Tuesday, May 26, 2009
Time For An Upgrade
With so many recent advancements in optics technology, is now the time to revamp your collection of lenses?
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 | Huge Zoom Range Despite its compact size, Tamron’s AF18-270mm F/3.5-6.3 Di-II VC LD Aspherical (IF) Macro features an amazing 15:1 zoom ratio, built-in Vibration Compensation and focusing down to 19.3 inches. It incorporates two LD, one AD and three hybrid aspherical elements to control aberrations and distortion, and was designed specifically for APS-C D-SLRs. |
Lenses designed for 35mm SLRs were optimized to cover an image area of 36x24mm. D-SLR cameras with a 35mm-SLR form fac-tor come with three basic sensor sizes: full-frame (FX to Nikon), APS-C (DX to Nikon) and Four Thirds System. Full-frame sensors are the same size as a full 35mm image frame, 36x24mm (diagonal measurement is 43.2mm). APS-C sensors are smaller, around 23.6x15.8mm (diagonal measurement is roughly 28.4mm, about two-thirds that of full-frame, hence the 1.5x “focal-length factor”). Four Thirds System sensors are smaller still, 17.3x13mm, with a diagonal measurement of 21.63mm, half that of full-frame (hence their 2x focal-length factor, but remember that full-frame and APS-C have a 3:2 aspect ratio, Four Thirds has a 4:3 aspect ratio).
There are variations. Canon’s EOS-1D Mark III uses an APS-H sensor measuring 28.1x18.7mm (diagonal 33.75mm, 1.3x focal-length factor); Canon mid- and entry-level D-SLRs use a slightly smaller APS-C sensor measuring 22.3x14.9mm (diagonal 26.82mm, 1.6x focal-length factor); and Sigma D-SLRs use a 20.7x13.8mm sensor (diagonal 24.88mm, 1.7x focal-length factor). You can calculate the focal-length factor by dividing the diagonal measurement of a D-SLR sensor into the diagonal measurement of a 35mm image frame (43.2mm): 43.2 divided by APS-C’s 28.4mm = 1.52x.
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| As of this writing, you can choose among three full-frame D-SLRs for well under $3,000. Modern high-tech D-SLRs perform their best with modern high-tech lenses. | ||
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| Image Sensors And Collimated Light Digital-imaging sensors contain millions of tiny picture elements or “pixels.” Each requires the light to hit it fairly straight on for maximum performance; light striking at too sharp an angle will hit the side of the “well” and bounce around, reducing image quality. Lenses designed specifically for digital imaging send the light to the pixels more efficiently than lenses designed for film photography. | |
Besides field-of-view changes with different formats (sensor sizes), there are several important considerations regarding lenses for D-SLRs. Newer lenses are designed with these in mind and, thus, produce better image quality with D-SLRs than their designed-for-film predecessors. Keep in mind, of course, that a top pro lens designed for a 35mm SLR will produce better results than a cheapie lens designed spe-cifically for a D-SLR.
It’s commonly thought that lenses designed for 35mm cameras won’t perform well with smaller-sensor D-SLRs because light rays coming through the edges of the lens have to be bent too sharply to fit the smaller sensor. That’s not so. A given lens bends the light a given amount when focused at a given distance, and that doesn’t change because you put a bigger or smaller image sensor at the focal plane. To say that a 35mm SLR lens won’t work well with a smaller D-SLR sensor because the light is being bent too sharply is to say that the central portion of a 35mm film image will be of poor quality—after all, that’s what the smaller sensor is seeing, the central portion of the image produced by the lens.
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