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?
Sobey says many of the wider-angle lenses are also designed to get light to the edges of the sensor to avoid vignetting. He continues, “Because sensors can be more sensitive to light striking pixels at sharp angles, many of Sigma's newer lenses, especially the wide-angles, have larger rear elements, which allow the light rays to hit the edges of a sensor at a more perpendicular angle so potential issues with chromatic aberrations and light falloff at the edges are minimized.”
While there are exceptions, such as the Hasselblad H3D, Canon EOS-1Ds Mark III and EOS 5D, and Nikon's new D3, most D-SLRs feature sensors that are smaller than a piece of 35mm film. Imagers typically range from 13mm to 19mm along their shorter side, compared to film's 24mm. This fact has presented some issues that manufacturers have had to deal with in the design of their lenses. The first, and perhaps the most critical, is the magnification factor and the need for shorter focal lengths (Fig. 1).
Crop Factors & The Need For Shorter Focal Lengths
If you use a 20mm film lens to project an image on a sensor that's 16x24mm, it will behave like a 30mm. This is because the sensor is 1.5 times smaller than a 35mm piece of film, which is 24x36mm. To maintain a 20mm focal length, you'd need a lens with a focal length of 13mm. D-SLRs with sensors even smaller than 16x24mm will need even shorter focal lengths because the magnification factor is even greater.
Nikon's Heiner says, “DX Nikkor lenses are designed specifically for digital with this in mind. In creating a lens exclusively for Nikon's DX-format digital SLR cameras, decreasing the size of a lens to two-thirds of what's necessary for the FX format, or 24x36mm, would seem simple, but it's not. There are many obstacles that have to be overcome. Otherwise, it would only make the size of a conventional lens smaller, but not improve aspects of lens performance such as sharpness or vignetting. Many DX Nikkor lenses have focal lengths similar to many non-DX Nikkor lenses, but they're designed to present a smaller image circle to the DX-format imaging sensor, thus solving the restrictions of more conventional lens designs.”
Since lens focal length is the distance from the optical center to the focal plane, the glass of a simple single-element 12mm lens, for example, would have to be physically inside your camera's body. This is impossible, obviously, so designers had a challenge on their hands.
The back focus had to be longer than the intended focal length, which required the use of an inverted-telephoto or retro-focus design (Fig. 2 and 4). The optical center on such lenses lies well to the rear of the lens element, and in many cases, completely outside the glass. This allows for very short focal lengths, even though the rear element remains quite a bit further from the image sensor's surface.
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