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 100”. 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 had a Schneider 90 mm f/4.5 Apo Componon enlarging lens lying around, so I decided to try it.
Here’s the setup:
The lens is mounted on a Cambo Actus, which is in turn mounted on a GFX 100S. The whole thing is mounted on a Cognisys rail, and is aimed at a backlit rasor blade.
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/4.5 through f/11
- Exposure time adjusted in M mode
- Cognisys rail 200 exposures, 20 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 f-stop setting of the lens was not visible with the Cambo setup, so I didn’t always get the f-stops I wanted. There are pairs of bars for five different locations on the sensor: in the center, 1/4 the way to the edge, halfway to the edge, 3/4 of the way to the edge, and near the edge. Unfortunately, not all locations are shown for all stops, but I think there’s enough data for you to get the idea of what’s going on. MTF50 in cycles per picture height is the y axis, and the x axis is f-stop. F/6.7 appears to be the optimum stop. One thing to remember: at 1:1, the effective f-stop is two stops narrower than the indicated f-stop, So f/5.6 is effectively f/11, f/8 is effectively f/16, and so on.
This is not great performance. It is not as good as the CV 125/2.5, for example. The lens was designed for a magnification of about 1:3.3, so it’s not to surprising that it doesn’t do well at 1:1, but I was still disappointed.
Here’s how the microcontrast looks:
It’s the same idea.
Comparing the enlarging lens to the Fuji 120 GF in the center of the frame:
The Fuji lens is a clear winner. It should be noted that the effective aperture of the 120 GF is one-third to two-thirds of a stop wider than the Schneider when the indicated f-stop is the same.
At the edge:
Now the Schneider lens looks better. The Schneider seems to have a flatter field than the Fuji lens, but since I’m planning on stacking, I haven’t looked at that in any detail.
So far, the best lens I’ve tested for 1:1 performance is the CV 125/2.5.