptable results can be obtained at even higher ISOs.
With film capture, changing ISOs meant changing a roll of film or, for some of us, carrying two cameras equipped with different films. But the twist of a dial or push of a button adjusts the ISO on digital cameras from frame to frame. Flexible ISO capability can resolve a number of photographic problems related to light, such as the need for a faster shutter speed to stop action, a smaller aperture to increase depth of field or even a slower shutter speed to suggest movement.
To alleviate low-light conditions, we typically add electronic flash, or floods and reflectors in a studio. In outdoor/nature photography, projected flash and reflectors might work, but not at great distance.
To use one of our favorite, and most elusive, examples, consider the moose. Active and more easily found at dawn or in cool, overcast conditions, a moose doesn’t welcome a close approach, has a light-absorbing dark coat and is constantly moving as it feeds. What’s needed in this situation is a fast shutter speed to stop the animal’s movement and enough depth of field to sharply render the entire large body. Even with a fast telephoto lens, the optimum ISO of 100-200 won’t offer sufficient light-gathering capability to capture a usable image. Digital ISO to the rescue! By “dialing” up ISO 400, or even ISO 800, your D-SLR will allow you to choose the faster shutter speed and smaller aperture needed without a significant loss of quality.
Landscapes captured before sunrise and after sunset can yield a unique mood and unusual palette of colors that sometimes can be captured by a simple long exposure taken from a tripod. The problem can be the subtle movement of wind-stirred foliage or the clouds, moon or stars in the dark sky. If a sharp image is desired, the only solution to this problem is the increased ISO that will enable a shorter light-gathering exposure.
So why would you want to slow down the exposure time? Some cameras offer ISO settings as low as 50. This one- or two-stop desensitizing of the sensor, coupled with a polarizing filter, reduces light and lengthens the exposure to accomplish an effect, such as emphasizing the silky flow of water in a brightly lit stream or emphasizing the movement of a subject. Significantly longer exposure capabilities can be achieved with the lower ISO in combination with neutral-density filters.
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