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Tuesday, April 5, 2011

Landscape Lens Tech


What makes a good landscape lens?

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This Article Features Photo Zoom
Coverage
An important consideration if you use filters for your landscapes is whether they will produce vignetting with your lenses. This is especially important with wide-angle lenses. If you use filters with wide-angles, you should use the special thin ones whenever possible to avoid or minimize vignetting. If you’re shooting digitally, you can correct some vignetting using RAW conversion software. Some cameras even correct it in-camera, but it’s best to avoid vignetting in the first place whenever possible.

Speed
Lens speed isn’t really a factor for a landscape lens since the subject isn’t moving (and, hopefully, neither is your camera). Fast lenses are more costly than slower ones, and also much bulkier. Especially when hiking to less accessible vantage points, lens bulk is a consideration. If you’re working from your vehicle, the lens bulk is less important, and a number of the pro landscape shooters we polled about their favorite landscape lenses do use fast zooms. Also, bear in mind that the best pro lenses tend to be fast and bulky.

Lens Aberrations Defined

Various aberrations affect lens sharpness, either overall or at different parts of the image. Lens designers generally counter these by using combinations of special elements in their designs.

Spherical Aberration: Parallel light rays passing through the edges of a spherical (evenly curved) lens element aren’t brought into focus at the same plane as light rays traveling through the center of the lens. This spherical aberration is most obvious in wide-angle and wide-angle zoom lenses. You can reduce it by stopping the lens down (and thus increasing depth of focus). Lens manufacturers use aspherical lens elements—elements on which the curve changes from center to edge—to reduce or nearly eliminate spherical aberration.

Chromatic Aberration (CA): Conventional glass elements tend to focus short (blue) wavelengths closer to the lens than medium (green) wavelengths and long (red) ones. This is called axial, or longitudinal, chromatic aberration. Such elements also tend to focus blue wavelengths traveling at an angle to the lens farther from the lens axis than green or red wavelengths; this is lateral chromatic aberration. Lateral chromatic aberration is a major cause of the purple/green color fringing you often see in digital images. This can be corrected when postprocessing RAW images, to a large degree, but the best results will occur with lenses that don’t produce it (or at least minimize it) in the first place. Stopping the lens down can reduce the effects of axial CA a bit, but not lateral CA. Manufacturers greatly reduce both types of CA by using low-dispersion and fluorite elements (and Canon’s DO Diffractive Optics elements). Note that CA affects black-and-white images, too, reducing sharpness.

Astigmatism: Just as with human eyes, an astigmatic lens produces differing degrees of focus in different parts of the image. Stopping the lens down can reduce it a bit, but better lenses are well corrected for it.

Coma: Coma causes light rays from an off-axis point passing through the edge of a lens to refract differently than rays from the point passing through the center, resulting in comet-shaped blurs rather than points in the image. Better lenses are corrected for coma.

Curvature of Field: A single curved lens element will focus an image on a curved plane, not a flat one, so that if the center of the image is focused, the edges will be unsharp, and vice versa. Stopping the lens down can reduce the effect; a good lens eliminates the effect through use of multiple elements.

Bokeh: This is a term for the way-out-of-focus elements—especially those in the background—that appear in an image of a nearby subject shot with the lens wide open. Some lenses have “pleasant” bokeh, others not-so-pleasant bokeh (pleasant/unpleasant depends on the observer’s personal taste, to a degree). Part of it is due to the lens design, its aberrations and how they’re corrected, and part to the aperture diaphragm—generally diaphragms with more blades create more nearly circular openings and more pleasant bokeh. If your landscape photography involves selective-focus images, bokeh is an important consideration.

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