You've no doubt seen or heard about new "designed for digital" lenses that are optimized for better performance with digital image sensors. If you're like most people, though, you're not quite sure what that means—just what makes a lens that's "designed for digital"? We'll cover the details and show you how the new lenses have helped overcome some of the challenges once posed by a switch to digital, and how these advanced optics can improve your images.
D-SLR Magnification Factors
One of the most important differences between film and digital SLRs is the smaller size of most D-SLRs' image sensors. With any lens you use, the angle of view seen by a D-SLR's sensor is less than that seen across the full span of the 35mm frame.
This narrower angle of view is equally well-described as greater magnification, and the image recorded with your lens on a D-SLR appears similar to one taken with a much longer lens on a 35mm camera. This effect gives us the magnification factor familiar to D-SLR shooters. (If you've ever shot 4x5 film, you've experienced something similar—a 90mm lens is a wide-angle with the big film format and a telephoto for the smaller 35mm frame.)
To gauge how your 35mm lenses will behave on a D-SLR, multiply the focal length by the camera's magnification factor. For example, a 200mm lens on a D-SLR with a 1.5x factor gives the equivalent of a 300mm focal length. It sure looks like a case of getting something for nothing—telephoto users certainly can get the extra reach of the longer lens without the increased weight or expense. The problem is that wide-angles' equivalent focal lengths are increased by the same amount.
On a film camera, a 20mm lens is nearly an ultra-wide. With a D-SLR's 1.5x magnification factor, though, it takes in no broader an angle than a moderately wide 30mm lens would on a film SLR. To get the same angle of view with a D-SLR as you'd get shooting a 20mm on a film camera, you'd need an ultra-short 12mm or 13mm lens, and these optics simply didn't exist much more than a year ago.
Wide Lenses For D-SLRs
Lens makers have given us back our wide-angle coverage with special optics designed specifically for the small-format image sensors found in most D-SLRs. Because these lenses don't have to cover the larger image area of a 35mm camera or a full-frame (or nearly full-frame) D-SLR, optical designers can create more radical lenses, including focal lengths of 12mm or even shorter. Mounted on a D-SLR, the lenses perform much like the wide-angle lenses we depend on with 35mm cameras.
Because of the recent advent of these designs, the lenses can capitalize on the latest in lens-making technology, including widespread use of low-dispersion (LD) glass and aspherical lens elements. LD glass minimizes the color fringing associated with chromatic aberrations in telephoto lenses. Because the ultra-short wide-angles use an inverted-telephoto design, LD glass is important for them, too. Aspheric lens elements minimize distortion and help improve sharpness in these technically demanding optics.
Examples Of Modern Digital Lenses
The Nikon DX series set the stage with lenses like the 12-24mm ƒ/4 G ED-IF AF-S DX Zoom-Nikkor. With the 1.5x magnification factor on Nikon D-SLRs, the 12-24mm lens provides a 35mm-equivalent focal length range of 18-36mm. The growing series of lenses includes a 10.5mm fish-eye, and also fits Fujifilm's FinePix S2 Pro and S3 Pro D-SLRs.
The Canon EF-S 10-22mm ƒ/3.5-4.5 USM is designed for the smaller imager in the EOS Digital Rebel and EOS 20D D-SLRs. With the cameras' 1.6x magnification factor, the lens offers the same angle of view as a 16-35mm lens on a 35mm camera, the widest non-fish-eye available on a D-SLR—for now.
Olympus uses the smaller 4/3 format for its D-SLRs' image sensors. Because of the sensors' size, the cameras have a 2x magnification factor, giving the Olympus 11-22mm ƒ/2.8-3.5 Wide Zoom an angle of view equivalent to a 22-44mm lens for the 35mm format. The zoom's two aspherical lens elements promote image quality.
The Sigma 18-125mm ƒ/3.5-5.6 DC offers an impressive focal length range running from moderate wide-angle all the way out to a strong telephoto. Along with Sigma's own line of digital SLRs, the lens comes in mounts for Canon, Nikon, Minolta, Olympus 4/3 and Pentax small-format D-SLRs. The equivalent focal lengths vary according to the magnification factor of the camera on which it's mounted—with Sigma's 1.7x cameras, it's approximately 30-212mm.
The Tamron SP AF11-18mm ƒ/4.5-5.6 Di-II LD Aspherical (IF) (available in Spring 2005) fits Nikon, Canon and Konica Minolta D-SLRs. It features an angle of view equivalent to a 17mm or 18mm lens on the wider end, depending on the size of the camera's image sensor, and zooming out to an equivalent of 27mm to 29mm.
The Tokina AT-X 124 AF PRO DX zoom fits Nikon and Canon small-format D-SLRs, providing a 12-24mm focal length range equivalent to 19-38mm for Canons and 18-36mm for Nikons. The lens has a constant ƒ/4 maximum aperture at all focal lengths.
The Pentax smc P-DA 14mm ƒ/2.8 ED (IF) gives *ist D and *ist Ds shooters the fast, wide equivalent of a 21mm ƒ/2.8 lens. Like other lenses with internal focusing, its front elements don't rotate—a real boon for users of graduated neutral-density filters and other position-dependent accessories.
Tokina (THK Photo)
Another difference between film and digital is the relatively mirror-like surface of image sensors. In some cases, the sensors' smooth face can reflect light back through the rear lens elements, where the beams bounce around until re-exiting the rear glass and striking the imager as flare or a ghost image. The series of ƒ/2.8-speed, L-series zooms, such as its CanonEF 70-200mm ƒ/2.8L IS USM, have optimized lens element shapes and anti-reflection coatings to minimize or eliminate these ghosts. Unlike the special small-format digital lenses, this lens series is a part of the regular 35mm lineup and provides outstanding image quality on both film and digital cameras. The Tamron Di and Di-II-series lenses, such as its SP AF17-35mm ƒ/2.8-4 Di LD Aspherical (IF), feature improved multi-coating and interior light baffling to combat off-the-sensor reflections. The lenses offer improved resolution, contrast and freedom from flare with film SLRs as well. This also is true of Sigma's new Super Multi Layer Coatings now used on allSigma lenses.
Canon and Nikon also have taken care to subtly curve the protective front filters on their super-telephoto lenses. The filters' new meniscus shape disperses the reflections from the image sensor that would otherwise bounce off the rear of the filter and straight back onto the sensor's face. The curved filters now are standard on all Canon fast IS super-telephotos, like its EF 500mm ƒ/4L IS USM; Nikon has introduced a curved filter on its 300mm ƒ/2.8G ED-IF AF-S VR Nikkor.
Digital photography still is photography, so other new lens technologies, not strictly for digital, are critical. Chief among them are the lenses that shift a group of their optical elements to compensate for camera shake. These lenses enable handheld photography at shutter speeds two to three stops slower than you'd use with a conventional lens of the same focal length. Look for Image Stabilizer (IS) lenses from Canon, like its EF 28-135mm ƒ/3.5-5.6 IS USM; Vibration Reduction (VR) lenses from Nikon, such as its 24-120mm ƒ/3.5-5.6G ED-IF AF-S VR Zoom-Nikkor; and Optical Stabilizer (OS) lenses from Sigma, like its 80-400mm ƒ/4.5-5.6 EX OS APO.
Another new technology, Diffractive Optics (DO), relies on applied physics, using a vastly different means to bend light rays than the curving glass elements we're used to. A special element within a DO lens directs light waves through a finely etched concentric lattice to achieve focus. By varying the lattice's spacing, the DO element can provide the advantages of an aspherical lens element and help control chromatic aberrations, too. Lenses with Diffractive Optics technology, such as the Canon EF 70-300mm ƒ/4.5-5.6 DO IS USM, boast improved image quality and can be made about one-third smaller than conventional lenses.
Nikon's Phase Fresnel lenses are similar, providing the same image quality and size advantages as Diffractive Optics. The Nikon TC-E3PF 3x telephoto converter lens brings this technology to its Coolpix 8400 advanced compact.