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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?

This Article Features Photo Zoom

ƒ/2.8 And Close Focusing Sigma’s 18-50mm ƒ/2.8 EX DC Macro HSM for APS-C D-SLRs is equivalent to a 27-75mm zoom on a 35mm SLR. This pro lens features a fast ƒ/2.8 maximum aperture throughout its zoom range, focusing down to 7.9 inches for a 1:3 reproduction ratio, a quick and quiet HSM focusing motor (Nikon-mount version only) and two aspherical elements, one ED element and one SLD.

What’s significant is that film emulsion can accept light from a wide range of angles with minimal loss of efficiency, while a digital sensor cannot. Digital sensors consist of millions of tiny “light wells” called pixels. If light strikes a digital sensor at too great an angle, it hits the sidewall of the light well and bounces around, instead of striking the light-sensitive bottom of the well. This can result in reduced exposure and sharpness, chromatic aberration and vignetting (microlenses over the pixels help, but they don’t eliminate the problem). Lenses designed specifically for digital imaging send the light to the pixels at a more efficient angle—even light coming through the edges of the lens—for better performance. Newer lenses from major manufacturers are designed with digital needs in mind and perform better on D-SLRs than older lenses designed before D-SLRs became popular.

Sealed For The Elements Pentax’s smc DA* 50-135mm ƒ/2.8 ED (IF) SDM is a high-end lens designed for Pentax D-SLRs, featuring three ED elements, weather- and dust-resistance, and a smooth and quiet SDM focusing motor. It’s equivalent to a 75-200mm zoom on a 35mm SLR.

Another consideration is that a digital-sensor assembly is much more reflective than a film surface. Thus, light can be reflected back into the lens from the sensor, causing flare, ghosting and loss of image quality. Lenses designed for digital use—regardless of format—have special coatings (and often special internal baffling) to minimize these effects. Some lens manufacturers even use a meniscus protective glass on their large-diameter lenses because flat glass can reflect light from the sensor, producing ghosting.

Lenses Designed For Smaller Sensors
Lenses designed for smaller-sensor D-SLRs don’t have to project as large an image circle as lenses designed for 35mm cameras. An image circle of around 28.4mm works for APS-C; for the even smaller Four Thirds System sensors, an image circle of 21.63mm—half that needed for 35mm—suffices. When you take into account the focal-length factors of the smaller formats, this can translate into big savings in both bulk and cost. For example, Olympus’ Digital Zuiko 300mm ƒ/2.8 lens for Four Thirds System cameras frames like a 600mm lens on a 35mm SLR (or full-frame D-SLR), yet is smaller and lighter, costs less and is a full stop faster than the maximum ƒ/4 aperture available in the 600mm lenses. (Of course, the smaller sensors mean the pixels are smaller for any given pixel count, which can produce image-quality issues, but that’s beyond the scope of this lens article.)

One thing to keep in mind about lenses designed for smaller sensors is that their image circles won’t cover a full 35mm image frame. If you use one on a 35mm SLR or full-frame D-SLR, it will vignette (the image won’t fill the frame and the corners will be darkened). Canon makes it physically impossible to mount an EF-S lens (Canon’s designation for lenses designed for the smaller sensors) on a full-frame or 35mm SLR. Nikon’s full-frame D-SLRs automatically switch to the cropped DX format when a DX lens is attached (if you mount a DX lens on a Nikon 35mm SLR, it will vignette). If you now use an APS-C D-SLR and anticipate upgrading to a full-frame model in the future, you may not be able to use your APS-C lenses on the full-frame camera.

Another consideration is diffraction. Smaller sensors require shorter focal lengths to produce a given angle of view. The shorter the focal length, the smaller the aperture diameter at any given ƒ-stop. The ƒ-number, by definition, is the focal length divided by the diameter of the opening at that setting: for a 24mm lens set at ƒ/16, the aperture diameter is 24/16, or 1.5mm. For the 16mm lens needed to produce the same angle of view with an APS-C sensor, the aperture diameter is 16/16, or 1mm. For the 8mm lens required to produce the same framing with a Four Thirds System camera, the aperture diameter at ƒ/16 is 8/16, or 0.5mm. Such small-diameter apertures produce lots of diffraction, which reduces image quality. For best image quality, try to avoid stopping down below about ƒ/8 with APS-C and Four Thirds System lenses. Fortunately, depth of field also increases as focal length decreases, so you don’t have to stop down as much to get a given amount of depth of field with a shorter lens.