IQ effects of Fuji 23/4 distortion corrections

This is the 71st in a series of posts on the Fujifilm GFX-50S. The series starts here. 

In the last post, I showed you some images with Lightroom’s distortion correction enabled and disabled for the Sony 12-24/4 FE lens. Several people had expressed the opinion that correcting distortion in wide rectilinear lenses in post should be avoided because of degradations in image quality (IQ). Some have said that such corrections “wreck microcontrast”. It  turned out, in the case of the Sony 12-24 and Lightroom’s lens profile, there was no problem. 

But what about the Fuji 23 mm f/4 lens on the GFX? It’s not as simple as with the Sony lens, since Lightroom doesn’t use a lens profile for distortion correction, but implements the process in such a way that it can’t be simply turned off from inside Lightroom.

However, Iridient’s XTransformer can make DNG files from RAFs with and without distortion correction. I used it both ways, and set the other options — noise reduction, vignetting correction, LaCA correction, etc — all to off. I also set the option to defeat several LR processing steps.

This is what I got:

A close look at the target at 100% magnification:

As with the Sony 12-24, these look very similar.

A closer look at greater than 100% magnification:

They look pretty similar in both the extinction and microcontrast regions. Note that we’re still getting false color well after the visual monochromatic extinction point. 

A slanted edge analysis of both samples follows. First, uncorrected:

Now, corrected:

The correction seems to introduce a real frequency axis zero just past the Nyquist sampling frequency. Other than that, they numbers look quite similar.

My conclusions: don’t worry about lens corrections with the Fuji 23/4.

By the way, don’t try to compare the numbers in this test with the ones in the preceding one. There are many uncontrolled things that could invalidate that comparison, such as LR profile sharpening (which was, I think, turned off for the GFX), target distance, focus accuracy, and alignment accuracy. The beauty of comparing two different developments of the same raw file is that you don’t have to be very precise about any of those things.

Addendum:

A reader suggested that the CA correction is more injurious to IQ than distortion correction. Here’s the crop of the target with CA correction applied, but without distortion correction:

And here’s the slanted edge analysis:

Looks fine to me. Note that the LoCA is not corrected, as you would expect.

Further addendum: 

Frans van den Bergh pointed out that the close reproduction of the false color in pairs of these images is not indicative of near-perfect correction because the distortion correction is almost certainly performed after the demosaicing. Frans suggested that the closeness of the MTF plots might result from the distortion correction resampling being performed on a nearest neighbor basis. I took a clos look, and that is not the case:

If you look at the part of the image indicated by the arrows, you can see differences that indicate that the corrected image has been resampled using something more sophisticated than nearest neighbor. 

If we take the difference between the corrected and uncorrected image and move the corrected one a few pixels to the left, we get this:

If we brighten that image, this is what we see:

I’m not sure what to make of that.