Let's not confuse the focal length definition of the lens itself with the apparent effect that a given sensor has when it's inserted in the cone of light projected by that lens. By physical design and definition, a 50mm lens is a 50mm lens no matter what use it's put to.
An APS-C sensor simply takes a smaller "bite" out of the projected cone of transmitted light and we associate the resulting image with
what we have grown to expect from a longer FL lens in the ol' familiar 135mm world view. A medium format sensor inserted in the same cone would appear to produce a wide angle effect (if it weren't clipped) simply because it would take a bigger/wider "bite". Early 35mm-centric advertising added to the confusion. Fortunately the medium format folks didn't join the media party too.
We (most of us here anyway) just happen to be trapped in a world that grew accustomed to the results we see when the FF 135mm format is placed in the image cone projected by any lens and we use that familiar impression as a
de facto standard for comparison. If you'd "grown up" in the medium format world you might have the same translation problem but the interpretation would be quite different. Just be glad you don't have a Minox 16mm in the mix too.
Try this:
ignore the FL of your lenses and assign them categorical descriptions relative to the results they provide
as you use them and regardless of the body-type -- UWA, WA, "Normal", medium tele, long-tele, macro, portrait, etc.
This is especially interesting if you use a given lens on more than one format. I sometimes use a Mamiya M645 150/3.5 with an extension tube on the DSLR's -- same lens but with quite different results and I feel no need to get tangled up translating FL's between 645, 135 and APS-C to understand the expected outcome.
For many years I thought of an 85/1.8 as "normal" and a 50/1.4 as "sort'a wide" on my Spotmatics and never gave a thought to the actual FL. To me, the lenses were task oriented irrespective of their FL. The same thought process worked equally well for the 2 1/2 years I carried a half-frame Oly Pen-F system even though the relative FL's were totally different -- just like APS-C but a quarter century earlier! Become "multi-lingual".
H2