Update 4:00 p.m.: I clarified two sentences and added a third to make it clear that Nikon and Canon never forced customers to buy small-frame lenses. Still relevant in my opinion, though, is the practical reality that customers already have and will again.
Canon beat Nikon to market by years with a high-end digital SLR whose image sensor is the size of a full frame of 35mm film. But while Nikon may have been late to the party with its new D3, I think it employed a much smarter approach to a lens compatibility issue.
One of the chief merits of single-lens reflex (SLR) cameras is their ability to accommodate multiple lenses, letting photographers adapt the same camera body to shoot everything from faraway birds to the confined quarters of a small house. It's annoying that one company's lenses generally don't work on another company's camera bodies, but it's even more annoying when a single company's products are incompatible, and the full-frame trend in higher-end cameras has put the spotlight on the issue.
Here's the background. The vast majority of digital SLRs sold today use smaller image sensors measuring roughly 24x16mm that are much less expensive to manufacture than full-frame 36x24mm sensors. Canon and Nikon both have made lenses specifically for these smaller sensors; Canon's small-frame lenses bear the EF-S label and Nikon's are called DX. Although neither company forced anyone to buy these lens lines, many have done so.
One big reason to make small-frame lenses is that wide-angle lenses need to be reworked for small sensors. For example, you need to use a 10mm lens on a small-frame Canon camera such as an EOS 40D to get the same field of view as a 16mm lens on a full-frame camera model such as an EOS 5D. Another big reason is that lenses for smaller sensors can be made smaller and lighter.
For many folks with digital SLRs, having a separate line of lenses for small-frame cameras isn't a big deal. They'll buy a lower-end, small-frame camera, and if they buy a later model, there's a good chance it'll be another small-frame model and their lenses will work fine. But for photographers upgrading to a full-frame camera, the lens compatibility issue rears its ugly head.
Canon shooters can't use small-frame EF-S lenses on full-frame cameras. Full-frame cameras physically can only accommodate EF lenses. Nikon, though, permits its small-frame DX lenses to be mounted on its full-frame FX cameras.
Nikon's approach isn't all smooth sailing. DX lenses are designed to shine light only a small sensor, so by default, a full-frame camera will use only the central portion of the larger sensor, meaning that many pixels are wasted. On the 12.1-megapixel D3, you only get a 5.1 megapixel image. And if you do enable the full sensor to work, in many situations light will only fall on the central pixels anyway, leaving a clear record of the lens' smaller image circle. (The DX-on-FX photo by D3 owner Rob Gruhl above illustrates the effect.)
Nikon's strategy meant that lens compatibility is a bigger issue for professional-level photographers. Its first full-frame camera, the $5,000 D3, has only just hit the market in recent weeks. That means pro-level photographers who wanted to go digital with earlier pro-level SLRs such as Nikon's D2Xs might well have felt no compunctions about investing in small-frame lenses and might have been enticed to do so to get a super-wide-angle zoom, for example.
In addition to the relatively inexpensive "kit" lenses that ship with lower-end SLRs, Nikon DX models include a $900 12-24mm f/4 wide-angle zoom, a $1,200 17-55mm f/2.8, a $600 10.5mm f/2.8mm fisheye, and a $700 image-stabilized 18-200mm f/3.5-5.6 super-zoom that's the company's best-selling lens ever.
Canon, on the other hand, introduced full-frame cameras many years earlier--2002 with the EOS-1Ds--when pros weren't as far along in their transition from film to digital and therefore might have been less likely to have made investments in EF-S lenses. Indeed, one EF-S lens useful in getting full performance out of a small-frame SLR, the 10-22mm f/4-5.6 zoom, wasn't even on the market until 2004.
But the market is different now. Canon introduced the full-frame 5D more than two years ago, and it's now available for $2,100. No entry-level buyer will be interested, but that's within the range of a lot of enthusiasts, and I wouldn't be surprised if a more powerful successor (the 5D Mark II? The 7D?) is announced in conjunction with the PMA photo show in January. This is an obvious upgrade path for people who bought solid small-frame models such as the Rebel XT or 20D.
Unfortunately, it's not just a matter of reworking the lens mount on the 5D successor. The approach Canon chose with its EF-S lenses means they physically can't work on full-frame cameras because the camera's reflex mirror, which flips out of the way when a photograph is taken, would strike with the rearmost lens element or at least a rubber ring that protrudes to protect it.
"The reflex mirror on a full-frame camera would definitely collide with the rubber ring on EF-S lenses if someone managed to jam one on," said Canon spokesman and tech guru Chuck Westfall. Canon is not currently rethinking its approach to small-frame lenses on full-frame cameras, he also said.
Canon stands by its approach of permitting only full-frame lenses on full-frame cameras. "In our view, it's more valuable to preserve a full angle of view wherever possible," Westfall said. "The image circle projected by EF-S lenses is only sufficient for EOS digital SLRs with APS-C format (small-frame) image sensors, so allowing such lenses to be mounted on cameras with larger imaging formats wouldn't be practical."
The "S" in EF-S refers to the shorter back-focus distance separating the lenses' rearmost element and the sensor. "Probably the most important potential advantage of shorter backfocus is smaller, lighter and more affordable lenses," Westfall said. "Case in point: EF-S 18-55mm f/3.5-5.6 is significantly smaller, lighter, and more affordable than a lens of the same zoom range and maximum aperture with full-frame coverage."
Canon has some good engineers and I'm sure it carefully evaluated its options when making small-sensor lenses. Its lens approach probably helped it be the first to market with a sub-$1,000 digital SLR, the original Digital Rebel.
But with full-frame SLRs now dropping in price--potentially even faster now that Nikon is exerting competitive pressure--I think it's too bad Canon didn't opt for a design with better lens compatibility. If you're the type of photographer who might consider upgrading to a full-frame camera, think carefully before laying out nearly $1,000 for a Canon 17-55mm EF-S f/2.8.