Originally posted by mlag I prefer on aps-c the 28-80mm
Which lens is that one?
It is true, by using a FF lens on APS-C the outer area of the lens is eliminated, which with most lenses will be where it yields its least quality performance. So you get that lens at its best potential by eliminating this area. But by doing this, a whole dimension of the wider FOV is also eliminated. I too have a 17-50mm f/2.8- by Sigma. For more reach I pair it with my DA* 50-135mm f/2.8 which is an excellent pairing. I use these extra high-quality lenses mainly when I need the greater aperture range, and/or their top-flight IQ. Otherwise I'll take a smaller but fine-performing all-around APS-C lens.
A lens having both a WA FOV and a TELE range as well is more difficult to design, therefore generally comes with more compromises in one way or another, yet some still provide fine performance. The idea behind such a lens is for versatility with a single lens. By eliminating the WA part of it, that half of its usefulness and versatility vanishes. Better results and far more versatility could be had from the DA HD 16-85mm WR or the DA 18-135mm WR lenses on APS-C.
As far as my use of FF lenses for APS-C, in my case I much prefer those in the tele range, as here there is the APS-C advantage of that lens's best quality, and the enhancement of effective tele in image size. The 50mm "normal" lens is a good case in point, as for me it is more useful on APS-C as a moderate tele portrait style lens than it is on A FF body. During my first 6 years of shooting back in the day, I only had a 50mm lens. I rather got tired of this FL, and was relieved and excited when I finally got a 28mm WA and a 135mm tele. Having all of these broadened my usage considerably. Then I got hold of a 40mm f/2.8 lens, and found I preferred its FOV over the 50mm lens for a "normal" perspective lens, as it let more into the frame, yet without an actual WA effect.. Years later, I also bought the FA 43mm f/1.9 Ltd. and loved it. Then the FA 35mm f/2 and loved it as well. These became my most-used versatile fast primes for one-lens compact carrying, without having to carry a larger zoom lens for general use.
Then came the DSLR cameras. APS-C, of course. I did not find my highly-valued general-use wonderful prime lenses to have the kind of versatility and appeal, as they'd had for years with my film cameras. My great Tokina 28-70mm f/2.6-2.8 zoom lens likewise had lost its versatility with excellence. So these lenses wound up sitting, as I found myself acquiring new DA lenses for my DSLR cameras, as these were far more useful for once again operating in the same FOV range. However, though these DA lenses have been of very good quality, and produce fine imaging, they are often not quite the equal, as in not as much aperture or of equal quality in the same FOV. My fine FA 28mm f/2.8 has the same FOV on APS-C as the FA 43mm Ltd has on a film body, so it can be dual-purpose in that sense, and I can even use my FA 43mm hood on it, but it does not have f/1.9 aperture. There is no 24mm f/2 that can function on APS-C with such excellent quality as the FA 35mm f/2 on a FF body. Not to mention an equivalent for the FA 31mm f/1.8 Ltd.
I have an old Sigma EX 24-60mm f/2.8 DG lens I bought as new closeout for a great price for film use, since the price was too good to pass up, and it offered more WA in a fast zoom lens than my Tokina 28-70mm, and it had received very good lab test reviews. If needing more tele, it would be the Tokina, if more WA, it would be the Sigma. This is one I've found to be still versatile and useful for APS-C as well, because the 24mm end will still yield some WA FOV. It is therefore a good dual-purpose, versatile lens for both APS-C and FF use.
I am very glad to be able to take full advantage of these fine lenses in their original FOV once again. Then there is the matter of upgrading image quality even more. Even if one could obtain an APS-C lens with the same FOV, it will not be able to deliver the same resolution at 24 or 26mp as the K-1 II does at 36mp. That is enough of a jump where the difference can be visible.
In the case of MSL, this difference would be even greater, when comparing the K-5, which still has the AA filter- and is at 16mp, then getting the K-1 II, which no longer uses the AA filter- and will be producing images at 36mp!
When it comes to going more into the tele range, the advantage shifts to APS-C, because in going more and more tele, with a FF equivalent setup, the lenses become more and more monstrous, and more and more expensive!.