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Wednesday, August 1, 2007

Full-Frame vs. Small-Frame Digital (Does It Matter?)

Digital sensors come in a variety of sizes. Is bigger better?

Canon D-SLRs from the EOS 30D down have sensors measuring 22.5x15.0mm or a little smaller, with a 1.6x magnification factor; a 28mm lens on these cameras frames like a 45mm lens on a 35mm SLR.

Sigma's SD14 has a sensor measuring 20.7x13.8mm and a magnification factor of 1.7x; a 28mm lens frames like a 47.6mm lens on a 35mm camera.

Four Thirds System D-SLRs (those from Leica, Olympus and Panasonic) have even smaller sensors measuring 17.3x13mm, with a 2x magnification factor; a 28mm lens frames like a 56mm lens on a 35mm camera.

You can see that a 28mm "wide-angle" 35mm camera lens isn’t wide-angle when used on any of the small-sensor D-SLRs. As a rule of thumb, to get the same field of view with an APS-C D-SLR as you would with a 35mm SLR, you need a lens two-thirds the focal length of the film camera lens; if you want the field of view of a 24mm lens, you need a 16mm lens on the D-SLR.

The new Canon EOS-1D Mark III, like its predecessors, has a sensor midway in size between APS-C and full-frame and a 1.3x magnification factor; a 28mm lens frames like a 36.4mm on a 35mm camera, minimally wide-angle.

Full-frame-sensor D-SLRs (today represented by the Canon EOS-1Ds Mark II and EOS 5D) have sensors measuring 36x24mm, the same size as a 35mm film frame, so there’s no magnification factor; any lens used on a full-frame D-SLR will frame just as it does on a 35mm SLR.

Digital Lenses

Fortunately for small-format wide-angle fans, SLR and lens manufacturers now offer a number of affordable super-short focal-length lenses designed specifically for the small-sensor D-SLRs. These bring true wide-angle capability to all D-SLR users at a reasonable cost. But note that these lenses can’t be used on 35mm SLRs or full-frame D-SLRs because they don’t cover the larger image area and thus vignetting will occur (see Figure B).

Since lenses designed to be used with smaller image sensors don’t have to project as large an effective image circle as lenses designed for 35mm cameras, manufacturers can optimize them optically for that smaller image circle. This means lenses can be smaller and can send the light more efficiently (directly) to the image sensor.

Each manufacturer has a special identifier for its APS-C lenses: Canon (EF-S), Nikon and Tokina (DX), Pentax (DA), Sigma (DC), Sony (DT) and Tamron (Di II). All lenses for Four Thirds System cameras are designed specifically for the 17.3x13mm Four Thirds System sensor.

Wings of SpringFigure B: APS-C image sensors, being much smaller than a 35mm image frame, don't need a 43.27mm image circle. Lenses designed for APS-C sensors produce a smaller image circle, just sufficient to cover the 28.37mm diagonal measurement of the APS-C sensor format. If used on a 35mm SLR or full-frame D-SLR, such a lens would produce vignetting: the image wouldn't fill the image frame. In many cases, APS-C lenses can't even be mounted on full-frame cameras because of potential for damaging the reflex mirror.

Pixel Size

Image sensors contain millions of photoactive CCD or CMOS "pixels" that collect light. The larger these cells, the more efficiently they capture light—but the fewer will fit on a given size image sensor. Larger sensors can hold more pixels of a given size (more megapixels) or larger pixels for a given megapixel count. The more pixels you have, the greater the sensor’s resolution—the more fine detail a sensor can capture. The bigger the pixels, the better the low-light and high-ISO performance.

D-SLRs by and large have found the sweet spot between maximum pixel count and optimum pixel size for the technology today. There’s also a lot besides pixel count and size that goes into image quality, including the camera’s A/D converter, image-processing engine and imaging algorithms. As a result, you can get excellent image quality with any of today’s D-SLRs.

As with film, some cameras are better for some tasks than others. A good rule of thumb is that a small-format D-SLR is ideal for things you’d use a 35mm film SLR for and a —medium-format— D-SLR is ideal for things you’d use a medium-format film SLR to photograph.


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