Warren_L:
The focal length of a lens on one type of camera body is the same as on another type of camera body, be it 35mm SLR or dSLR. What you see through the viewfinder is of the same magnification, but you will see less of that image with a cropped sensor. Because of this cropped image on cropped sensor dSLRs, it is related to a longer focal length on a 35mm SLR because of it's similar angle of view.
I'm not a fan of the term *cropped* sensor. Cropped is a printing term, and by definition indicates that part of the original image capture, and therefore resolution is lost. Is a 35mm image cropped just because one could have shot the image using medium format, and had more to begin with?
Cropping is when one takes a 3:2 aspect ratio (35mm full bleed), and creates a 5:7 aspect ratio print for example. In that case one losses a good chunk of the original image, and by association a good chunk of resolution as well. In digital one captures an image at full resolution using focal length X, and the resulting image is NOT cropped, it's as captured, and full res.
I like the term focal length multiplier better, but in reality people need to get used to thinking in terms of their camera format rather then in terms of 35mm values. This may never happen, but that thinking is a somewhat limited way of approaching the subject.
So why do people think of focal lengths based on a given arbitrary format just because it's popular? I guess it's kinda like learning Spanish by associating words in English rather than the object they are describing. At some point to goal is to think in Spanish without the internal translation involving another language.
So what is impacted by a focal length multipler? The answer is Field of view, and DOF. The field of view for a 200mm lens on a Nikon Digital sensor is equal to a 300mm lens on a 35mm film camera, however the DOF of the 200mm lens on a Digital camera is greater than the DOF of the 300mm lens on the film camera.
Food for thought!
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