Tuesday, May 26, 2009
What’s under the hood of modern photography optics?
Lenses containing such elements are well corrected for chromatic aberrations. Many such lenses are achromatic: they bring two of the three primary colors into focus at the same plane (usually red and blue). The best lenses are apochromatic, bringing all three primary colors into focus at the same plane. (Bear in mind that even apochromatic lenses may not bring all wavelengths into focus at the same plane.)
Canon’s Diffractive Optics (available in just two DO lenses thus far) essentially compensate for chromatic aberrations by using diffraction to cancel it out. Besides minimizing chromatic aberrations, DO elements allow for much more compact lens designs (the EF 70-300mm ƒ/4.5-5.6 DO IS USM zoom lens is 30% shorter than the EF 70-300mm ƒ/4-5.6 IS USM zoom).
Each lens element surface reflects a portion of the light striking it, reducing the amount transmitted. With some lens designs containing more than 20 elements, some with two surfaces (some elements are cemented together in groups and thus have only two surfaces for the group), the light loss due to reflections can be great. Additionally, the reflected light bouncing around in the lens can cause flare and ghosting—especially a concern in digital cameras and their highly reflective sensor-assembly surfaces.
Lens manufacturers long ago developed anti-flare coatings to reduce these reflections and thus increase the amount of light transmitted by the lens, but today’s newest lenses employ coatings designed to combat reflections from the image sensor assembly, as well as internal reflections—one good reason to buy newer lens designs if you’re shooting with a D-SLR instead of a film camera.
Lens coatings also increase contrast and improve color balance.
When the AF SLR era began in the mid-1980s, most manufacturers added a focusing motor to an adapted film-camera body to provide autofocusing. Canon went with a whole new system, using an electronic lens mount and AF motor in each lens rather than in the body. The benefits included motors optimized for each lens and no mechanical linkage between lens and camera body for AF and diaphragm operation. The drawbacks were that the lenses were a bit heavier, and you couldn’t use your old Canon lenses on the new EOS cameras.
While most other brands of D-SLRs still have AF motors in the bodies, most today also offer at least some lenses with built-in AF motors. Canon USM (Ultrasonic Motor), Nikon AF-S, Olympus SWD, Pentax SDM, Sigma HSM and Sony SSM lenses provide quick, quiet autofocusing and the ability to fine-tune focus manually without switching to MF mode. If you use AF, you’ll probably want these lenses for their AF performance, quiet operation and ability to fine-tune focus manually.