This is a continuation of a test of the following lenses on the Sony a7RII:
- Zeiss 85mm f/1.8 Batis.
- Zeiss 85mm f/1.4 Otus.
- Leica 90mm f/2 Apo Summicron-M ASPH.
- AF-S Nikkor 85mm f/1.4 G.
- Sony 90mm f/2.8 FE Macro.
The test starts here.
Yesterday I looked at on-axis focus shift with the Sony 90mm f/2.8 G macro lens and the Zeiss Batis 85mm f/1.8, both on a Sony a7RII, and found a lot of it. Both of these lenses were designed for mirrorless cameras, where focusing is often done at the taking aperture and thus focus shift isn’t as much of a problem as with lenses designed for SLRs, where the focusing is usually done wide open and thus focus shift is more of a problem.
I’m looking at two SLR lenses today, the Otus and Nikkor 85s. Will we see as much focus shift?
I mounted a Sony a7RII to the Cognisys computer-driven focusing rail. I set the controller up to use 192mm of travel and make 49 exposures 4mm apart. I set the assembly 3.3 meters from the target, mounted the lens, focused mid-rail (actually a little toward s the back from mid-rail) and exposed at 49-shot series from 1/1.4 through f/8. For the Otus, I went all the way to f/11. I developed the sets in DCRAW using AHD, and had it convert the files to 16-bit sRGB TIFFs. I told Imatest to calculate the on-axis MTF50 for luminance for a horizontal edge, and plotted the results in cycles per picture height vs distance from the point furthest away from the target.
There is plenty of focus shift; so much that I missed the best focus for the f/8 and f/11 series. The focal point moves away from the subject as the lens is stopped down. We can now see that f/2.8 is the best aperture of on-axis sharpness, which we couldn’t see before I started making these measurements with the motorized rail. To be fair, f/2, 2.8 and 4 are all essentially the same sharpness. It is clear that focusing this lens wide open will not yield optimal results, even at f/8.
The Nikon is not as sharp. Focus initially moves closer to the subject as the lens is stopped down, then turns around and goes the other way at the transition from f/2.8 to f/4. If you’re going to be shooting at f/5.6 or f/8, it’s probably OK to focus wide open. The Nikon curves are more asymmetrical than the Zeiss ones; I don’t know what that means.
I am surprised by how much focus shift there is with these two lenses. It’s one thing to see it in mirrorless lenses, but entirely something else to see this much shift in SLR lenses.
I am now using the entire length of my focusing rail and still don’t have enough travel to make the curves reliably. I would show you the curves for the Summicron, but the focus shift went away from the f/2 setting as I stopped down, and I missed most of the peaks. The only thing I can think to do about that in the short run is to get closer to the target. However, I think that I’ll find that the inkjet-printed slanted edge target I’m using is not sharp enough for some of these lenses if I do that.
What can be said from results at 10 feet or a little closer about focus shift at other subject distances? At this point, I don’t know.