Tuesday, October 25, 2011
Create dramatic and dynamic landscape images when you explore the ultrawide end of the spectrum
There also are chromatic aberrations, which cause different wavelengths of light to focus at different distances from the lens. Extra-low-dispersion elements are used to correct this. These have designations like ED (Nikon), UD and SUD (Canon), FLD (Sigma), SD (Tokina), LD (Tamron) and Super ED (Olympus). Like aspherical elements, these are found in the higher-end ultra-wide-angle zoom lenses.
Vignetting. The image produced by a lens tends to be brightest at the center, where the light rays travel straight through, and dimmer at the edges of the frame, where the light rays are bent the most. This darkening of the corners and edges of the image is vignetting, and it's especially apparent in wider-angle lenses. Most ultrawide zooms are corrected for vignetting, but it's not possible with current technology to completely eliminate it. You can reduce vignetting by stopping the lens down.
Focus Shift. A true zoom lens maintains focus as you zoom it. Most zooms on the market change focus as they're zoomed, however. This isn't a problem when using autofocus because the AF system automatically corrects for it. But when you're focusing manually, you need to be careful as you zoom. If you zoom the lens to change the framing, be sure to refocus at the new focal length.
Aperture Range. Constant-aperture zoom lenses maintain the aperture as they're zoomed, but they tend to be expensive. In variable-aperture zooms, the maximum aperture size shifts as the focal length is increased. TTL metering automatically compensates for this, but you need to pay attention if you're shooting with a fast aperture to keep the background out of focus.
Depth Of Field. Depth of field refers to the distance in front of and beyond the point of focus in which objects in a scene appear acceptably sharp in the image. It depends on a number of things, including the viewed image size, viewing distance, camera format, lens focal length, focused distance and aperture.
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