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 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.
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.
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.
Figure 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.
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.