Originally posted by awscreo I thought this setting was only for in body stabilization to know how to behave. I don't think it corrects anything? Also, 15mm vs 14mm in Uwa is very substantial regardless, look up 12mm laowa and notice how much wider it is. The wider you get, the more each mm means, at least that's my understanding of it.
Yes, the setting is for IBIS, and EXIF. You have to let the sensor adjustment mechanism know how much correction to apply. So, for the Samyang/Vivitar design which is somewhere in the 13-14mm focal length range, the settings of 12mm and 15mm available yield incorrect correction. I would rather under-adjust than over-correct. Down at those focal lengths, every mm counts (even more so for FF bodies).
I should mention that I have enough UWA lenses that it is pretty clear to me that uncorrected for distortion Samyang 14mm seems wider than indicated when I compare it to others. (I tend not to correct barrel on UWA, as I find rectilinear projection an unpleasant type of distortion for most of the scenes I shoot.) When corrected to rectilinear, I suspect the lens is 14mm or even a little less wide. However, the proper correction applies to the real focal length, not any adjustments.