Pentax 645 120/4 smc FA macro on GFX 100S at 1:1

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.

I’ve been posting a lot recently about the 120 mm f/4 GF macro lens for the GFX. I found it a good performer at 1:2, at minimum focusing distance (MFD) with no extension tubes, but that it had really soft edges and a lot of focus curvature at MFD with 36 mm of tubes. In this post, I tested it at 1:1 with a 45 mm tube,a and found it credible on-axis but soft on the right edge of the frame.  I reported those results numerically and visually, using the time honored sharpness target of a banknote. The 120/4 GF was soft in the corners and edges at MFD with 18mm, 36mm, and 45mm of extension by tubes.

For copy applications, and for some 3D subjects that require extension tubes, the 120/4 GF just isn’t cutting it. I tried the CV 125/2.5 and it was a big improvement. I’d tried a Schneider 90 mm f/4.5 Apo Componon enlarging lens, and it was a disappointment at 1:1. I’d heard good things about the Pentax 645 120mm f/4 smc FA macro lens. It focuses to 1:1 natively, so I thought it might do better at 1:1 than the native GFX 120/4 macro, which needs a 45 mm tube to get to 1:1, and suffers off-axis when extended.

I adapted the Pentax lens to the GFX with a Cambo Actus.

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A simple dumb adapter would have worked as well, but I had the Cambo set up already, and just needed to change lens boards

Here’s the test procedure:

• GFX 100S
• Foba camera stand
• C1 head
• Cambo Actus
• Lens focused to get to 1:1 magnification
• ISO 100
• Electronic shutter
• 10-second self timer
• Indicated f/4 through f/11 in whole-stop steps
• Exposure time adjusted in M mode
• Cognisys rail 100 exposures, 40 um step size
• Initial focus short of target
• Convert RAF to DNG using Adobe DNG Converter
• Extract raw mosaics with 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

The results:

The vertical axis is MTF50 in cycles per picture height. Higher is sharper. The horizontal axis is f-stop.

1. The blue and red columns are for the Pentax lens on axis, with, respectively, a horizontal and a vertical edge.
2. The yellow and purple columns are for the Fuji 120/4 macro lens on axis, with, respectively, a horizontal and a vertical edge.
3. The green and light blue columns are for the Pentax lens on the right side of the 33×44 mm frame, with, respectively, a horizontal and a vertical edge.
4. The brick red and blue columns are for the Fuji 120/4 macro lens on the right side of the 33×44 mm frame, with, respectively, a horizontal and a vertical edge.

Indicated f/8 gives the best compromise performance for both lenses. At f/8, the Fuji lens is slightly sharper in the center, and quite a bit weaker at the edge.

Looking at micro contrast, which I’m defining for this post as contrast at 0.25 cycles per pixel.

Again, f/8 indicated is the best compromise stop, and the Fuji lens is better on axis and worse off axis.

Comparing the Pentax lens to the CV 125mm f/2.5 Apo Lanthar:

At indicated f/8, the CV is a hair better, but there’s really not that much difference between the two lenses. This is good news, since the Pentax lens is =much cheaper and d=easier to find than the CV.

We can look at on-axis MTF50 versus defocus distance for each of the raw channels to get an idea of longitudinal chromatic aberration (LoCA).

At the right edge, for horizontal edges:

And, finally, a lot at the point spread function cross-section at f/8 on axis:

Subject distance runs from top to bottom, with the top being front-focused, and the bottom being back-focused.

At the edge for a horizontal (radial) edge: