This is the 30th in a series of posts on the Fujifilm GFX-50S. The series starts here.
There has been a lot of talk on the ‘net that the resolution differences between the a7RII and the Fuji GFX 50S don’t amount to much, so why suffer the increased hit on the pocketbook, loss of features, and increased weight to get the GFX?
Let me be clear that I see a big improvement in the resolution of the GFX, both with native and adapted lenses. Whether it’s enough to get you to pony up the cash for the GFX is a question only you can answer. Whether it’ll make a difference that you can see in the prints that you make with the field techniques that you use is another thing I can’t help you with. But there is a step up in resolution there.
I thought I’d do a quantitative test of on-axis sharpness with the same lens on the GFX and the a7RII. This will allow us to zero in on the differences in the sensor. There are three:
- The GFX sensor is 33×44 mm versus the a7RII’s 24×26 mm. This means that the physical picture height of the GFX is 37.5% taller than the A7rii.
- The GFX sensor is 8256 x 6192 pixels while the a7RII sensor is 7952 x 5304 pixels. This means that the picture height in pixels of the GFX is 17% more than the A7rii.
- The a7RII has nearly 100% fill factor because of its back-side illuminated (BSI) construction and the way its microlenses are designed. The Fuji has less coverage, since the microlenses are deliberately made small than the pixels in the sensor. This will increase sharpness at the cost of more aliasing.
You probably noticed that I normalized by picture height, not diagonal. That’s because I usually prefer images between 4:3 and 1:1 in aspect ratio. If you like long skinny images, you might normalize by the picture width; that will give results more favorable to the a7RII.
I set up my motorized rail and backlit razor blade sharpness rig, set the Otus to 5 feet and F/2.8, set both cameras to base ISO, turned EFCS on in both cases, and made two series of exposure runs with 1 mm steps between exposures. I looked at the resolution as measured by MTF Mapper in each of the raw planes as decoded, but not demosaiced, by dcraw. The metric for sharpness that I chose was MTF50 in cycles per picture height.
Here’s what I got:
The green channel in the a7RII case peaks at 2144 cy/ph. For the GFX, the corresponding number is 2988. That’s a 39% improvement. The red channel improves by 32%, and the blue channel by 40%. I think the fact that the ratios are different may have something to do with the dye spectra in the tow cameras color filter array (CFA) being different. Not that the blue channel is the sharpest for the GFX, while the green channel is the winner for the a7RII. Another possible source of difference is the sensor stack thickness and location.
Here’s the Imatest summary of the green raw channel of the GFX image in which that channel was the sharpest:
The reason that the MTF curve looks so flat is that the G1 channel image has only a quarter the pixels and half the resolution of the entire raw image.
The sharpest green channel image with the a7RII looks like this:
Even on a per-pixel basis, the a7RII image is slightly less sharp. Some of that has to do with the fact that the a7RII pitch is less. Some probably has to so with the proportionally smaller microlenses in the GFX. I have no way of sorting out which is which.
If we let dcraw demosaic those two images with the Imatest default settings, here’s what we get: