Tuesday, August 23, 2011
Macro Lens Guide
A proper macro lens is designed to eliminate aberrations, focus colors and attain maximum sharpness on close-up subjects
When we talk about magnification regarding macro lenses, we’re talking about magnification at the image plane. You can blow up the image well beyond that on your computer monitor or in a print, of course, and that can make the macro subject much bigger than in real life—even if you shot it with a nonmacro lens. But the macro lens gives you the advantage of more pixels recording the subject instead of being thrown away. The image of the subject will be recorded bigger in the frame, with the potential to reveal more detail.
Some confusion exists regarding a lens’ magnification on full-frame vs. APS-C-format DSLRs. The lens’ magnification doesn’t change when you switch it from one camera to another—say, from a full-frame Sony A900 to an APS-C Sony A580. A given lens focused at a given distance produces a subject image of a given size at the image plane. This doesn’t change because you place a larger or smaller piece of film or image sensor at the image plane—the subject’s image is still the same size at the image plane. A 100mm macro lens focused at its minimum focusing distance reproduces a 1⁄2-inch subject a 1⁄2-inch high at the film plane, and that image will be a 1⁄2-inch high regardless of film or sensor size.
The confusion arises because the size of the image frame varies with sensor format. A full-frame sensor measures 24x36mm, or about 1.0x1.5 inches. An APS-C sensor measures around 15.7x23.7mm, or about 0.6x0.9 inches. That 1⁄2-inch-high image of the subject will take up half the height of a full-frame sensor, but nearly all of the height of an APS-C sensor. So for practical purposes, you do get more magnification with the smaller sensor—the subject fills up more of the image frame. But the actual magnification produced by the lens at the image plane doesn’t change—it’s the amount of that image that each sensor size sees that changes (see the macro lens diagram).
Macro Lens Focal Lengths
Macro lenses come in a number of focal lengths. In 35mm film days, these generally were normal (50-60mm), short tele (90-105mm) and tele (180-200mm). This still holds true today, and the results are the same with full-frame digital SLRs.
The full-frame macro lenses also can be used on APS-C DSLRs (assuming compatible mounts, of course), the smaller sensors’ 1.5x-1.6x crop factor effectively turning them into 75mm, 150mm and 300mm lenses for framing purposes. There also are macro lenses designed specifically for APS-C DSLRs; some of these come in shorter focal lengths to account for the APS-C crop factor and provide equivalent fields of view to the full-frame trio.
In any event, you’ll choose a normal, short telephoto or telephoto macro lens to suit your needs. Why the different focal-length categories? Each provides a different working distance, and thus a different perspective.
A normal (50mm for a full-frame camera) macro lens produces its 1x magnification at a distance of around seven to eight inches, a short tele macro lens (100mm for a full-frame camera) at around 12 inches and a tele (200mm for a full-frame camera) at around 19 inches. Shooting closer to the subject expands perspective, while shooting from farther away compresses it.
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