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Tuesday, April 5, 2011

Landscape Lens Tech


What makes a good landscape lens?

Labels: LensesGear


This Article Features Photo Zoom

This cutaway illustration shows the complex optics and mechanics of a modern lens. To get a sharp, corrected image with good contrast and accurate colors, all of these precision parts must work perfectly in concert with one another. Zoom lenses like this have benefitted so much from computer-aided design that they’re now the norm for landscape pros.

Finding your perfect landscape lens is a matter of defining the priorities.
Because everything in photography is a trade-off, it’s largely a matter of deciding where you can make a sacrifice in order to gain a key benefit. Let’s look at the most critical attributes for taking scenics.

Sharpness
The single most important attribute has to be sharpness. Landscape images require fine detail and are often printed large. But no matter how fine-grained your film or how many megapixels your digital camera has, if the lens can’t provide appropriate sharpness, the images will suffer. You want a landscape lens to be sharp across the frame (although soft corners/edges can be tolerated with some subject matter) and across the zoom range. Generally, this means using the higher-end lenses, as they provide better performance. (It also requires using a tripod or other steady camera support—camera movement will destroy image quality even with the sharpest of lenses.) For a zoom lens, you want one that’s sharp at all focal lengths, not just the widest one or the longest.

Distortion
After sharpness, the lens’ distortion characteristics come into play. The ideal landscape lens will have minimal distortion: Straight lines should remain straight, especially the horizon when placed high or low in the frame. Fisheye lenses curve straight lines that don’t pass directly through the center of the image; non-fisheye wide-angles should not (although most do, to some degree). You can check for distortion by nearly filling the frame with a rectangular object like a newspaper page, or by shooting scenes with the horizon high or low in the frame, or even framing with a wall-ceiling intersection high or low in the frame (in both horizontal and vertical formats).

Sensor Formats And Lens Choice

What a given focal length “sees” depends on the size of the sensor or film with which it’s used. Smaller sensors and film formats “see” a smaller portion of the image projected by a given lens than larger sensors or film formats do (see the diagram, right).

For a given format, the normal lens is considered to be one with a focal length approximately the diagonal measurement of the image format. For example, a 35mm film frame measures 36x24mm, with a diagonal of 43.2mm. Thus, a normal lens for this format is around 43mm (50mm is the most widespread normal lens for the 35mm format).

Lenses shorter than the image diagonal are wide-angle; they produce a wider angle of view than the normal lens. A 28mm lens on a 35mm (or full-frame) DSLR is wide-angle; it produces a wider angle of view than the normal 43mm or 50mm lens.

The digital APS-C format, used by many DSLRs and new mirrorless, interchangeable-lens models, measures around 23.5x15.7mm, with a diagonal of 28.9mm. Thus, a normal lens for APS-C is around 29mm; a 29mm lens on an APS-C camera will “see” what a 45mm lens “sees” on a 35mm or full-frame digital camera. If you put a 28mm lens on an APS-C camera, it “sees” like a 42mm lens on a 35mm SLR: normal, not wide-angle.

Four Thirds System sensors measure 17.3x13mm, with a diagonal of 21.6mm, half that of a 35mm or full-frame DSLR.
Thus, a given focal length used on a Four Thirds System camera frames like a lens twice its focal length on a 35mm or full-frame digital SLR.

Each SLR manufacturer offers a wide range of lenses for its cameras, whatever their format.
But since shorter focal lengths are required to produce a given angle of view with the smaller sensors, wide-angle lenses for these cameras have very short focal lengths; to provide the angle of view of an 18mm superwide-angle on a 35mm or full-frame digital SLR, an APS-C DSLR would require a 12mm focal length, and a Four Thirds camera, a 9mm focal length. It’s more difficult to produce a 9mm or 12mm lens that’s sharp and distortion-free than an 18mm, one reason why digital landscape photographers tend to prefer larger formats.


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