A generation ago, debate raged over the presumed benefits of medium format (big cameras using 2¼-inch roll film) compared with the 35mm film format used in standard SLRs and rangefinders.
A version of that debate continues today. At issue: the relative merit of the "full-frame" DSLR-one whose sensor dimensions closely match those of the 35mm film frame-against those with smaller sensors. Which is better overall-or, more to the point, better for your kind of shooting?
We've looked hard at all the angles and tallied a scorecard that may surprise you.
SENSOR SIZE MATTERS
Three sizes of DSLR imaging sensor are in wide use today: full frame (named for the 35mm film frame), APS-C (about the size of the standard frame of Advanced Photo System film), and Four Thirds (developed without reference to a film size). Canon, Nikon, and Sony offer both full-frame and APS-C models; Pentax and Samsung, APS-C only; and Olympus and Panasonic, Four Thirds.
The bigger the sensor, the more of an image it can cover-in other words, it will have a wider angle of view. Put a lens of the same focal length on cameras of the three formats and, if it provides the view of a moderate wide-angle lens view on a full-frame camera, it will serve as a normal on the APS-C camera, and a short tele on the Four Thirds model.
This has led manufacturers and publications to state "equivalent" focal lengths for lenses intended for smaller-than-full-frame cameras. So a 35mm lens is equivalent to 52.5mm on an APS-C camera and 70mm on a Four Thirds camera. While this may cause more confusion than it clears up, it provides a useful comparison of lenses across different formats.
THE MAGNIFICATION FACTOR-OR IS IT?
A quick way to compare lenses across formats is to use an arithmetic multiplier. Called a magnification factor, crop factor, or 35mm lens factor, this is simply a number multiplied by the focal length of a lens on a smaller-sensor camera to get a full-frame equivalent.
Most APS-C cameras have a lens factor of 1.5X. Canon APS-C models, which use a slightly smaller sensor than other brands, have a factor of 1.6X. And Four Thirds models have a 2X lens factor. So a 200mm lens on an APS-C model gives you the reach of a 300mm or 320mm lens on a full-framer. On an Olympus or Panasonic, it would give you the reach of a 400mm.
BUT DOES IT REALLY?
Some photographers say that this extra reach amounts to a crop of the full frame-equivalent to simply blowing up the center portion of the full-frame image.
But this is a film analogy that doesn't hold true in the digital world. The APS-C camera concentrates all its pixels in that smaller frame, whereas if you crop the image from a full-frame camera, you lose pixels.
For example, if you put a 200mm lens on a 12.1MP full-frame Nikon D3, then cropped the picture to a field of view equivalent to what you'd get using a 300mm lens, your image would wind up just 5MP in size. But if you put the same lens on a 12.3MP APS-C sized Nikon D300, you'd get a 300mm-equivalent image-with the full 12.3 megapixels.
With today's high-resolution full-framers, that compromise isn't as striking, but it doesn't disappear. Put a 200mm lens on a 24.6MP full-frame Sony Alpha 900, then crop, and you'd get a 10.5MP image-respectable, but still less than you'd get with a 12.2MP APS-C Sony Alpha 700.
So the telephoto advantage of smaller-format DSLRs is very real. Sports and wildlife shooters in particular can benefit from a smaller-sensor DSLR, either by getting long reach with relatively compact lenses, or by getting huge reach with big "legacy" glass-full-frame lenses originally developed for film SLRs.
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