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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

Wide-angle lenses are essential landscape photography tools for their ability to take in epic vistas and allow you to move in close to emphasize a foreground object while still showing enough of its surroundings to give a feel for the location.

Clockwise from top left: Nikon AF-S Zoom-Nikkor 14-24mm ƒ/2.8G ED; Tokina 17-35mm ƒ/4 PRO FX; Tamron SP 10-24mm F/3.5-4.5Di II LD Aspherical; Canon EF 17-40mm ƒ/4L USM
Wide-angle zooms offer a number of advantages. For one, they provide an entire range of focal lengths in a single, convenient package. A 16-35mm zoom not only gives you 16mm and 35mm, but every focal length in between, including the popular 18mm, 21mm, 24mm and 28mm. A second wide-angle zoom benefit is instant access to any of these focal lengths. Just twist the zoom ring, and you can crop the scene as you wish—much quicker than removing one lens, attaching another, and seeing if that meets your needs. With DSLRs, equally important is that zooms minimize lens changes in the field, which, in turn, minimize dust spots on your image sensor.

To choose the best wide-angle zoom for you and your photography, there are some key factors to keep in mind.

What's Wide Angle For Your Camera Format?
Longtime 35mm photographers think of lenses from 35mm and shorter as wide-angles because these focal lengths produce a wider angle of view than a focal length equal to the format's 43.2mm diagonal measurement (or the 50mm focal length popularly considered "normal" for the 35mm format). But a focal length's angle of view also depends on the format of the camera with which it's used. With film, this wasn't a big deal. All 35mm SLRs produced 36x24mm images, so a given focal length on one 35mm SLR would produce the same angle of view on any of them.

With digital SLRs and mirrorless digital cameras, sensors come in different formats, and each produces a different angle of view with a different focal length because it "sees" a different portion of the image produced by the lens. The sensor in a "full-frame" DSLR is the same size as a standard 35mm image frame, so a given focal length on a full-frame DSLR will produce the same angle of view as it would on a 35mm SLR. The sensors in APS-C DSLRs are smaller (23.6x15.6mm or so vs. 36x24mm), so they "see" a smaller portion of the image produced by a given lens. APS-C digital cameras have a 1.5x crop factor, so a given lens on an APS-C camera shows the field of view of a lens 1.5 times longer on a full-frame camera. A 35mm "wide-angle" lens becomes a 53mm "normal" lens when used on an APS-C camera for framing purposes.

Sweet Spot
Every lens has a sweet spot, an aperture at which it provides its best performance. That's because various aberrations reduce sharpness at large apertures, and diffraction reduces it at small ones. With most lenses, the sweet spot is a stop or two down from wide open (for example, ƒ/5.6 or ƒ/8 for an ƒ/4 lens), and it's best not to stop down beyond that unless you really need the increased depth of field. When you stop a lens way down, depth of field increases, but diffraction reduces overall resolution—objects in front of and beyond the point of focus will appear sharper relative to the focused subject, but the focused subject itself will be less sharp.


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