Originally posted by Col Flanders THAT was NOT my question!
I realize that a 90mm lens designed for a 6x7 camera "appears" the same as a 90mm lens designed for a DSLR.. I am asking about mounting a 90mm lens designed for my Pentax 6x7 mounted on a DSLR with an APS-sized CCD image sensor (such as the K10D.)
I truely appreciate everyone's help and patience...
Let me try asking this way...
If I put a 50mm lens designed for a 35mm camera on a DSLR with an APS-sized CCD image sensor, I no longer have as wide a field of vision. effectively, the 50mm lens has become a 75mm lens.
Correct?
So, what happens when I mount a 6x7 90mm lens on a K10D using the S-mount adapter, of course.
90mm is 90mm is 90mm. The lens focal length does not change, regardless of what kind of camera it was designed for. A 90mm lens designed for 6x7 will give the exact same magnification and results as a 90mm lens designed for a DSLR, and the exact same results as a 90mm designed for 35mm, with only the following differences:
Due to the fact that the 6x7 lens is designed to cover a very large image circle, it's pretty much guaranteed that there will be no vignetting whatsoever when used on a DSLR. (If there is, it would have been an utterly horrendous lens when used on a 6x7 camera).
Since lenses tend to be sharper at the centers of their image circle than at the edges, the 90mm 6x7 lens will have the advantage of much more consistent sharpness throughout the entire coverage of an APS-C sensor.
In short, you will see absolutely no differences in terms of field of view between lenses of focal length X, regardless of what kind of camera they were designed for. You may see differences in terms of vignetting and quality since you're shooting through a lens that was designed to provide good quality over a much larger image circle.
Of course, for the same reason, a 90mm 6x7 lens is going to be MUCH larger and heavier than a 90mm lens designed specifically for a DSLR.
Also note that while APS-C DSLRs require far less image circle coverage, because the sensor of all DSLRs is much more reflective than film, DSLRs have more stringent requirements than film cameras in terms of internal reflections within the lens.
For many higher-end lenses (such as Pentax SMC lenses), this doesn't matter much since Pentax SMC is about as good as you'll get in terms of reducing reflections from optical elements, and I believe most of the SMC Pentax lenses also take care to ensure that no other element of the lens is too reflective. Lenses from other manufacturers may not perform as well though. This is why you now see lenses that provide full frame image circles that are marked as "Optimized for Digital" or "Digital Integrated" - extra care has been taken with such lenses to reduce internal reflections.
Examples of such full-frame-with-digital-taken-into account are the Sigma DG series and the Tamron Di (not Di II) series.
An example of a lens that provides acceptable results on film but horrendous contrast on a DSLR due to too many internal reflections is the first generation Sigma 28-200 3.8-5.6. It served me well on my old PZ-70 but was unusable on my K10D.
Chances are that most if not all 6x7 lenses will have good multicoatings though.