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.
I’ve been testing the lenses for focus shift, and I was unable to get representative captures for the entire aperture sequence for the Batis and the Summicron using the 200 mm rail and a 3.3 m target distance. But now I know that the Batis focus distance moves closer as you stop the lens down and the Summicron focus distance moves further away as you stop the lens down, so I have a better chance of getting good data. So I repeated the testing.
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 a little toward s the back from mid-rail for the Batis and a little toward s the front from mid-rail for the Summicron using manual focusing, and exposed at 49-shot series from f/1.8 through f/8 for the Batis and f/2 through f/8 for the ‘cron. 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.
As the lens is stopped down, the focus point moves closer to it. Peak sharpness is achieved at f.2,8 through f/4. The total focus shift from f/1.8 to f/8 is about 110 mm, or a bit over four inches in 10 feet. Focusing wide open is definitely not the plan here.
You can read off the depth of field in mm if you know what degradation of the peak MTF50 you’re willing to tolerate. Consider the Batis wide open. If MTF50 = 1000 cy/ph is your criterion, at 3.3 meters your DoF is 56mm. Stated in English units, at 10 feet your DoF is 2 inches. Not a lot.
As the lens is stopped down, the focus point moves away from it. Peak sharpness is achieved at f/4. The total focus shift from f/2 to f/8 is about 80mm, or a bit over three inches in 10 feet. Focusing wide open is not the plan here.
Note that, when mounted on a rangefinder camera with no live view, like an M8, M9, or one of the film M-series Leicas, that there is no way to set the rangefinder to deliver maximum sharpness over the range of apertures from wide open to f/8. A compromise might be to set up the rangefinder for best performance in the middle, at f/4, but that would mean that then lens would be delivering an MTF50 of around 600 when rangefinder focused wide open, instead over over 1000, which could be achieved if the lens were indeed focused on the subject at f/2.
If you’re interested in why focus shift happens, you might want to take a look at this post on spherical aberration.