Monday, January 29, 2007
Lens Buying Guide
Everything you need to know about focal lengths, maximum apertures, new technologies and more!
Magnification Factors. As much as angle of view is controlled by lens focal length, it's also governed by the size of your image sensor. If you use the same focal length on two sensors of different size, then the angle of view, or magnification, changes.
On a 35mm film camera, a 50mm is a normal lens. When a lens of that focal length is used on an 8-megapixel advanced compact, it gives an angle of view equivalent to 200mm on the 35mm camera. That's because the advanced compact's image sensor is four times smaller than 35mm film, so its angle of view at a given focal length is four times narrower and its apparent magnification is therefore four times greater.
Since the image sensors of most D-SLRs are smaller than 35mm film, lenses naturally have a narrower angle of view when they're mounted on D-SLRs than they do with film. The apparent magnification for most D-SLRs is from 1.3x to about 2x that of the 35mm film format. A 20mm lens on a D-SLR with a 1.5x magnification factor, for example, sees the same angle as a 30mm lens would when it's mounted on a film camera; your 50mm behaves like a 75mm, and a 200mm acts like a 300mm.
In many respects, a digital SLR's magnification factor is good news. It increases the reach of your existing telephotos, with neither the light-loss penalty of a teleconverter nor the bulk of using longer focal-length lenses. Until recently, though, magnification factors were a problem on the wide side-when you mount your 28mm lens on a D-SLR, it sees the same angle of view as a 42mm lens does with film. This problem is now answered with new ultra-short lenses for D-SLRs.
Perspective. You can use distance together with focal length to expand or compress your picture's perspective at will. Move back and use a long lens for compression; come in close and use a wide-angle for a sense of depth. This works because distance controls perspective-the closer you are to something, the more perspective foreshortening there will be, with foreground objects looming large and background objects receding into the distance. When shooting up close, then, the wide-angle's job is to encompass the enlarged foreground objects so you can capture the resulting impression of depth. On the other hand, if you back up and use a telephoto, the same foreground object now will appear similar in size to the background, making the scene compressed in perspective.
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