Tuesday, May 22, 2012
The Big Glass
Long-focal-length super-telephotos are obviously well suited to wildlife work. They also can give you a new perspective on your landscape photography.
Magnification is a function of the subject's size, its distance from the camera and the focal length of the lens used to make the photo. Using a longer lens, moving closer to the subject or choosing a larger subject will make the subject appear larger in the image frame.
Many wildlife photographers who use DSLRs and long lenses prefer "cropped-sensor" cameras to full-frame ones because the smaller sensors crop in on the image formed by the lens, producing a tighter framing than the same lens would produce on a camera with a larger sensor. For practical purposes, this is "magnification," although technically, it's not: A given focal length focused at a given distance produces a given magnification at the image plane. If you put a 300mm lens on a full-frame camera and focus on a moose at a certain distance, the moose's image will be 0.5 inches high at the image plane. Since a full-frame sensor measures approximately 1x1.5 inches, the moose's image will take up half the height of the frame. If you then put the same lens on an APS-C (1.5x) DSLR and take a photo with that camera from the same spot, the moose's image will still be 0.5 inches high at the image plane, but the APS-C sensor measures approximately 0.6x0.9 inches, so that the 0.5-inch-high moose image will occupy nearly the entire height of the photo. For practical purposes, putting the 300mm lens on the APS-C camera effectively turns it into a 450mm, but really it doesn't—the APS-C sensor just crops the image produced by the lens at the image plane more tightly.
It's commonly believed that long lenses compress perspective and short ones expand it. Actually, it's the shooting distance that determines the perspective; the focal length just determines the framing. You can prove this to yourself by putting your camera on a tripod, shooting an image with a long lens, then removing that lens, attaching a short one and shooting another shot from the same spot. Blow up the short lens shot so it covers the same field of view as the long lens shot, and you'll see that the perspective is the same. (Perspective refers to the spatial relationships among portions of a scene: how large one object appears relative to more distant or closer ones and how far apart the objects appear to be.)
We think of "telephoto compression" because we generally use long lenses at great distance—and shooting at great distance does indeed flatten perspective. And we think of "wide-angle expansion" because we generally use short lenses at close shooting distances—and shooting at close range does indeed expand perspective. From a landscape standpoint, choose your shooting distance (when possible) to produce the desired perspective and the focal length to frame the image as desired from that distance. Of course, in landscape (and wildlife) work, shooting from a desired distance isn't always possible, so you choose a focal length to frame, as desired, and live with the resulting perspective.
Depth Of Field
Long lenses produce less depth of field than shorter ones. This makes them great for shallow-depth-of-field selective-focus shots, but requires stopping them down when more depth of field is needed. But even stopping all the way down may not produce the desired depth of field, and stopping down increases the adverse effects of diffraction, reducing overall image sharpness. Due to the limited depth of field, when working with a long lens (especially a fast one wide open), precise focusing is imperative. (Tip: For wildlife subjects, focus on the near eye.) You'll have to work with your long lens for a while to get a "feel" for its depth-of-field characteristics.
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