Off-axis testing of the Fuji 23/4 GF on the GFX 100S

This is one in a series of posts on the Fujifilm GFX 100S. 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 100S”. Since it’s more about the lenses than the camera, I’m also tagging it with the other Fuji GFX tags.

In the previous eight posts, I tested the off axis performance of the Fujifilm 110 mm f/2, 80 mm f/1.7 , 250 mm f/4, 63 mm f/2.8, 45 mm f/2.8, 50mm f/3.5, 30mm f/3.5, and 120 mm f/4 macro GF lenses on a GFX 100S. Now I’ll do the same with the 23mm f/4 GF lens.

Here’s the test protocol:

RRS carbon fiber legs
C1 head
Target distance 2 meters
ISO 100
Electronic shutter
10-second self timer
f/4 through f/11 in whole-stop steps
Exposure time set by camera in A mode
Focus bracketing, step size 1, 120 to 60 exposures
Initial focus well short of target
Convert RAF to DNG using Adobe DNG Converter
Extract raw mosaics with dcraw (I’ll change this to libraw and drop the DNG conversion when I get the chance, but the Matlab program that controls all this is written for dcraw)
Extract slanted edge for each raw plane in a Matlab program the Jack Hogan originally wrote, and that I’ve been modifying for years.
Analyze the slanted edges and produce MTF curves using MTF Mapper (great program; thanks, Frans)
Fit curves to the MTF Mapper MTF50 values in Matlab
Correct for systematic GFX focus bracketing inconsistencies
Analyze and graph in Matlab

I analyzed a horizontal edge in the center of the frame, and both a horizontal and a vertical edge on the far right side of the frame. The horizontal edge is oriented in a radial direction, and that’s how I’m identifying it in the plots. The horizontal edge is oriented in a tangential direction, and that’s the way I’m tagging it. I also redid the tests with the target on the lens axis. I looked at both horizontal and vertical edges. The edges are identified on the graphs as, respectively, radial and tangential edges. Those terms don’t make sensor on axis. The numbers ought to be the same, but I’ve included both to give you an idea of the variation possible with my test method.

Here are the MTF50 results in cycles per picture height:

I measured the MTF50 on each of the raw channels, and am reporting on the MTF50 on a white-balanced composite of those channels, which is mostly the green channel. The MTF50 for the right side radial edge is markedly lower than the other two.

Here is the microcontrast at 0.25 cycles per pixel.

And at the tougher =standard of 0.33 cycles per pixel:

What about longitudinal chromatic aberration (LoCA)?

The above plots the shift of the red raw image plane (on the sensor side of the lens) compared to the white-balanced raw plane. Red channel LoCA is low for such a wide lens.