Monday, November 26, 2007
The Digital Lens Revisited
In a world of marketing buzzwords like 'optimized' and 'designed for digital,' what's really going on behind all the hype?
Ghosting & Flare
Although chromatic aberration isn't exclusively a digital issue, ghosting and flare is unique to the format (Figures 5a and 5b). Canon's Westfall explains, “One of the big differences between a digital SLR and a film camera is the fact that you're using this image sensor with usually a low-pass filter in front of it. And it's a shiny surface. So if you're using a lens that isn't optimized for an image sensor, there's a risk—depending on which lens—that there will be some internal reflections that bounce back and forth between the image sensor and the element in question, which could be any element in the lens.
“What we've done over the last seven or eight years now is to work on developing coatings and make sure that the shapes of the individual lens elements are such that the flare and ghosting are minimized or eliminated.”
Nikon's Heiner says, “Well before the new Nikon D3 was announced, our lens designers were busy in advance producing Nikkor lenses with new technologies to optimize their use with both our FX- and DX-format D-SLRs. Some of these include Extra-low Dispersion glass elements for minimal chromatic aberration and aspherical lenses, including large-diameter PGM (Precision Glass Molding) elements to reduce coma and other types of aberration even at the widest aperture. These lenses also benefit from Nikon's exclusive Nano Crystal Coat—an extra-low refractive index coating that virtually eliminates internal lens element reflections across a wide range of wavelengths and is particularly effective in reducing ghosting and flare.”
In the case of super-telephoto lenses, which have a large protective glass filter in front of the lens elements, both Canon and Nikon use a subtly curved meniscus design that prevents light bouncing back off the imager from returning into the camera as a ghost image. Older designs that used a flat-glass filter were known to suffer from this spectral ailment.
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