Telecentricity

Digital sensors perform best if light reaches them perpendicularly (Figure 7.4). At the corners of a picture taken with a conventional wide-angle lens, there is likely to be color fringing from light striking the Bayer matrix at an angle, even if film would have produced a perfect image.

A telecentric lens is one that delivers bundles of light rays to the sensor in parallel from all parts of the image. The obvious drawback of this type of lens is that it requires a large-diameter rear element, slightly larger than the sensor.

Telecentricity is one of the design goals of the Olympus Four Thirds system, which uses a lens mount considerably larger than the sensor. Other DSLRs are adapted from 35-mm SLR body designs, and the lens mount is not always large enough to permit lenses to be perfectly telecentric.

MTF curves do not tell you whether a lens is telecentric, and some excellent lenses for film cameras work less than optimally with DSLRs, while some mediocre lenses work surprisingly well. It's a good sign if the rear element of the lens is relatively large in diameter and is convex (positive); see for example the Olympus 100-mm f /2.8 and the digitally optimized Sigma lens in the left-hand

Figure 7.4. With a telecentric lens, light from all parts of the image arrives perpendicular to the sensor.

Conventional lens Telecentric lens

Figure 7.4. With a telecentric lens, light from all parts of the image arrives perpendicular to the sensor.

column of Figure 7.12, p. 85. Telescopes and long telephoto lenses are always close to telecentricity because all the elements are so far from the sensor.

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