AF fine tune and focus shift for fast lenses

Started Sep 12, 2016 | Discussions thread
Ferguson Senior Member • Posts: 1,826
Re: AF fine tune and focus shift for fast lenses

Nikon DSLR's, when using phase detection focus (i.e. through the viewfinder focus) focus at the widest aperture of the lens.  I'm not sure of the context of not working wider than F2.8.  I know the viewfinder screens do not give an accurate DOF preview wider than about F2.8, and I suspect the AF system is designed to target F2.8 as a sweet spot, but that's guessing.

But the lens is wide open, so rather by definition it is focusing wide open.

So if you fine tune at the widest aperture it will be affected by focus shift.  Since the DOF gets deeper as you go up in f stop though, this is usually the best idea -- you have the Fine Tune nailed when you need it most.  Of you don't shoot wide open ever, though, just back/front it a bit so it lands where you want it.

My 85/1.4 shifts backwards, for example.  At about F2.8 the DOF range, centered at F1.4, is back so the very front of the DOF range is on the prior focus target.  So I have to decide if I want to shift a bit forward to compensate, and if so, I tend to get noses not eyes in focus at 1.4.  That lens is problematic in so many ways -- it also has a very different (visibly different) fine tune at portrait distances than sports (say basketball) distances.  But it's a delightful lens if you get the focus right, so I put up with the idiosyncrasies.

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Sony a7R IV Sony a9 II Sony FE 24-70mm F2.8 GM Sony FE 70-200mm F2.8 GM OSS Sony FE 85mm F1.4 GM +2 more
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