This is the sixth post in a series of Nikon D850 tests. The series starts here.
I decided to do my downhill-with-a-tailwind test for autofocusing accuracy on the D850 with the Nikon 105 mm f/1.4 lens. This test uses a static flat, easy-to-focus-on target and bright light (in this case, f/1.4 at 1/800 seconds at ISO 64).
Here’s the setup:
The camera is on the right, mounted to an Arca Swiss C1 which is mounted to the camera stand. The wire to the Nikon remote snakes across the frame. The remote itself is resting on the top of the copy stand main pole. The target is the letter size print with the two black rectangles and the Siemens Star in the center. The star was centered, and the focus point set to the center, where it almost covered the star.
I made three sets of exposures:
- EFCS, MUP (PDAF)
- EFCS, Liveview (CDAF)
- Electronic shutter, Liveview (CDAF)
Here’s the rest of the protocol:
- ISO 64
- Release priority: focus
- Aperture exposure mode
- f/1.4 through f/11 in whole stops
- 32 exposures at each f-stop
- Wescott LED panels set to 5500 K.
- Target distance at the center of the rial, 3 m.
- NEFs 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 one of the green raw channels:
As the caption indicates, this is using Mup mode, but not liveview, so we’re using the phase-detection autofocus (PDAF) capability of the D850.
The vertical axis is MTF50, measured in cycles per picture height (cy/ph). I plotted the average of the 32 images, the average plus the standard deviation (aka sigma), and the average minus sigma. I also plotted the best and the worst results of each 32-image set.
Now let’s look at the two live view plots:
You can see that the PDAF doesn’t do very well wide open when compared to the CDAF. This may be because the calibration of the lens to the camera is imperfect. But I don’t think so because the PDAF is slightly better than the CDAF at f/2 and f/2.8. You can also see that whether you use the EFCS or the electronic shutter makes little difference. Take a look here to see how these results compare with manual (or robo-manual) focusing.
This is a pretty darned good performance in terms of the spread of the results. Whether it’s good in comparison to what we’d get if the lens were focused perfectly is unknown at this point. I’ll consider doing some more testing.
Now let’s look at the red raw channel:
It’s pretty clear that the lens has a lot of longitudinal chromatic aberration (LoCA), and that the PDAF at f/2 and f/2.8 is optimizing for the green channel, while the CDAF is aiming more for a blended optimum. I like making the green channel as sharp as possible, but there are arguments in both directions.
Here are the blue channel results:
Pretty similar to the green channel results.
In a comment to this post, Horshack asked to see the data for each exposure. Here it is for the four widest f-stops:
You can see that at f/1.4 and f/2, the PDAF tends to get stuck in a rut; where it’s focused when the shutter release is depressed influences where it’s going to end up. That probably explains why, once it’s in that rut, that it’s more consistent than the CDAF, which you can see starting over if you look at the live view display when the sequences are running. To see what happens when the PDAF system is forced out of its rut, look here.