Quote:
Originally Posted by Matthias099
The biggest difference that I am aware of is that DSLR sensors are smaller than a 35mm film frame. That results in greater focal length. For example a 100mm lens on a 35mm SLR has a focal length of ~140mm on a DSLR (that's for my Nikon D80, but others are similar I am sure).
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This isn't quite correct; the focal length is an inherent property of the lens and doesn't change when you plug it into different cameras.
However, one of the most noticeable effects of changing focal length is field of view — the amount of a scene which fills the frame. Field of view changes linearly as you change focal length, so if you double focal length, you something that was only half of the picture now fills it up. Zoom lenses, of course, do exactly that.
But the other way to fill up a frame with something half the size is to simply have a smaller frame — and for size and production cost reasons, most DSLRs do. In the case of Pentax dSLRs (and most Nikons and Sonys too) it's 1.5× smaller. So the effect on field of view is the same as increasing the focal length. (But everything else remains the same.)
This is important in the context of the original poster's question. Your original 35-80mm lens will be "shifted" towards the telephoto side and will have the field of view of a 52.5-120mm lens on a film camera. But if you buy the new designed-for-digital 50-135mm DA★, the same thing will apply, and you'll get framing like what you'd be accustomed to from a 75-202mm lens on a film camera.
There are other differences too — digital sensors are more reflective than film, so lens coatings are designed to better take that reflected light into account.I think there are other coating-related differences,too.
And, while film doesn't care what angle light comes from, digital sensors really need it to be straight on, so new lenses are designed with that in mind. (This is more of a problem near the edge of the image, so the crop factor helps with older lenses here.) The K20D's new sensor is supposed to have features that make this less of problem.