**Clarification added at the bottom of the post.
We all know that a 200mm lens is always a 200mm lens whether it is installed on an Full Frame DSLR or on an APS-C DSLR (you can replace the 200mm with any focal length, it all works the same). It is the crop factor of the APS-C sensors that changes as we are losing part of the image projected from the image circle. The angle of view changes, but never the focal length. As an example, shooting with a 200mm lens using the K20D , one would have an image similar, (in angle of view), to a 300mm lens. Right? Yes indeed, but the focal length would still be 200mm.
Some are concerned that since part of the image is actually lost, the cropping diminishes the quality of the picture. It doesn't. If you were to crop an image, taken with a full frame DSLR with, say, a 14.2MP sensor, to the size of an APS-C sensor, yes there would be a loss of details. You would wind up with an image re-sized to about 9.5MP. If you take a picture with the K20D equipped with an APS-C sensor, the crop actually happens outside the image circle of the sensor and the image remains that of a 14.2MP sensor. So isn't it a benefit to be able to extrapolate 300mm out of a 200mm lens? Oh yes, the next argument I usually hear is that wide angle lenses are also affected the same way and a 20mm wide angle lens would act like a 30mm lens. That's true, but since the maximum aperture of the lens installed remains the same when used on an APS-C sensor, don't you think that it's a positive thing? You can purchase a super wide angle lens for $300.00 to $600.00, but try to purchase a 300mm f/2.8 lens for under $4,000.00. You can purchase the Pentax DA* 200mm f/2.8 for around $800.00 and on your Pentax DSLR, it's like having a $4,000.00 300mm f/2.8 lens on a Full Frame DSLR.
Theoretically, there are other things to consider. The bokeh of a 300mm lens is different than that of a 200mm lens. It is usually smoother (but not necessarily as the lens construction and the number of Diaphragm Blades also make a difference). The compression effect of a 300mm is greater than that of a 200mm. In other words, a 300mm makes objects appear closer to each other that in reality, more so than a 200mm and certainly a lot more so than a 50mm lens or wide angle lens. I wanted to test the above and find out for myself what were the real tangible differences. I am lucky enough to have the DA* 200mm and the DA* 300mm lenses. I don't have a Pentax FF DSLR (nobody does thus far), but I figured that if I were to take some photos with both lenses using my K20D, then adjust the results so that the final images were the same sizes with the same framing, I would see what the differences were. Here are my results.
Difference in Bokeh! The pictures above show an image taken with the DA* 200mm @ f/4 (top photo) and with the DA* 300mm @ f/4 (above photo). The images were re-sized to show approximately the same image framing. There is a small difference between the bokeh. The 300mm produced a little creamier bokeh. Is the difference enough to prove a point? You decide.
Difference in compression! Above, I took roughly the same picture using the DA* 200mm and the DA* 300mm both set @ f/4. The darker portions of the images represent about the same framing. The two images below show the results taken with the 200mm and the 300mm when matched to the same framing sizes. I cannot see a big difference between the two. Can you?
I ask you, did I misunderstood something or did I misunderstood everything?
** See this article about DEPTH-OF-FIELD AND FOCUS, FOCAL-LENGTH AND PERSPECTIVE, OBJECT ISOLATION, SEPERATION, COMPRESSION and LAYERS. Pay particular attention to the Perspective and Compression.
** Looking at the comments and emails received, it seems that I have confused a lot of people with this post. Maybe my "Franglish" is getting in the way. My point is simply that the pictures taken with a 200mm lens used on an APS-C sensor will be similar to the pictures taken witth a 300mm used on a full frame DSLR. If there are differences, I don't think that they are noticeable enough to bother with. The K20D with the 14.6MP CMOS sensor gives me better results than I had using "Full Frame" 35mm films. Agreeing that the picture quality of the K20D meets your requirements, one is better off with an APS-C sensor when it comes to costs. The advantage of gaining on the telephoto side outweights the benefits of gaining on the wide angle side with a FF camera. A 35mm format wide angle lens (14mm @ f/2.8 or 21mm f/2.8 when used on APS-C) ~$300 to $600 - 35mm format telephoto (200mm @ f/2.8 or 300mm still at f/2.8 when used on APS-C) ~ $1400 to $5000. It gets even better when using a 35mm format telephoto of 300mm f/2.8 on an APS-C DSLR.