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Monday, April 28, 2008

Fast Lenses On Sub-Full-Frame Sensors

Quick, inexpensive and light, sub-full-frame lenses aren't just for the amateur market anymore

Optimum Optics
“In creating a lens exclusively for Nikon's DX sub-full-frame digital SLR cameras,” points out Lindsay Silverman, Senior Technical Manager at Nikon, “decreasing the size of a lens to two-thirds of what's necessary for the full-frame 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.”

Nikon's AF-S NIKKOR 24-70 ƒ/2.8G ED is a fast-aperture lens, considered by many to be “standard equipment” in their gadget bag. This must-have lens offers a versatile wide-angle to medium-telephoto range for either full-frame FX- or DX-format sensor cameras. The super-fast Silent Wave Motor allows quick and quiet autofocusing, plus a wide, constant ƒ/2.8 aperture for fast work, especially in low-light situations. The wide ƒ/2.8 aperture is essential for working photojournalists; sports, action and wedding photographers; and others forced to shoot in difficult lighting situations where a fast aperture is required to achieve high enough shutter speeds to stop any action or movement. The aspherical glass and extra-low- dispersion (ED) elements help the lens to maintain not only a compact size, constant ƒ/2.8 aperture and unparalleled sharpness, but also the color, clarity and proper contrast even at this widest aperture.
—Steve Heiner

If you were to attach a pro-level lens made for full-frame sensors, the area of coverage on the focal plane for the smaller camera is compensated for, and even magnified, but almost always at a loss of image quality from a design meant for the film plane of classic 35mm full-frame sizes. Conversely, lenses made distinctly for a sub-full-frame digital SLR can't be put on a full-frame camera because the size of the image projected on the focal plane is smaller than what's needed for an APS- or similar-sized image sensor.

Sigma utilizes many elements to speed up its lenses designed specifically for the APS-sized image sensor (what they refer to as DC lenses). They have used a combination of aspherical glass elements, internal focusing and the HyperSonic Motor (HSM), which allows fast autofocusing, to obtain a constant wide aperture while maintaining the focus, speed and quality of images for their lenses made specifically for sub-full-frame D-SLRs.

Tom Sobey from Sigma adds, “The HyperSonic Motor and internal focusing allow a faster autofocusing. Because the optics don't have to cover a full 35mm format, the lens itself can be somewhat more compact and lightweight.”

“Lenses for APS sensors use construction that realizes a smaller image circle because they don't have to cover the same sensor area,” points out Mark Weir, Senior Technology and Marketing Manager at Sony. “As a result, they can be smaller in size. In theory, there's no relation between image circle and wide-aperture or constant-aperture design. However, in practice, a smaller image circle would allow larger apertures to be realized within a given overall size.”

When developing a lens, more than just the sensor size can matter. Like Sony (currently), Olympus offers only sub-full-frame sensors, available in its Four Thirds System, with a digital-specific E-System Zuiko Digital line of lenses developed specifically for the sensor.

“If you look at the lens-mount relationship to the size of the sensor,” says John Knaur, Senior Product Manager of D-SLRs for Olympus, “part of the Four Thirds System dictated that the sensor area, the 23mm image circle, and the actual mount would be about twice that size. By having the larger lens mount, we can keep the light columnated as it comes back from the lens, so that you get sharp edge-to-edge resolution at wide-open apertures. This allows us to create a telecentric type of lens, even when we're doing ultra-wide angles, like the 7-14mm.”

Other Considerations
Digital lens design also incorporates more than just the optics. Detailed communication between the lens and the camera's focus system are just as important as anything else. As cameras become more and more like small computers, updates aren't just for your software anymore. Firmware updates for cameras, and even for some lenses, streamline focus and correct issues that would have been solved previously through a new design. Third-party lens manufacturers like Tamron, Tokina and Sigma have to pay particular attention to these other considerations when making a fast lens for more than just one sensor type.


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