Originally posted by ATLphotog Thanks a lot - that was a big help. And the article you referenced above ("The Crop Factor Unmasked - Field of View vs Focal Length on various formats") really helped illuminate the whole concept of full frame vs. APS-C (I wish the latter had a better name - I know I'm going to call it the wrong thing half the time, with the C before the P or S after the C, etc
) and how the lenses interact with each.
The technical explanations, regarding focal length being the same but the different sensor size results in a cropped image for the smaller APS-C, etc. while true, makes the practical application more complicated to predict the resulting image difference than necessary. As has been said, understanding it from the field of view explanation is quite easy. The larger FF sensor will present a larger FOV compared with the smaller APS-C sensor. The concept of the APS sized format actually started with film cameras, that is a film size of 16x24mm instead of the 24x36mm of "35"mm film. Those smaller-format cameras did not do well in the marketplace, eventually failing altogether. The case with DSLRs has been very different. The smaller format has been very successful. The DSLR sensors are likewise 24x36mm for FF and 16x24mm for APS-C.
The difference in FOV with the same lens can be easily calculated for the end result of the image you will get, which is approximately 1.5x (or with Pentax more precisely 1.53x) for APS-C over FF. For example, if using the 1.5x factor as your guide, a 50mm lens on an APS-C Pentax body will produce images that are of similar-appearing size in the frame as an image would be if coming from a 75mm lens on a FF body if shot at the same distance. (using a precise for Pentax 1.53 factor, it would be a 76.5mm lens similarity) The FOV is narrower with the smaller sensor. There are smaller sensors yet on other styles of digital cameras, where this factor is much greater. This allows smaller lenses and smaller cameras to produce more tele-like images using more compact ensembles.
But in taking that principle too far, the disadvantages began to become very evident in terms of higher noise with ISO increase, much greater difficulty in achieving a wide angle FOV, and DOF issues. The APS-C size seems to have arrived as the best compromise with the least disadvantage, as evidenced in the latest camera designs.
So simply using the 1.5x factor will quickly give you a good prediction of what a lens of any FL will look like when viewing scenes through the VF on a camera of APS-C sensor size, vs. that of a FF body. The DA* 200mm f/2.8 lens on the APS-C body will appear similar to having a 300mm f/2.8 on a FF body, at a fraction of the weight, size, and cost. The DA* 50-135mm f/2.8 will again present similarly on APS-C bodies which it is designed for, to having a 70-200mm f/2.8 lens on a FF body, again at a fraction of the weight, size, and cost.