This is the 64th in a series of posts on the Fujifilm GFX-50S. The series starts here.
Yesterday I reported on longitudinal chromatic aberration (LoCA) and focus shift with the Fuji 23 mm f/4 lens on the GFX 50S. I found the focus shift to be minimal, and the LoCA about average for a high-quality lens. Now I’m going to do a test of autofocus accuracy, using the same target as yesterday, but from a much greater distance, 2 meters. This is as far away as I cant get from this particular target and still have enough pixels to so the sharpness analysis accurately. Of course, I could print a bigger target and get further away, but I think 2 meters is far enough to get the measure of the lens in its comfort zone.
I used the bottom of the central slanted square, and the AF area covered that and the zone plate to the left. The details:
- ISO 100
- Release priority: focus
- Manual exposure
- Wescott LED panels set to 5500 K.
- Cognisys computer-driven focusing rail
- 51 exposures 1.6 mm apart
- Target distance at the center of the rial, 2 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 results, for each raw channel:
The vertical axis is MTF50, measured in cycles per picture height (cy/ph). The horizontal axis is camera position shift in cm. The points on the left side of the graph are with the camera farther away from the subject than the points on the right.
These results are far more consistent than I saw with the Fuji 63 mm f/2.8 lens. They are more consistent than I saw with the Fuji 120 mm f/4 macro lens.
How do these results compare to manual focusing? I made a series of images with the lens set at a constant focus distance and the camera on the rail.
- At f/4, the green channel peak was 2000 cy/ph, as compared to and average of 1950 cy/ph using AF. I call that a wash.
- At f/5.6, the best MF green MTF50 was 2500 cy/ph. Autofocus fell well short of that, with an average of 2100 cy/ph. The best out of 50 shots was 2250 cy/ph. That is very good performance, but you can do better using manual focusing.
- At f/8, the best manual focus image was 2200 cy/ph. The AF average result is only 100 cy/ph less than that, an inconsequential difference.
- At f/11, you can get to 1800 cy/ph focusing manually, and AF gives you an average of only 50 cy/ph less than that.
- This is an easy target for contrast-detection autofocus (CDAF) cameras like the GFX.
- It would take you many tries to get the manual focusing nailed the way you can with a computer controlled rail.
- The camera might do better — or worse — at other distances.
- As I said in yesterday’s post, don’t compare the absolute sharpness reported here with that in the 63 and 120 macro tests, where I used a different target that produces higher numbers.
This is top-notch performance at all apertures save f/5.6. It’s not bad at f/5.6, but it’s probably worthwhile to use manual focusing at that aperture if conditions permit. It is a little disappointing that all three lenses that I’ve tested, the 23, 63, and 120, all show their worst AF performance at their best f-stops.