© MARZANNA SYNCERZ / FOTALIA.COM
Crop Factor in action: Even when both the viewpoint and lens focal length stay the same, if you shoot the same scene with three different sensor sizes-Four Thirds (1), APS-C (2), and full-frame (3)-you get three different angles of view.
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.
THE WIDE-ANGLE CONUNDRUM
At the other end of the lens spectrum we have the wide-angles, and here the full-framers have-or at least had-an advantage. That 17mm lens that gives you a wild, wide view on a full-frame DSLR? On an APS-C camera, it's equivalent to a ho-hum 27mm.
The playing field is being leveled, though: Witness Canon's 10-22mm f/3.5-4.5 EF-S (16-36mm full-frame equivalent), Nikon's 12-24mm f/4G DX Nikkor (18-36mm equivalent), and Sony's 11-18mm DT f/4.5-5.6 (16.5-36mm). We even have curved-field fisheyes for smaller-sensor DSLRs. We'll likely see even wider lenses for smaller formats in the near future.
So unless you already have a stash of legacy wide-angle lenses for full-frame, there is no real wide-angle advantage to a full-frame DSLR.
THE ACHING-BACK FACTOR
Because smaller-than-full-frame sensors don't take in as much image area, camera makers are able to design lenses for them that cast a smaller image circle onto the focal plane. That means lenses can be made using smaller-diameter elements, for less size and weight.
And if you're willing to forego some brightness, these lenses can be smaller and lighter still.
That's how manufacturers can design those ultrawide zooms we mentioned above, as well as long-range zooms like 18-250mm optics, at relatively moderate cost. These lenses are sometimes termed "digital only" and can be identified by manufacturers' trade names: Canon EF-S, Nikon DX, Pentax DA, Sony DT.
Now, here's the rub: If you opt for a smaller-sensor DSLR system, and later want to add a full-frame DSLR, your digital-only lenses will have limited utility, at best. Canon EF-S lenses won't even mount on Canon full-frame models. Nikon and Sony APS-C lenses will work at reduced resolution, as we've noted. Adding full-frame glass along with a full-frame body ends up an expensive proposition.
However, going the other way-using full-frame lenses on smaller-sensor cameras-is easy street. These lenses will work on either format. You gain tele reach with the long lenses on the APS-C body, and if you want a wider angle of view than you get with the full-frame lenses, you can always add an APS-C ultrawide at reasonable cost. (For a real-life example of how this works, see the sidebar on the next page.)
"For oldsters like me who have a closet full of legacy glass, it was a real delight to be able to use my existing lenses on the new cameras," says Douglas Rea, a professional photographer and professor at Rochester Institute of Technology in Rochester, NY.
Another presumed advantage of using full-frame lenses on APS-C cameras is that the smaller sensor is using only the central "sweet spot" of the image circle, thereby providing sharper images with less light falloff at the edges.
In Pop Photo Lab tests, though, the sweet spot is proving a mythical beast. It may have held true for older legacy lenses, but we now see very similar SQF numbers for full-frame lenses tested at full-frame and APS-C sizes. And the better digital-only lenses perform on a par with equivalent full-frame optics.
While digital-only lenses have been criticized as slower than their full-frame counterparts, this, too, is changing as manufacturers build up these lens lines.
HOW DENSE ARE THOSE PIXELS?
For a smaller-sensor DSLR to have the megapixel count of a full-frame DSLR, the pixels have to be packed tighter. For instance, the 12.1MP full-frame Nikon D700 has just under 14,000 pixels in a square millimeter of its sensor, while the 12.3MP APS-C Nikon D300 sensor has about 33,000. Even the full-frame 24.5MP Nikon D3x has less pixel density than the D300-about 28,300 per square millimeter.
With more loosely packed pixels, manufacturers can make the pixels bigger, and that's the case here: A D700 pixel is about 8.5 microns across, while a D300 pixel is about 5.5 microns. Bigger pixels can catch more light, which can make for images that have less noise and greater dynamic range.
But now we come to the real world. In our tests in the Pop Photo Lab, the Nikon D700 and D300 produced very similar image-quality results. They were neck-and-neck in noise control through ISO 1600, with the D700 slightly better at ISO 3200 and 6400. (The D700 also has ISO settings of 12,800 and 25,600; the D300 does not.) So, given the same megapixel count, the smaller sensor can perform on a par with the larger one.
Up to a point. The Canon EOS 50D, with its 15.1MP APS-C sensor, produced more resolution in our tests than the Nikon 12MP-range cameras, but did not really exceed them in overall image quality, chiefly due to noise. To get that many pixels on the smaller chip, the 50D needed a pixel size of 4.7 microns. The full-frame 21.1MP Canon EOS 5D Mark II, on the other hand, has room on its sensor for larger pixels, and its image quality blows away the 50D.
Sometimes there is no substitute for square millimeters.
THE BOTTOM LINE
If you've been tallying the scorecard, you've probably reached the same conclusion we have: that for most serious amateurs, and many pros, there is no compelling reason for going to a full-frame DSLR. The APS-C cameras and lenses in the Canon, Nikon, and Sony systems can provide comparable imaging quality at less cost and almost always reduced size and weight. Pop Photo has run many ooh-inducing double-page spreads of photos taken with smaller-than-full-frame DSLRs.
But if you require extremely high-resolution images, if you make really big enlargements, or if you want great leeway in cropping deep into an image, you'll want a high-megapixel full-frame DSLR. Given the current state of the technology, APS-C cameras reach a point of diminishing returns at around 14-15MP.
To put it simply, full-frame is becoming the new medium-format.