Digital Lenses: The f stops here

|| |—| | | RX FOR DIGITAL SLRs? Some lenses exclusively for digital SLRs: (clockwise from top left) Nikon, Sigma, Olympus, and Canon. All claim higher edge definition at wide- to ultrawide-angles, but do they deliver?| Listening to various pundits, you’d think digital SLRs should come with warning labels about possible side effects. If these critics (and, to a lesser extent, several camera and lens manufacturers) had their way, they might read: Caution! On this digital SLR, wide-angle and ultrawide-angle lenses may not deliver wide and ultrawide fields-of-view. Furthermore, older lenses designed for 35mm SLRs may produce images with lower resolution, contrast, and brightness, especially toward the edges.

By now, most Pop readers are familiar with the difficulty of capturing true wide-angle photos with most digital SLRs. Blame smaller-sized CCD and CMOS imaging sensors (relative to a 35mm film frame), found in all but the most expensive digital SLRs. We always point out a digital SLR’s 35mm lens factor in our tests, which range from 1.3X (on the Canon EOS-1D) to 2X (on the Olympus E-1, which doesn’t accept 35mm lenses). This factor can be multiplied by the focal length marked on the lens to determine its new “digital” field of view. So, on these cameras and all that fall in between, a 28mm lens (a true wide-angle on a 35mm SLR) now delivers the field of view of a 36-56mm lens, while an 18mm ultrawide-angle lens now acts like a 23-36mm, or true wide-angle lens. Unfortunately, existing fisheye lenses cause too much distortion to be used as ultrawide-angle lenses on a digital SLR; hence the introduction of zooms that go down to 11mm for digital SLRs.

Factor or fiction?
The beneficial side-effects of using a telephoto lens on a digital SLR far outweigh the loss of ultrawide perspective. For example, a relatively inexpensive 100-300mm f/3.5-4.5 lens acts like a 160-480mm f/3.5-4.5 lens on the Canon EOS Digital Rebel and EOS 10D (both with 1.6X 35mm factors). This extra reach is a real plus, but what about other possible side-effects, such as lower corner resolution, contrast, and brightness, that some manufacturers claim you get when older lenses are used on digital SLRs? Are these real problems or a marketing ploy?

According to Olympus and several other manufacturers, resolution and light falloff around the edges of an image are caused by the extreme angles of light from a non-aspheric lens striking the smaller image sensor found on a typical digital SLR (see our test of the Olympus E-1, November 2003). This is particularly noticeable at wide apertures on wide-angle lenses. Image contrast is also lowered by increased lens flare, which is caused by two things: First, light bouncing off the internal components and sides of a lens adds unwanted light to shadow areas. Second, light striking the protective glass filters on the front of a CCD or CMOS sensor reflects back to the rear lens elements and is mirrored back to the image sensor, again adding light where it doesn’t belong. On film SLRs, a coating on the back of the film minimized this part of the flare problem, so few lenses were designed with anti-reflective coatings on the rear elements. Newer digital lenses include more aspherical lens elements to straighten out the light path and more anti-reflective coatings on rear elements.

Are these new lenses worth the extra investment, or are there ways to minimize the side effects? To find out, I gathered various new “digital” lenses and compared them with their older, similar focal-length siblings (if they existed) to see how corner resolution, light falloff, and contrast were affected across a wide range of apertures. These results are tabulated in the chart, but note some surprises I found.

Lenses: family feud

|| |—| | | NARROW DIFFERENCE? A 35mm Canon zoom lens (top) vs. its digital-only 18-55mm EFS lens (right). The EFS has a smaller lens core, optimized to cover an APS-size CMOS sensor. For now, the EFS works only with the Digital Rebel.| As a whole, the new digital lenses outperformed their older 35mm film lenses at ultrawide and wide-angle focal lengths-but the differences in resolution and light falloff were less noticeable at higher apertures such as f/8, f/11, and f/16. Tip: If you have an older lens and want to maintain resolution and reduce light falloff, stop down to f/8 or smaller. But get a tripod if your shutter speed gets too slow to handhold the camera. Boosting the camera’s ISO sensitivity to get faster shutter speeds magnifies resolution and contrast differences between the center of the image and edges.

Sigma’s new digital lens outperforms its moderately priced 35mm sibling in terms of contrast at all aperture settings-proving the advantage of anti-reflective lens coatings. But several of the more expensive ultrawide-angle zoom lenses, such as the 16-35mm f/2.8L USM EF Canon lens ($1,390 street), contain aspherical lens elements with anti-reflective coatings. That may be why this lens outperformed the only Canon digital lens, the 18-55mm EFS lens ($100) at comparable 18-, 28-, and 35mm settings.

Nikon’s new 12-24mm f/4G ED DX-series Nikkor is a digital-only lens. At 24mm, it just edged out (pun intended) the lower-priced 24-85mm f/3.5-4.5G IF-ED Nikkor lens in terms of resolution and light falloff at the widest aperture settings (see chart). Results evened out at higher apertures, and there was little contrast difference between the two lenses at any aperture.

There’s no equivalent Nikon lens to challenge the new DX series at the 12mm setting, so if you want an ultra-to-wide-angle-zoom field-of-view for the D100, D2h, or D1x, this lens is your best choice. However, don’t expect miracles at the 12mm setting. Images shot at this focal length show a 50-percent resolution dropoff and noticeable light falloff around the edges at f/4. By f/16, however, corner resolution was within 10 percent of the center. (So if you’re shooting in low light, bring that tripod.)

The bottom line? The “digital-only” lens category is relatively new and growing rapidly, but it’s no marketing ploy. If you have good 35mm ultra- and wide-angle lenses, as well as normal-to-telephoto lenses, keep them. But if you’re starting out, opt for digital-only or digitally optimized lenses for better edge-to-edge sharpness, contrast, and brightness.

Download our Digital vs Conventional Lenses Chart
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