Lens fine tuning

Started Feb 19, 2013 | Discussions thread
Ferguson Senior Member • Posts: 1,137
Re: Lens fine tuning

There is still a bit more art than science to this I think, but I'll offer my experience.  I have the 85/1.4G which is also subject to focus shift.  My shift is not enough to bring the focus point out of focus, but is enough to shift the DOF markedly back as you go from 1.4 to about 2.8 (or put another way, if at F1.4 the focus point is 40% of the distance from the front to back of DOF, at F2 or so it is maybe 5%, just barely still in focus, then as you go up to F4 and beyond it becomes more normal primarily because the DOF gets so much longer; numbers made up not measured).

But that said, I fine tune wide open, because that's where it is most critical.  If I were to tune it for F2 to re-center the DOF range, at F1.4 the focus point would actually not be in focus any longer, the much thinner DOF range would not cover it.  If I always shot at F2 or higher I might tune there.

The other thing I've found on the faster lenses is more variation by distance. The 85/1.4G does not have a good happy medium AF fine tune between portrait distance and (say) concert or sport distance, it varies too much (at F1.4), and so I fine tune it more toward portrait distances where I really want F1.4, and shoot sports at F1.8 or F2.   And sometimes I adjust that and try to remember to put it back for portrait distance.

It's all a compromise, and if you work at very fast fstops I see no real choice than to pick a compromise, be very aware of it, and adjust your work accordingly.  E.g. I might focus on the bridge of someone's nose instead of the eye if I am shooting at F2, but focus on the eye if F1.4.

But as wide open is where it's most critical (i.e. thinnest DOF), if you do shoot wide open, my recommendation is tune there.

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