Originally Posted by zuiko85
How would any lens with noticeable focus shift be useable on a RF? Perhaps if the amount of focus shift was so small that it was less than the DoF increase from stopping down?
Seems that lens would work best on a TTL type viewfinder camera.
With a lens like the Zeiss 50/1.5 Sonnar, as james.liam above mentioned, the manufacturer traditionally calibrated the optimum RF focusing a couple stops down from wide open. This meant that at f/2 the lens slightly front focused, but might still be reasonably within DOF (and on film, maybe not so noticeable), while at f/4 it would be slightly back focused, but the increased DOF at f/4 would hopefully cover much of that error. Shooting wide open would have the greatest focus error, but the lens was already pretty glowy and the SA would mask some of this. Again, on film it was likely less noticeable. My understanding is now the ZM50/1.5 is RF calibrated for wide open use, given this is now the popular way to use the lens on digital. To compensate for focus shift, one would have to intentionally guesstimate intentional front focus when shooting at f/2, 2.8 and f/4 before DOF masks the shift.
I also have the 7A 50/1.1 and fine-tuned the RF cam for correct RF focus with my camera at f/1.4, the point at which it sharpens up a bit yet still has nice SA glow. TBH, I have not checked infinity. I primarily use the lens at nearer distances as a character lens. It takes stopping down to f/11-16 to really bring up the edges for more technical applications. Instead I rather use a more capable design for such applications (i.e. ZM50/2 or 50 Lux ASPH).