This is a continuation of a series of posts on the Fuji GFX 50S. 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 “GFX 50S”.
I couple of days ago, I published a study of the focus shift and autofocus errors of the Fuji GFX 50S using the Fuji 45 mm f/2.8 lens. Now I’ll repeat the test with the 110 mm f/2.
I’ve plotted the three Adobe RGB color channels. The graph presents displacement of the image projected on the sensor from the desired green-channel focal plane, not the error in the object field in front of the lens. The reason for doing this is to allow easier comparison of lenses of different focal lengths at various target distances.Negative numbers indicate front-focusing. The image-plane shift is in micrometers (um). The separation of the focal distances of the three color planes is because of the longitudinal chromatic aberration (LoCA) of the lens. The dots indicate the results for each of the ten exposures at each f-stop. I’ve plotted lines indicating the average (aka mean or mu) of the sample set bolder and added thin lines above and below the means that are one standard deviation (sigma) away from them. We can see steady focus shift as you stop down.
Let’s look at the diameters of the circles of confusion (CoCs) implied by the above focus errors:
It looks like most of the troublesome shift occurs between f/2 and f/2.2.
Here are the errors that result from focusing at f/2, f/2.8, f/4, and f/5.6, stops and exposing at narrower ones.
It should be OK to focus this lens at f/4 for shots made at narrower apertures, but if you’re really picky you’ll focus at f/5.6 for apertures narrower than that.
Turning on autofocus, setting the mode to AF-S, the release priority to “focus”, and the spot size to two clicks smaller than the default, we get this:
The camera is overcorrecting for focus shift in the middle f-stops.
But the resultant errors in terms of the diameters of the blur circles are quite small.
We see the same kind of thing in AF-C mode:
I consider this good performance.