This is the 66th in a series of posts on the Fujifilm GFX-50S. The series starts here.
In this post, I reported on Fuji GFX 50S systematic autofocus errors at certain f-stops (mostly one, actually) using the 23 mm f/4 lens. I have seen similar behavior with the 63 and 120 macro. When I made the photographs for yesterday’s visual comarison of the 110 on the GFX and teh Otus 85 on the Sony a7RII, I shot some AF pix with the GFX just to see what would happen. In all cases, at least one one of my manually focussed images was sharper. At f/2.8, the manually focused ones were a lot sharper.
Thus, it was with some trepidation that I set up my controlled af test. I used the following low-contrast target:
I have marked the region of interest that I used in red. Here’s the procedure:
- ISO 100
- Spot and Zone both tested (this is new; I just tested spot before)
- Spot set on the ROI, which included a bit of the zone plate.
- Zone set the same plate which included all the zone plate and some or all of several squares.
- Release priority: focus
- Pre-trigger: 500 milliseconds, which was long enough for focus confirmation green square to flash and confirmation beep to sound in all cases
- Manual exposure
- Wescott LED panels set to 5500 K.
- Electronic shutter
- Cognisys computer-driven focusing rail
- 32 exposures 4 mm apart
- Target distance at the center of the rial, 3 m.
- RAF converted to DNG in Adobe DNG Converter
- DNG exported as TIFF mosaiced file in dcraw (document mode)
- TIFFs cropped and raw channels selected in Matlab program
- MTF50 of cropped TIFFs measured with MTF Mapper
- Data assembled in Matlab
- Data plotted in Excel
The spot mode data for the red and blue raw channels, and the Gr one:
What you are looking at is a standard measure of sharpness, MTF50, as measured in cycles per picture height (cy/ph) at the whole stops from f/2 through f/11. The orange line is the average, or mean, of the 32 exposures at each aperture. The gray line is that average, plus the standard deviation for the 32 exposures, and the blue line is the average minus that standard deviation. If the data were Gaussian (which it isn’t) two thirds of the values would be between the gray and the blue lines.
For comparison, here’s a run of 100 images 1.6 cm apart with the focus fixed using MF at f/2.8, the sharpest aperture for this lens, and the one that seems to give the AF algorithm the most trouble:
You can see that peak sharpness is about 1400 cy/ph. This is much lower than we get with the high contrast target, but we’re just interested in relative sharpness here. The average sharpness using AF at f/2.8 is less than that, but not by much. Focusing at f/2.8 is not very consistent, though. AF is pretty consistent at the other apertures. This is the best AF performance I’ve seen in hte four lenses tested.
Now let’s look at the results in zone Ad MODE:
At f/2.8, the AF performance is much more consistent, and the results are very close to the best you could do focusing manually.
This means that I’m going to have to go back and run the other three lenses through the same tests in zone mode.
Note that the blue channel, which was weak at f/2 in the manual focusing tests using the razor blade target, is fine here. I’m not sure what that’s about.