Probably the most frequent argument I’ve seen over the years in photo discussion groups is about what a full-frame lens “becomes” when used on crop sensor, in terms of reach and depth of field. Nothing “becomes” anything. However, the ratio of linear depth of field to frame size increases.
Except when a lens is designed to perform otherwise, as with the 23mm Summicron-TL. Look at the DOF chart on the data sheet for the lens and you’ll surmise that wide open it isolates about the same as a 35mm lens at f/3.2 would on full frame. But according to what I read in an interview with one of its designers, it was made to perform isolation-wise exactly as a 35mm f/2 lens would on full frame. Images bear that out.
The 35mm Summilux-TL wasn’t mentioned in that interview. And to my eyes, it appears to isolate pretty much the same as a 50mm Summicron would do on full frame. But not a ‘lux. It makes me a bit wistful that I passed up a chance at a bargain-basement price on a 50mm Summilux-SL in favor of the APS-C lens, chosen for being about 40% the weight of the SL lens.
Funny thing is, without birding glass to lug around everywhere, some of these “heavy” short lenses are looking a bit lighter . . .