Mr. Bill wrote in post #13290472
If one has gone FF, what is the best replacement for EF-S 17-55
I really would like to get something that has IS yet still have the wide angle + 2.8 or better.
I currently have a 17-40L and keep finding that it just doesn't cut it in low light situations, even if when bumping up the iso.
Actually, the nearest equivalent lens to the EF-S 17-55 on a FF camera is the EF 24-105/4L.
Here is why a lot of people don't quite grasp this:
1) 24-105 on a FF sensor gives the same FOV range as a 15-65 would on 1.6X, so the 24-105L on FF is actually both wider and longer.
2) f/4 on a FF sensor gives the same DOF (at equal framing) as f/2.5 does on 1.6X. So the 24-105L is actually capable of very slightly shallower DOF (but they are nearly equal).
3) The FF sensor has better high ISO performance by at least one stop for equal technology, so the f/4 of the 24-105 is at worst equal for light gathering capability.
So there you are. Incidentally, the 24-70 is capable of much shallower DOF and light gathering on FF than the 17-55 on 1.6X, but it does lack IS. For moving subjects, the 24-70 on FF is hands down better.