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Tuesday, August 10, 2010

Lens Tech: Specialty Lenses For DSLR-Shooting Pros

For every job, there’s a perfect tool. Most photography can be handled by your core set of glass, but every once in a while, you need something different. These lenses don’t come out of your bag every day, but when you need one, nothing else will do.

This Article Features Photo Zoom

DSLR Format, Angle Of View And “Magnification Factor”

Full Four Thirds Format

APS-C Format

Full-Frame 35mm Format
Note that the angle of view produced by a given lens focal length depends on the format (size of the sensor) of the camera with which it’s used. A 50mm “normal” lens on a “full-frame” digital SLR provides a 46° angle of view (measured diagonally, corner to corner). A smaller APS-C sensor “sees” a smaller portion of the image produced by the lens, so the 50mm lens produces an angle of view of 31°30’, about what a 75mm lens would “see” on a full-frame or 35mm camera. On a Four Thirds System camera, with its even smaller sensor, the 50mm lens will produce a 23° angle of view, equivalent to what a 100mm lens sees on a full-frame camera.

This means a 35mm lens is “wide-angle” on a full-frame DSLR, but would be “normal” (52.5mm-equivalent) when used on an APS-C DSLR, and “portrait telephoto” (70mm-equivalent) when used on a Four Thirds System DSLR. In “telephoto” terms, a 200mm short tele on a full-frame DSLR frames like a 300mm tele on an APS-C camera, and like a 400mm super-tele on a Four Thirds System DSLR. You have to think in terms of angle of view for the format, or “equivalent focal length,” rather than just the raw millimeter count, when considering a lens for a specific project.

Some consider the effect of smaller sensors on angle of view to be a “magnification,” but this is not technically the case. A given focal length produces a given image size at the focal plane, and this doesn’t change because you change the size of the image sensor or film at the focal plane. If a 100mm lens focused at infinity produces an image of a specific distant tree a half-inch high at the image plane, it’ll be a half-inch high at the image plane whether there’s a 17.3x13.0mm Four Thirds sensor there or a 36x24mm full-frame sensor there. The magnification produced by the lens doesn’t change. However, that half-inch image will take up nearly the entire height of a horizontal Four Thirds System image, and just half the height of a horizontal full-frame image. So for practical purposes, you can consider the smaller sensor’s effect to be magnification, but technically, it isn’t.


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