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Tuesday, August 1, 2006

Mastering The Wide-Angle

Exploring with the wide end of the focal spectrum opens up a world of creative compositional possibilities

Wide-Angles With D-SLRs

With the exception of the Canon full frame EOS-1DS Mark II and EOS 5, all of today's popular digital SLRs have image sensors that are smaller than a full 35mm film frame. These smaller image sensors see a smaller portion of the full image produced by the lens than a 35mm film frame (or full-frame image sensor) does, in effect cropping the image into what a somewhat longer lens sees on a 35mm camera. What this means to the wide-angle fan is that you need a shorter focal length at a given angle of view. When used on one of the popular APSC digital SLRs (so-called because their image sensors are about the size of an Advanced Photo System C-format film frame), a 28mm wide-angle lens sees what a 42mm lens on a 35mm SLR would see.

To deal with this factor, camera and lens manufacturers have designed a series of lenses specifically for these smaller sensors. Since these sensors are smaller, the lenses can be smaller, and smaller wide-angle lenses aren't as costly as those that must cover a full 35mm film frame. So you can choose from a whole variety of superwide-angle lenses and wide-angle zooms for very reasonable prices. As an added bonus, these APSC lenses send light to the image sensor more efficiently than lenses designed to cover 35mm film frames do.

Each manufacturer has a special designation for its APS-C lenses: Canon uses EF-S, Nikon and Tokina use DX, Pentax uses DA, Sigma uses DC and Tamron uses Di II. These lenses offer excellent results with the small-sensor D-SLRs, but bear in mind that they can't be used with 35mm or full-frame D-SLRs because their smaller image circles won't cover the whole image frame; they'll vignette.


Fish-eye lenses have an exceptionally wide angle of view, generally 180 degrees. There are two types: circular fish-eyes produce a round image in the rectangular image frame, and full-frame fish-eyes fill the frame, with a 180-degree angle of view measured diagonally (corner to corner). Note that circular fish-eyes produce round images with 35mm and full frame D-SLRs; when used on APS-C D-SLRs, the smaller image sensors crop in on the circular image, filling the frame.

Full-frame fish-eye lenses for full frame SLRs have focal lengths of 15- 16mm. There are also non-fish-eye superwide-angle lenses in this range. The difference is that "regular" superwide—angles are corrected for barrel distortion and in theory render straight lines as straight lines, no matter where they appear in the image frame. Fish-eyes render straight lines that go through the center of the image as straight lines, but bend lines that don't go right through the center outward: they do exhibit barrel distortion.


For 35mm cameras, superwide-angle lenses are those with focal lengths shorter than 24mm; lenses from 24mm to 43mm are just plain old wide-angles. This is an arbitrary classification, though; all are rectilinear (meaning they render straight lines as straight lines, no matter where they appear in the image), and the only difference is the field of view, the shorter ones taking in more of the scene spread before the camera. For APS-C-sensor D-SLRs, the dividing line is 16mm instead of 24mm (multiply the 35mm camera focal length by 0.67 to find the focal length that provides the equivalent field of view on an APS-C D-SLR). For the Olympus Four Thirds System D-SLRs, divide the 35mm camera focal length by two to find the focal length that will provide an equivalent field of view on Four Thirds cameras.


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