Most APS-C DSLRs, on the other hand, not only accept smaller, more affordable "digitally optimized lenses" such as Nikon's DX, Canon's EF-S, Pentax's DA and Sony's DT series, but are also backward-compatible with the respective brand's full-frame lenses that may harken back to the film days—albeit with a 1.5X to 1.6X cropping factor (see the sidebar). When full-frame lenses are mounted on a compatible APS-C model, the sensor is covered by the central sweet spot of the lens, resulting in the best performance match and minimal edge distortion or light falloff. However, on full-frame DSLRs, you're more likely to notice the shortcomings of a full-frame lens—especially lenses designed during the film days—at the widest apertures and toward the corners of the image. You'll also notice that the AF zone area on a full-frame camera generally covers less of the field of view than it does in an APS-C camera. That's because camera manufacturers tend to use similar focusing engines across both DSLR formats, and the wider field of view provided by a full-frame camera means a proportionally smaller AF zone in the center of the viewfinder. However, that gives a focus-tracking advantage to the APS-C DSLR, since the AF zones cover more of the image area and can lock onto moving subjects on the extreme right or left before a full-frame camera's AF system can.
For nature and sports shooters, the added 1.5X or 1.6X cropping factor afforded by any lens on an APS-C DSLR is a plus, as it allows them to get closer to a subject without spending more on a heavier supertelephoto lens (for example, a 100-300mm ƒ/4 lens becomes a 150-450mm ƒ/4 equivalent lens at half the size and weight) or turn a bright 50mm ƒ/1.4 lens into a great 75mm-equivalent ƒ/1.4 portrait lens. In addition, adding a sensor-based image-stabilization system to a DSLR, such as those found in Sony and Pentax models, or even a lens-based optical-stabilization system like those sold by Nikon and Canon, can be done more effectively and at a lower cost with an APS-C camera or lens than a full-frame camera or lens. You also may notice that the image-stabilization system and the autofocusing system are slightly quieter on APS-C cameras and lenses due to the mechanical differences.
Improved lens designs for APS-C DSLRs also have increased the number of ultrawide-angle offerings, allowing wider fields of view that once were the exclusive domain of full-frame DSLRs. In fact, a typical image-stabilized 18-55mm ƒ/3.5-4.5 kit lens, with an equivalent wide-angle to telephoto field of view of 24-80mm, now can be purchased for $100 to $200, yet ranks as one of the sharpest lenses available for APS-C models. Compare that to the full-frame Nikkor 24-85mm ƒ/3.5-4.5G ED VR kit lens introduced for the Nikon D600, which costs $500.
One last note on the lens front: As manufacturers continue to pack more pixels into their full-frame sensors than their APS-C sensors (and they will, won't they?), photographers may start to notice a greater drop in resolution at apertures above ƒ/11 due to lens diffraction. Unfortunately, diffraction affects all lenses equally due to the physics of light and becomes more noticeable in high-resolution sensors (above 20 megapixels) and at apertures above ƒ/11—something to consider when you're trying to squeeze out more depth of field in a scenic or macro shot.
The bottom line? The APS-C class of HDSLRs looks like it's going to be around for a long time based on its relative cost, size and several performance advantages over full-frame HDSLRs. In addition, there are more than enough APS-C and full-frame lenses available to satisfy most pros and advanced amateurs who want to stick with APS-C—and those same lenses can be used on a growing number of APS-C-class camcorder models and several interchangeable-lens compact cameras.
Michael J. McNamara is a renowned expert on digital cameras, printers and color management systems. During his 20-year editorial career, McNamara has written hundreds of articles on all aspects of photography and technology. He's also the editor in chief of the McNamara Report, www.mcnamarareport.com.