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Tuesday, February 25, 2014

Super Telephoto Zooms

We decipher the technobabble behind these nature photography mainstays

Labels: Gear
This Article Features Photo Zoom
The variable-aperture designs are also more likely to shift focus as you change the focal length. If you're using AF, this doesn't matter, as the system will compensate automatically. But if you focus manually with a variable-aperture zoom, you'll have to focus at the focal length you'll be using for the shot. It's nice to zoom to the longest focal length to get a magnified image for focusing, then zoom back to the desired composition, but with lenses that shift focus as they zoom (which includes some constant-aperture zooms), you can't do this because the focus will shift when you change the focal lengths. Test your zoom(s) to see if focus shifts when you zoom. If in doubt, it's always safer to focus at the focal length you'll be using for the shot.

Angle Of View
Shorter focal lengths provide wider angles of view than longer ones. But just how wide or narrow a given lens' angle of view also depends on the format of the image sensor. Smaller sensors "see" less of the image produced by a given lens, producing a narrower angle of view. Larger sensors see more of the image, producing a wider angle of view. See the diagram at left.

Smaller DSLR sensors generally are assigned "crop factors" based on how their angles of view compare to that of a full-frame sensor (36x24mm, the size of a 35mm film image frame). In the early days of digital, this helped film shooters quickly understand what a given lens would do when used on a smaller-sensor camera (early digital SLRs had smaller APS-C sensors, so named because they were approximately the size of an Advanced Photo System "Classic-"format image frame). As mentioned earlier, a full-frame DSLR image sensor measures 36x24mm and has a diagonal of 43.2mm. An APS-C sensor measures around 23.6x15.6mm and has a diagonal of around 28.3mm. Since the full-frame diagonal is about 1.5X longer than the APS-C diagonal, a given lens on an APS-C camera produces the same field of view as a lens 1.5X longer on a full-frame camera. For example, a 200mm lens on an APS-C camera frames like a 300mm lens on a full-frame camera—great for telephoto fans. But a 28mm lens on an APS-C camera frames like a 42mm lens on a full-frame camera—not so good for wide-angle fans. That's why the kit zooms sold with APS-C cameras start at 18mm: An 18-55mm zoom on an APS-C camera frames like a 27-83mm on a full-frame camera.

Four Thirds System sensors measure 17.3x13.0mm, with a diagonal measurement of 21.6mm—half that of a full-frame sensor. So a given focal length on a Four Thirds System camera frames like a lens twice as long on a full-frame camera: A 200mm lens on a Four Thirds DSLR (or Micro Four Thirds mirrorless camera) frames like a 400mm lens on a full-frame DSLR.

Note that this is a crop factor, not truly a magnification. When focused at a given distance, a given focal length produces an image of a given subject as a given size at the focal plane. For example, a 100mm macro lens focused on a 20mm-high object at 1:2 produces an image of the object 10mm high at the image plane (sensor or film). This size doesn't change just because you put a larger or smaller sensor at the image plane. A smaller sensor just crops more tightly, so the object's 10mm image takes up more of the frame. For most practical purposes, it can be considered magnification, but in reality, it's not.

Several manufacturers offer lenses designed for full-frame cameras (these include 35mm SLR lenses) and lenses designed specifically for the smaller APS-C sensor. The full-frame lenses cover an image circle 43.2mm in diameter—the diagonal measurement of the full-frame sensor (or 35mm film frame). APS-C lenses cover an image circle of around 28.3mm—the diagonal measurement of an APS-C image sensor. If you use an APS-C lens on a full-frame camera, the image will vignette. Canon calls its APS-C lenses EF-S, and you can't even physically mount one on a full-frame camera. Nikon calls its APS-C lenses DX, and if you mount one on a full-frame Nikon DSLR, it will crop to the smaller DX format automatically. Sigma calls its APS-C lenses DC, Sony—DT, Tamron—Di II and Tokina—DX. (Pentax doesn't make full-frame DSLRs, so their DSLR lenses are APS-C.)


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