Լ����Ƶ

Although digital SLR (DSLR) cameras are the tool of choice for professional bird photographers, beginners are often daunted by the prospect of learning how to operate these complex devices. Fortunately, by understanding just a few simple concepts, you make the best use of your DSLR’s capabilities. In particular, learning how to control exposure will ensure that your subject is well lit, has good color and detail, and stands out from the background.

There are three major factors influencing exposure: the “f-stop,” or aperture (the size of the hole light travels through to reach the digital sensor); the shutter speed (dictating how long the shutter is open), and the ISO (which mimics the sensitivity of film to available light). All affect image brightness, though they affect other things, too. Aperture, for instance, affects depth-of-field (how in focus the background is); shutter speed affects sharpness (faster speeds reduce motion blur), and the ISO settings influence the amount of digital “noise,” or graininess, in the image. The challenge is choosing settings that produce good exposures while minimizing blur and noise.

Automatic exposure modes can help simplify this task. A popular choice is “aperture priority” mode: You set the aperture to the smallest f-stop your lens will allow (i.e., wide open, to isolate the bird from the background), set the ISO to a reasonably high number like 800, and let the camera choose the shutter speed. If it chooses a speed that is too slow to freeze the bird, you can increase the ISO to increase the shutter speed. So instead of having to actively manage three settings (aperture, shutter speed, ISO), you need to manage only one (ISO).

The main problem with this approach is that the metering system in the camera may choose an exposure that looks good for the background but not the bird. Selecting a different metering mode can help. Evaluative/matrix metering uses readings from the entire scene, while spot metering uses one spot (for example, the center). However, since the spot meter may be larger or smaller than the bird, even if you point the spot at the bird, the camera may not choose settings that optimally expose the whole bird. A quick fix is to dial in an “exposure compensation” that tells the camera to shoot brighter or darker than the meter is telling you is right.

A more advanced solution is to use manual mode and do the metering yourself. Set your aperture to wide open, and then set the shutter speed to about 1/500th of a second and the ISO to around 800. Now take a series of test shots, checking brightness on the LCD after each shot. Turn on highlight alerts (“blinkies”), to see if you are overexposing; if you are, reduce the exposure before the next shot (by turning down the ISO or increasing your shutter speed). Once you’ve found settings that work well, you’ll often need to make only small changes after that—for instance when the light changes or you move from a white bird to a black bird. By using the LCD to judge exposure, you can see how the bird looks rather than blindly trusting the camera’s metering system to get it right.

Hopefully, someday manufacturers will offer a “bird mode” to make all of this simpler, but until then, I recommend that beginners start with one of the automatic modes and then progress to manual mode when they feel more confident.

William H. Majoros has been studying birds and bird behavior for more than 20 years. This article is adapted from his online reference guide, .