|
FACTOR THIS: What really happens when you place a 35mm lens on a digital SLR? That depends on the size of the CCD or CMOS sensor in the camera. If it’s a full-frame sensor, you get the same field of view as 35mm film (full frame, A, from Canon EOS-1Ds Mark II). A DSLR with a smaller sensor than film, such as the Nikon D70, captures a narrower field of view equivalent to 1.5X the focal length (red box, B) while maintaining the same 2:3 ratio, perspective, and depth of field. That’s actually in-camera cropping and not focal-length magnification. An even smaller sensor, such as the Four Thirds standard CCD found on the Olympus Evolt, captures a field of view equivalent to 2X the focal length, but changes the size ratio to 3:4 (white box, C).
If digital SLRs were the only game in town, choosing a lens would be easy. You’d simply pick one based on how wide or how tele it acted and be done with it. But if you already own lenses for a 35mm SLR, you might develop a love/hate relationship with them when you switch to a DSLR. On the one hand, you’ll love how your telephoto lenses become superteles. On the other hand, you’ll hate how true wide-angle lenses turn into semiwide or even normal lenses, forcing you to buy a new super- or ultrawide-angle lens to get a real wide-angle field of view. Some photographers think it’s a conspiracy on the part of DSLR manufacturers to sell more lenses.
This lens conundrum raises several more questions: Do DSLRs somehow magnify the focal length of existing 35mm lenses? Why do DSLRs have 35mm lens factors? And are there ways to compensate for the ways that 35mm lenses behave (or misbehave) on a DSLR?
The first question is the easiest to answer. No, digital SLRs do not change or magnify the focal length of 35mm lenses. However, most DSLRs capture a narrower field of view (FOV) than a 35mm SLR. The reason and the extent to which this occurs are based on the size of the CCD or CMOS sensor used in a particular DSLR. (Lens focal lengths, on the other hand, are determined without consideration for the size of the film or sensor in a camera.) This form of in-camera cropping was readily apparent in the masked viewfinders of early-model DSLRs (adapted from 35mm SLR bodies) and can still be seen in the Sigma SD10 DSLR.
|
