This is the seventh post in a series of Nikon D850 tests. The series starts here.
Actually, this isn’t really a D850 test. It’s a test of a Nikon 105 mm f/1.4 lens that happens to be mounted to a D850. The performance of the combination is what I’m testing. We saw some anomalies in the D850 autofocus test with that same 105/1.4 lens, and the purpose of this test is to take the measure of the lens on the D850 so we can judge the autofocus results in context.
I made a series of exposures at each whole f-stop from f/1.4 through f/11, using a motorized rail and this protocol:
- Nikon D850
- Double-edged razor blade backlit with Wescott LED panels aimed at white diffuser
- ISO 64
- Manual exposure
- Manual focus, set at one position
- Cognisys computer-driven focusing rail
- 100 exposures 1.6 mm apart
- 2 meters to target in the middle of the rail
- Document mode decoding with DCRAW
- MTF50s for all the raw color planes calculated using MTF Mapper
- Graphing in Excel
Here’s a picture of the test setup:
The controller for the rail is in the basket attached to the tripod legs. The rail is mounted to a Arca Swiss C1, and the camera is mounted to the rail. The tripod in front of the camera has the razor blade. Behind that is the white paper diffuser. You can see one of the Wescott LED panels in the upper left; the other is hidden behind the camera.
Here are the results at f/1.4:
Camera motion in cm is plotted as the horizontal axis, with the left-hand side having the subject further from the camera than the right-hand side (The camera moves closer to the subject by 16o micrometers after each exposure). Total travel was 160 mm, or 16 cm. I’ve chopped off the nearest 6 cm since there was nothing of interest happening there. The vertical axis is MTF50 in cycles per picture height (cy/ph), which is a good proxy for sharpness.
The lens is only moderately sharp wide open. There is a fair amount of longitudinal chromatic aberration (LoCA) that shows up as the red channel peaking at a different distance than the green and blue ones. The red is also not as sharp; that is usually the case for a fast lens wide open.
Stopping down in one-stop intervals:
It is not until at least f/8 that the depth of field is adequate to cover up the LoCA.
Plotting all the green channel data on the same graph allows us to look at focus shift, and also to easily see the sharpest f-stops. I’ve expanded the distance axis for easier viewing:
The plane of focus moves further away from the camera as the lens is stopped down. The lens is very sharp from f/2.8 to f/8. There is a lot of focus shift. It’s going to be very difficult for PDAF autofocus systems that focus at an effective aperture of f/4 or narrower to get decent performance that wide apertures. I don’t know the details of the Nikon D850 PDAF system; perhaps someone can enlighten me. For manual focusing, I recommend focusing at the taking aperture for f/4 and wider shots. For narrower apertures than f/4, you can safely focus at f/4.
The absolute numbers for MTF50 are not comparable to the AF results published earlier, for two reasons. The most important one is that the razor blade provides a sharper target than the printed slanted edge I used for the AF testing. The less important one is that these tests were performed at 2 meters, and the AF tests were done at 3 meters. Still, the differences are so large that I suspect that even with CDAF the camera’s autofocus system is not getting all the sharpness out of the lens that it can deliver if optimally focused.
I tried testing CDAF autofocus system on the razor blade target. It was totally unable to acquire focus at any of the 7 apertures used in this test.
If you look hard at the focus shift above, it’s an obvious question how the AF system did as well as it did in the preceding post. Here’s the answer, from Marianne Oelund, who is a Nikon expert:
…if you need to move back and forth between the widest apertures, and moderate ones, AF will be problematic, especially with camera bodies released prior to 2014 (starting that year, Nikon incorporated focus-shift compensation into the AF system – without telling anyone, of course).
Here are results tests of two excellent lenses with the same target at similar distance with different cameras:
The a7RII has slightly lower resolution than the D850, so it is at somewhat of a disadvantage in the above test. Note how well it does wide open in comparison to the 105/1.4.
The GFX has more resolution than the D850, and also smaller microlenses, so it has an advantage. Both of these lenses have far lower focus shift than the Nikon on tested here.