There's been a lot of talk about DSLRs, but less seems to get said about their changeable lenses. That's odd, because the ability to swap lenses is a defining feature of DSLRs, one that brings enormous advantages with it. And the nature of the lens market has changed as extensively as the camera market itself over the past few years. In a sense, people should be talking about the lens systems more than the cameras, buying the lenses they like, then a camera body to match. Because the lens is the eye; the camera is only the recording system of what it sees.
Yet if the market patterns today are like those when 35mm [film] dominated the SLR, people are buying the bodies first, based on brand name, and most never go on to buy second or third lenses.
Today's "standard" lens, the kit lens that comes with most DSLRs, is a zoom with a range of something like 18-55mm or more (a "35mm equivalent" of around 27-82.5mm or more). That's a lot of performance for a hundred-dollar lens, a wide-angle range that would have been exotic and bold when film was at its height.
But since today's "standard" lens provides so much range, there's less of an urgency to add that second or third lens. When the non-zooming 50mm lens was the standard on a new camera, a user requiring wide-angle viewing was forced into another purchase, a separate 28mm (or so) lens. Now it's all one with the OOBE (out of box experience).
Most of the manufacturers now offer an interchangeable lenses with a zoom range of 10x or higher-14x in the case of the recent Tamron and Pentax releases. Considering that the first digicams usually had 3x zooms at the best, 10x to 14x is positively breathtaking. But if you run from 18mm to 250mm (per the Tamron/Pentax), what could the customer possibly accomplish with more lenses?
Several suppliers do offer DSLRs in two-lens kits, which could be said to cut both ways: with both a wide-angle zoom and a telephoto zoom in the initial purchase, what's the incentive for a customer to keep buying? But also, now that the customer has the two lenses, the clamshell of his mind is pried open. He's more receptive to the principle that lenses are good things to have, because his experience tells him so. He's seen how having two lenses permitted him to master a broader range of conditions, to take more and better pictures. It's not just marketing hype that claims that having lots of lenses is a good thing.
Reshuffling the Deck
The DSLR market began taking off around 2003, when under-$2,000, followed by under-$1,000 models, began cascading into the stream. We began hearing a lot of talk about lenses designed specifically for digital imagers. CCDs and CMOS imagers require the light to travel in more of a straight direction than film does, closer to perpendicular to their receiving surface. Film is flat, so light striking it from an angle can still make an exposure. But the diodes on imaging chips are down in something of a well, and light coming from an angle strikes the wall of the well and is no help at all.
Olympus was one of the first to proselytize on this, when they presented the FourThirds system in the E-1. Theirs is the only DSLR line with no ties to the optical past. All the others share film-camera lens mounts, and even some film-camera lenses.
It wasn't long before everyone was in accord with the thesis, however. Canon, Nikon, Pentax, Sigma, Sony, and Tamron all offer lenses said to be digital-specific-that is, not only do they have the straight-back flight path, they also project a smaller image that suits the miniature APS-size imager.
With the exception of Olympus, all the manufacturers declare that film-camera lenses are capable of quite excellent digital pictures, and our experience bears them out. Yet all acknowledge that lenses designed for digital optimization should produce snappier images, everything else being equal.
There are further benefits to digital-specific lenses, both real and theoretical. Since the image they're forming is smaller, they themselves can be smaller and more convenient to lug around. And, since the "normal" focal length is shorter for a digital lens, digital photos naturally tend to show a greater depth-of-field, because all other things equal, depth-of-field increases as focal-length decreases. It's also easier to make shorter lenses with faster apertures. Most people would consider all of these to be plus factors of digital-specific lenses.
This was all new just a few years ago, in a segment of the market that had been pretty stable since the acceptance of the zoom-the 1970s. Along with these new concepts, we also had to get our arms around "35mm equivalencies" and "magnification factors," which we've done pretty well, considering.
The effect of focal length depends in large part on the size of the image it's projecting. The 35mm frame has remained the same size from the late 1920s till the present, so the effects of different focal lengths are fairly standardized and easy to remember. A 50mm lens is generally considered "normal," so we all know what to expect from a 28mm lens compared to a 16mm, a 200mm lens compared to an 800mm.
These rules-of-thumb fall apart in the digicam market, because imaging chips come in many sizes. Canon alone has used three different size chips that are considered "APS size." Point-and-shoot cameras tend toward even smaller imagers, again in multiple sizes. So how are people to easily conceive of the effect of a lens on their particular camera?