Posted as part of
another post in another thread. Fit here also, I think.
Another example without LCC.
K-1,
(FF) Square Mode, DA15 Limited w/o hood (retracted), f/16, crop in post 4700 x 4700 px. In APS-C the square crop yields to only 3200 x 3200 px and shows significant lesser wide angle.
You can't get this image on APS-C, since the sensor size isn't sufficient. If you want a square image and crop the APS-C area accordingly, you get much lesser angle and so a different image with a lot lesser pixels! In FF mode the weakest areas are cropped away. APS-C standard mode even shows worse corners and borders on the short sides!
I got some pretty nice square images using the DA15 on the K-1 in Square Mode and a slight crop in post. If you own the lens and the K-1 I'd suggest try it yourself. You may get some nice images you otherwise wouldn't. Isn't that what we photographers are after? - Let's not only be gear heads who may again and again discuss if a lens is category APS-C or FF.
What you can use a lens for mainly depends on its image circle. The sensors aspect ratio of 3:2 by far isn't the only rectangle that you can build positioning corner points on the circles rim. In fact the APS-C sensor size restricts in a way that from the lenses' perspective isn't appropriate! So let's use FF mode if it's needed and crop in post.