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Depth of field: The small-sensor difference

Smaller image sensors change lens behavior on an SLR. Here's a look at one consequence, depth of field.

Having struggled on many occasions to explain the difference between cameras sensors the size of a full frame of 35mm film and the vastly more common smaller sensors, I thought I'd point readers toward this lucid explanation of one aspect by Canon tech guru Chuck Westfall in his October column with the Digital Journalist. One reason you might want to pay attention: the vast majority of digital SLRs use smaller sensors than film SLRs, though Canon and Nikon have some high-end exceptions.

Westfall details in his article how depth of field changes in relation to sensor size. I'm drawing attention to the issue as an excuse to explain why a common practice in the camera business raises my hackles.

An illustration of the smaller field of view common to digital SLRs. CNET

But first, the background. Depth of field describes how much of an image will be in focus--for example, if you're focusing on a subject's eyes, will the tip of the nose and the back of the ears also be sharp? Sometimes photographers want a deep depth of field, as in landscapes with an interesting foreground and background, but sometimes not, as in portraits where a shallow depth of field makes the background recede into a distraction-free blur.

Depth of field is a function of a lens' focal length and aperture settings and of the distance of the subject from the lens, Westfall explains. That's old hat to those familiar with photography. But the new era of smaller sensors, such the Canon's APS-C size and Nikon's DX, has added a new wrinkle because lenses behave differently with smaller sensors.

The smaller sensors crop off the outer portion of an image that would be recorded on a larger full-frame sensor. Suppose you and your pal both want to photograph a car with your SLRs equipped with 50mm lenses, but you have a small-frame camera and your pal has a full-frame camera. Your friend will be able to fit the whole car in the frame at a certain distance, but you'll have to step farther back to get a wide-enough field of view.

Because of that behavior difference, it's common to describe lenses on smaller-frame cameras in terms of their equivalent performance on a full-frame camera--a "50mm-equivalent" lens is a 30mm lens on a Canon Rebel XTi, a 33mm lens on Nikon D40x and a 25mm lens on an Olympus E-3. So Canon's small-frame SLRs have a 1.6x focal length equivalence, Nikon is 1.5x and Olympus is 2.0x.

But here's what gets my goat about the "equivalent" term: there's more to the equation than just field of view.

Increasing your distance from the car, or using a zoom lens to select a wider-angle focal length so you don't have to step back, also increases the depth of field. Other things change when you step back, too, including what exactly is visible in the background and the magnitude of the foreshortening effect that makes backgrounds appear closer when using telephoto lenses.

A more extreme illustration of the depth-of-field issue comes with compact cameras or mobile phones. With their even smaller image sensors, it's easy to get plenty of depth of field but hard to blur out distracting backgrounds when desired.

The camera industry has faced changes like this before, during moves from large-format to medium-format to 35mm film. There's no new standard size for smaller sensors in SLRs, much less swarms of compact models and cell phones, so 35mm is likely to live on as a reference point even if it fades as a format from most of the market.

I don't see an easy solution to this lens nomenclature problem, though.

Some advocate using the term "field-of-view crop factor" rather than focal length equivalence factor (it's the same actual number, happily). That term is more accurate, but people are accustomed to describing lenses in terms of focal length, not field of view, so I'm not optimistic.

I suspect we're mostly stuck with focal-length equivalence, which is workable if not perfect. But do me a favor and realize that it's just not fully equivalent.