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?
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.
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