Digital Lenses: The f stops here

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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.

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