Monday, January 29, 2007
Lens Buying Guide
Everything you need to know about focal lengths, maximum apertures, new technologies and more!
Owners of advanced compact digital cameras still can extend their focal-length range significantly with accessory, or add-on, lenses. These optics attach to the front of your existing lens, reducing or lengthening the built-in's focal length. Wide-angle adapters shorten your focal length by about 20% to 30%, while telephoto adapters can provide anywhere from a 50% to a 300% focal-length increase, depending on the adapter's make and model. A 50% to 70% increase is common.
Because these accessory lenses use the built-in lens' focusing system, diaphragm and electronic components, size and expense are kept to a minimum while still providing strong image quality. Modern accessory lenses make use of the latest lens-making technologies, including low-dispersion glass and even phase-fresnel designs (see next page).
LOW-DISPERSION GLASS. As light passes through glass elements, it tends to disperse, or separate out, by color (prisms are an example of dispersion taken to extremes). The separated red, green and blue wavelengths focus at slightly different points and with unequal magnifications, causing fuzzy images and color fringing. This is known as chromatic aberration, and it's mainly a problem for telephotos. Lenses specifically designed to correct all wavelengths are known as apochromatic, or APO, lenses.
Optical elements made of special glass, such as low- (LD), extra-low- (ED) and ultra-low- (UD) dispersion glasses, help minimize chromatic aberrations. Since telephoto lenses always have been particularly prone to these aberrations, they have made increasing use of the premium glasses. Very short focal-length lenses for SLRs and D-SLRs rely on optical designs that are basically inverted telephotos, so they also use the low-dispersion glasses to fight chromatic aberration. That's why so many "designed for digital" lenses feature LD, ED or UD elements.
ASPHERIC LENS ELEMENTS. Aspheric lens elements help deliver sharper images and prevent straight lines from bending in your photos, as they sometimes do with wide-angle lenses and some zooms. Because aspherical elements can take the place of multiple conventional lens elements, they allow lighter, more compact designs. Advanced lens-making technology has overcome the challenges in manufacturing aspherical lens elements, which until recently had been prohibitively expensive or impossible to make. They're now common even in lower-priced compact digital cameras.
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