Originally posted by mediaslinky I know that the small image sensor on the dslr gives a simulated "built-in-zoom" effect because ot crops the edges of light let in by 35mm lenses. So, if you shoot 50mm lens on a dslr you "see" a 75mm image, that is it's like you zoomed into 75mm instead of the 50mm the lens actually was.
But, with my DA 18-55... isn't it designed for my sensor? Will every DA lens be designed for my smaller sensor? If I see a DA 200mm, it will "see" 200mm on my camera, right?
slinky
The focal length of the lens does not change with camera format. But the field of view changes with a given focal length depending on the size of the film frame or imaging sensor. The sensor in the Pentax digital SLR is about 18x24mm, or roughtly 2/3 the dimensions of the 35mm film frame (roughly 24x36mm). So you see 2/3 of the field of view. A 6x7 medium format camera (like the Pentax 67) has a film area that is roughly double the dimensions of the 35mm film frame (at 60x70mm).
So, a 200mm lens gives an approximate (diagonal) field of view of about 8 degrees on the digital, 12 degrees on the 35mm film camera, and 24 degrees on the 6x7 film camera. The lens is still a 200mm lens, whether it is a DA lens with a small image circle, a FA lens with a bigger image circle, or a 6x7 lens with a larger image circle. A DA 200mm, an FA 200mm, and a Pentax 67 200mm lens (with adapter) all give exactly the same field of view on the digital SLR.
The "35mm equivalence" is just a point of reference for photographers who are used to 35mm film cameras and associate a particular focal length with the associated field of view. To get the 12 degree field of view that comes from a 200mm lens on a 35mm film camera, you need a 130mm lens for the digital SLR, and a 400mm len for the 6x7 film camera. That is the only point of the "35mm equivalens." Unless you have a lot of 35mm experience, it is a meaningless reference.