This is a continuation of a series of posts on the Nikon D850. The series starts here. You should be able to find all the posts about that camera in the Category List on the right sidebar, below the Articles widget. There’s a drop-down menu there that you can use to get to all the posts in this series; just look for “D850”.
In a previous post, I reported on my progress in developing a software tool to process image sets made of a LensAlign target to produce the sort of results that FocusTune does: focus distance versus AF Adjust setting. Yesterday, I applied that technique to the Nikon 105/1.4 on a D850, with a subject distance of ten feet. Today, I’ll do the same thing with the excellent Sigma 85 mm f/1.4 Art lens.
I set the target distance to 8.5 feet. I focussed manually at the beginning of the series, and making 10 images at the one-third f-stops from f/1.4 through f/4, when I got bored and switched to whole-stop intervals for f/5.6 and f/8. Here’s the result for the AdobeRGB green channel:
All image points are plotted, 10 for each f-stop. Back-focusing is indicated by positive y-axis numbers and front-focusing by negative ones. I focused the first image with CDAF wide open, and got a small amount of front focusing, but I’m not totally confident in the location of the target plane right now, so don’t put much stock in that. The green line is the average for each stop, but I screwed up and only calculated it for the whole stops. As you can see, the spread in the data for each stop is quite small, and we don’t really need the average line. The forward movement in the subject focal plane at f/8 may be an error; the technique that I’m using becomes less accurate as the DOF increases.
You can also see about 2 cm of focus shift as the lens is stopped down, which I’ve observed using other analysis methods. By f/2.8, the shift has virtually ceased.
All three AdobeRGB channels:
The red channel is back-focused with respect to the green channels, and the blue is even further back-focussed. This is a consequence of the 85/1.4’s longitudinal chromatic aberration (LoCA).
When I use the D850’s contrast-detection autofocus (CDAF) on every shot in a similar series (but with all whole-stop spacing), here is what the green channel data looks like:
CDAF shows some shift as the f-stops change. The green unfilled circles have diameter proportional to the standard deviation of the data set at each stop.
The shift in subject focal plane is far more pronounced. The spread is greater, too, but that it somewhat masked by the different vertical scales
Here’s one more lens for which you need to decide what f-stop is most important to you, and use that f-stop when you collect tuning images.