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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?

Like film cameras, digital SLRs come in a wide range of formats. But with D-SLRs, the format is based on the size of the image sensor, not on the size of the film. Sensor size has several ramifications for the photographer. First, larger sensors cost a lot more than small ones, in part because of the difficulty in manufacturing them. Second, larger sensors "see" more of the image formed by a lens and thus provide a wider field of view with any given focal length. Third, for a given pixel count, larger sensors contain larger pixels, which, all other things being equal, collect light more efficiently for better low-light and high-ISO performance. Finally, larger sensors generally require larger camera bodies.

Today’s D-SLRs employ image sensors ranging in size from 17x13mm to 48x36mm and in resolution from 5 to 39 megapixels. The popular D-SLRs that look like 35mm SLRs use sensors up to full frame (36x24mm, the same as a 35mm film frame); sensors larger than that are found in D-SLRs based on medium-format film cameras.

Digital sensors also come in different aspect ratios (the ratio between the long dimension of the image and the short dimension). Most feature the 3:2 ratio familiar to 35mm photographers. Four Thirds System sensors, as the name implies, use a 4:3 aspect ratio that more nearly matches that of common print formats like 8x10 or letter (8.5x11).


There’s not much you can do about sensor cost. The cameras with the largest sensors cost a lot more than those that use smaller ones. But you don’t need a huge sensor to get great image quality; today, working pros do very well with sensor sizes throughout the range.

Angle Of View

Figure A shows how sensor size affects angle of view with any given lens focal length. Basically, the smaller the sensor, the narrower the angle of view you get with a given lens because a smaller sensor sees less of the image formed by the lens than a larger sensor does. For example, on a 35mm SLR, a 28mm lens is noticeably wide-angle, with a 75-degree angle of view. Put that lens on a D-SLR with a smaller APS-C image sensor, and it provides an angle of view of around 47 degrees, about equivalent to what a 50mm lens provides on a 35mm SLR.

Most of today’s popular D-SLRs have APS-C image sensors, so-called because they’re about the size of an Advanced Photo System Classic-format image frame. (For the record, an actual APS-C film frame measures 25.05x16.7mm; APS-C image sensors are a bit smaller.)

Fujifilm, Nikon, Pentax, Samsung and Sony D-SLRs use sensors measuring around 23.7x15.6mm. These cover the area of a scene that a lens 1.5x longer would cover on a 35mm SLR; a 28mm lens on one of these D-SLRs frames like a 42mm lens on a 35mm camera.

Figure AFigure A: The angle of view of a given lens depends on the image format with which it's used. A lens of a given focal length produces an image of a given size (magnification) at the focal plane when focused at infinity. How much of this image a given camera "sees" depends on the size of the imaging element, be it film or a digital sensor.

Lenses for 35mm cameras produce an image circle 43.27mm in diameter (the diagonal measurement of a 36x24mm 35mm image frame). A 23.7x15.6mm APS-C image sensor sees a smaller image circle than a full 35mm frame sees, hence the reduced field of view. A 4x5-inch film sheet sees much more than a 43.27mm image circle, so the 35mm camera lens wouldn't fill the 4x5 image.


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