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
Macro is the photo op that’s always available: You can find good close-up subjects just about anywhere. All you need is a way to make your camera focus close enough.
Dedicated macro lenses are the best option because they can focus from infinity down to close enough to produce a life-size (1:1) magnification at the image plane. Macro lenses also are optically optimized for close focusing distances, so they produce better results at such close range than nonmacro lenses used with extension tubes (and, of course, lenses with extension tubes attached can no longer focus out to infinity). Most macro lenses also are well corrected for flat fieldwork, such as photographing stamps and coins, which may or may not be useful for nature photography. The main drawbacks of macro lenses are that they’re generally bulkier and more costly than nonmacro lenses of equal focal length, although the differences today aren’t nearly as great as they were some years ago.
There are a number of things that make macro lenses different from nonmacro lenses, but the biggest is that macro lenses can focus much closer than nonmacro lenses. Additionally, macro lenses are optically optimized for very close focusing distances, although most also perform very well at “normal” shooting distances, too.
Early macro lenses had extended focusing mounts, which allowed the user to position the front elements farther from the image plane, by rotating the focusing ring, than was possible with nonmacro lenses. Some macro lenses today still use this model. This is simple and produces increased magnification, but means the physical length of the lens increases considerably as it’s focused closer—and that could result in the lens casting a shadow on the subject or even striking it.
Newer macro designs employ optical formulas that allow for life-size magnification without moving the front element farther from the image plane. Internal focusing moves inner elements to focus. Doing this with appropriate elements and groups also allows for a closer minimum focusing distance and a more compact design. Since the internal elements are smaller and lighter than the front element, this also makes things easier for the AF motor, producing quicker autofocusing.
Macro lenses generally employ floating designs, with additional groups of elements that move during focusing to counter aberrations at close focusing distances. Floating elements and internal focusing alter the focal length, decreasing it as focusing distance is reduced, but for practical purposes, this doesn’t matter: A macro lens still will provide 1:1 magnification at its minimum focusing distance.
Curved lens elements project a flat subject as a curved one, so when the center of the image is in focus, the edges are out of focus, and when the edges are sharp, the center isn’t in focus. Stopping down increases depth of field and depth of focus, and thus reduces the problem, but most macro lenses are better corrected for this curvature of field than are nonmacro lenses. This correction requires additional lens elements and groups, increasing the bulk and complexity of the lens, but it’s also one reason why macro lenses are much better than nonmacro lenses for photographing flat subjects.
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