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Tuesday, February 28, 2012

Wide-Angle Zooms

The ins and outs of a landscape photographer’s most used lens

Labels: LensesGear
This Article Features Photo Zoom

LEFT: Sigma 12-24mm ƒ/4.5-5.6 DG HSM II; RIGHT: Sony Carl Zeiss 16-35mm ƒ/2.8
The Four Thirds System (and Micro Four Thirds System, which uses the same 17.3x13.0mm sensor size) has a 2x crop factor. Any given focal length used on a Four Thirds camera frames like a lens of twice its focal length on a full-frame camera. That is, a 35mm lens used on the Four Thirds System camera frames like a 70mm lens on a full-frame camera.

The point here is that which focal length constitutes "wide-angle" varies with camera format. For 35mm SLRs and full-frame digital cameras, lenses of 35mm and shorter are wide-angle, and lenses of 21mm and shorter are superwide-angle. For an APS-C camera, the equivalent focal lengths would be two-thirds of those for full-frame—wide-angle starts at 24mm and superwide-angle at 14mm. For Four Thirds cameras, the equivalent focal lengths would be half of those for full-frame—18mm for wide-angle, 11mm for superwide.

Exotic Glass Elements
Wide-angle lenses are prone to spherical aberration (light rays traveling through the edges of the lens don't focus at the same plane as rays traveling through the lens closer to its center), especially fast wide-angles with their large front elements, so lens designers include one or more aspherical elements to minimize this. Designers also employ extra-low-dispersion elements with such designations as ED, UD, SUD, FLD, SD, LD and SED to minimize chromatic aberrations (different wavelengths of light focusing at different distances behind the lens or at different distances from the optical axis). All of the wide-angle zooms listed in our lens chart incorporate both aspherical and extra-low-dispersion elements.

Distortion & Perspective
In theory, true wide-angle lenses are rectilinear; they reproduce straight lines in the scene as straight lines no matter where they appear in the image frame. In practice, wide-angle lenses, and especially superwide-angle lenses, do bend straight lines near the frame edges: barrel distortion. Fisheye lenses aren't rectilinear; they bend straight lines outward unless those lines pass right through the center of the image.

Focal Ratio
Lenses are commonly designated by focal length and maximum aperture. For example, a 17-35mm ƒ/4 wide-angle zoom provides focal lengths from 17mm through 35mm and has a maximum aperture of ƒ/4 at all focal-length settings. What does that "ƒ/4" mean? The ƒ-number is simply the ratio between the diameter of the entrance pupil (the apparent size of the aperture as viewed from the front of the lens, not the physical diameter of the diaphragm opening) and the focal length. When the 17-35mm zoom is set at 17mm and wide open, the effective aperture diameter is 17mm divided by 4 = 4.25mm. When the lens is zoomed to 35mm, the effective aperture diameter is 35mm divided by 4 = 8.75mm.

While most pro zooms have fixed maximum apertures like the 17-35mm ƒ/4, lower-priced zooms have variable apertures; the effective aperture gets smaller as the lens is zoomed to longer focal lengths. For example, the 18-55mm ƒ/3.5-5.6 "kit" zoom sold with many entry-level DSLRs has a maximum aperture of ƒ/3.5 at 18mm, shrinking to ƒ/5.6 at 55mm. The diameter of the entrance pupil is 5.14mm at 18mm (18mm divided by 3.5) and 9.8mm at 55mm (55mm divided by 5.6). If the lens maintained the ƒ/3.5 aperture throughout its focal-length range, the entrance pupil would have to be 15.7mm at 55mm (55mm divided by 3.5)—much larger, resulting in a larger and more costly lens.

TTL (through-the-lens) metering automatically compensates for the change in aperture as you adjust the focal length of a variable-aperture zoom lens, but it's something to keep in mind should you use a separate handheld exposure meter à la Ansel Adams.


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