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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?



Collimation Of Light Rays
You've likely heard different opinions regarding whether light rays need to strike the sensor as straight as possible. Some manufacturers say this isn't an issue because of a sensor's micro lenses. Other manufacturers will argue that the collimation of light rays is still an important part of digital lens design, regardless of the sensor's micro lenses.

 

the digital lens revisited

Schneider Apo-Digitar 120mm ƒ/5.6 with a Rollei #0 shutter release
Sally Smith Clemens of Olympus, says, “When you think of a piece of celluloid and you think of the silver-halide crystals, they're like little grains of sugar or sand. They're dimensional, and they can collect light from stray angles because they're sensitive all the way around. An electronic imaging sensor has a flat surface, and each of the photo diodes have depth, much like a bunch of shot glasses lined up in a row.

“The light has to actually enter each one of those photo diodes at a perpendicular angle all the way across the sensor, even to the edges. The optical design has to be able to bend the light so it hits the sensor at a more perpendicular angle so the light goes all the way to the base of each diode.”

The fact that most sensors now have micro lenses on each pixel to help direct the light is certainly evidence that collimation of light is a concern for digital capture.

Says Canon's Westfall, “The bottom line is that with the micro lenses that we have over the individual pixels on the sensors, we're gathering as much light as we possibly can. So even if light hits the sensor at oblique angles, it's not going to result in any aberrations like extra chromatics. And it also doesn't have any strong effect on vignetting.”

Resolution
Although it isn't likely everyone will ever agree on how straight light rays need to be when they hit an image sensor, there's consensus on how well light is resolved. Generally, most would agree that it does have to be higher for an image sensor compared to film.

The same could be said of lenses for medium- and large-format cameras that can accommodate an optional digital back or film magazine, or vice versa. Take a Mamiya RZ67 Pro IID, for example. It uses 6x7cm and 6x4.5cm interchangeable film magazines, plus you have the option of using the ZD digital back or others on the market. The ZD back has a CCD that's 36x48mm, which is 1.25 times smaller than 6x4.5cm (60x45mm). That's roughly the same as using a 35mm film lens on a digital camera with an APS-H-sized sensor, which is 18.7x28.1mm.

So if you're shooting with a medium-format camera and switching back and forth between capture formats—film and a digital back—you want lenses that have a big enough image circle to cover 6x7cm or 6x9cm and still have enough resolution for the digital back. The Mamiya Sekor Zoom AF 75-150mm ƒ/4.5 D and Sekor AF 28mm ƒ/4.5 D Aspherical lenses are specifically designed for the demands of both formats.


 

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