Friday, May 25, 2007
Lens Design And Technology In The Age Of D-SLRs
Lens quality has grown exponentially in recent years. The optics being produced for professionals today incorporate some high-tech miracles to get the job done.
Apochromatic (APO) elements improve image quality in any application, but they're particularly important in telephoto lenses. Telephotos and tele-zooms are especially susceptible to chromatic aberrations, wherein the various color wavelengths come to sharp focus at different points inside and outside the image plane. This results in unacceptable contrast and sharpness, hence the need to include apochromatic elements. Apochromatic elements are expensive to produce and their quality varies widely. Most professional-level telephoto zooms feature two or more apochromatic elements. Wide-angles and ultra-wide-angles are now also being built with APO elements to reduce lateral color shifts.
Professional-series lenses from the major manufacturers feature a variety of exotic glass. For example, Canon L-series lenses utilize Canon UD glass to minimize chromatic aberration and color fringing. Many of these lenses also incorporate fluorite elements, which have the corrective power of a pair of UD glass elements.
Non-Imaging Light Reflection
Digital SLRs have more issues with reflections, which cause flare and ghosting, than their film counterparts did. The image sensors tend to reflect a certain amount of light. Those reflections bounce off the elements and inside of the lens barrel before finding their way back to the sensor, robbing the image of contrast and sharpness, as well as introducing flare and ghosting. Manufacturers reduce these reflections through several methods.
The first line of defense is in the form of a lens coating. The latest coatings are dramatically superior to those found on older lenses. Normal uncoated glass reflects as much as 5% of the light at each glass-air boundary. If the lens has more elements, the problem is magnified and the accumulated reflections reduce contrast and performance and produce extreme flare issues. Advanced coatings of an extremely thin transparent film on the element surfaces reduce the amount of reflection down to 0.5% or less.
To prevent light from being reflected along the inner lens barrel, extremely fine material is applied to those surfaces. Called electrostatic flocking, the treated surface will absorb most of the reflections.
The lens barrel's overall construction plays a key role in preventing reflection, too. Flare-cutting diaphragms block stray light from entering the rear of the lens. Baffles and the other construction elements also help to keep non-imaging light from coming to the sensor and harming the image.
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