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What Is Full-Frame Anyway?

Last updated on 12/03/2010

We posted an essay a few weeks ago about our reviewer's ongoing affair with the full-frame Nikon D700 dSLR. We got a few questions about what a full-frame camera is in the first place, so Ben is back to clear it up for you.
By Ben Keough

What Is A Full-Frame Camera Anyway?

When a camera is a full-frame model, that means its imaging sensor is precisely the same size as a 35mm film negative (36 x 24mm). Compared to the APS-C sized sensors found in most dSLRs (varied in size, but generally somewhere around 23.5 x 15.5mm), a full-frame sensor covers approximately 2.25 times the area. This greater sensor size has a number of side effects, both technical and a practical.

On the technical side, a full-frame sensor is more difficult to produce, and therefore proportionally more expensive. This higher component cost is reflected in the massive price gap between full-frame and APS-C cameras.

Pixel size is another key difference between APS-C and full-frame sensors. In short, a larger sensor with the same amount of pixels will have room for larger pixels, or “photosites.” The ratio of pixels to physical sensor area is often referred to as “pixel density.” It’s generally accepted that a lower pixel density will result in improved image quality thanks to a range of factors including greater dynamic range and lower image noise.

While these technical differences are largely academic, the practical photographic fallout from the difference in sensor size is immediately apparent to most users.

Crop Factor: Old Lenses Are New Again

Any dSLR user who has progressed beyond the sandbox of the “kit lens” has probably come across the phrase “crop factor.” This concept states that since a smaller APS-C sensor occupies the same position in a camera as a larger full-frame sensor, it essentially functions as a “cropped” version, with the edges snipped off.

Thus, a photo taken with an APS-C sensor can be considered as a cropped version of a full-frame photo. To quickly visualize what this means, imagine a wide-angle photo of a forest. Now imagine cropping thick band off of all four sides of the image. All that remains is a central chunk of the original image -- suddenly your photo isn’t a wide angle shot anymore. Rather than a forest, you have a couple trees. The sky is missing, and the grassy foreground is gone. Your field of view (FOV) has been drastically reduced. In this way, lenses behave differently on cameras with different sensors.

Crop factor was a concept invented to help film camera owners understand how their legacy lenses would behave on new digital (APS-C) bodies. A simple multiplication factor of 1.5x was produced to give a steady frame of reference. Let’s say you want to know how your 50mm prime lens will look on an APS-C body. Well, just multiply the focal length by 1.5, and you’ll see that a 50mm lens is equivalent, in terms of FOV, to a 75mm lens on film or a full-frame digital SLR.

The end result of all of this arcane math is that APS-C bodies make long lenses longer, which makes them wildly popular with sports and wildlife photographers, who need lots of telephoto reach. Meanwhile, full-frame bodies make it easy to go very wide, giving a boost to those who like fisheye effects or shooting architecture.

Depth of Field, Or “How Much Of My Image Is In Focus?”

The second major photographic difference between APS-C and full-frame cameras lies in their depth-of-field capabilities. In general, at the same focal length and aperture settings, a lens mounted on a full-frame dSLR will produce shallower depth of field than the same lens on an APS-C camera. Shallower depth of field results in subject/background separation, which gives images more “pop.” Meanwhile, deeper depth of field is preferred for landscape shots, where everything has to be in sharp focus.

Hope this clears it up for you a bit.