[Brightcolours]

 

I took a perfect (artificially created) distortion chart.

Then I distorted it to 7% and corrected it back (manually - not 100% accurate but good enough).

 

I've attached a crop 700% enlarged that illustrates the effect.

The "perfect image" does not show any grey values - just black & white, of course.

You may see that one pixel row got pretty dark (loss of edge contrast). Or in other words - loss of resolution.

 

It's, of course, no surprise but it clearly shows how lossy a correction of a rather "typical" MFT wide angle distortion really is there.

How did you distort it to what your test software calls "7%"?

 

Photoshop uses a different metric.

 

You have to make a distorted chart from the outset, not make one, then distort it (because the distortion will lose resolution, if you do that). 

 

This is what I end up with. I made a 6000 x 4000 image in PS. I put a chart in the top to "eyeball" about 6% using the Oly 12-100mm f4 PZ chart to judge the curvature.

 

I then pasted a pattern into the corner, of which you can see a part in the crops I made. I "undistorted" the chart again, and this is what the corner looks like after that. 

Left is the pattern I pasted onto the distorted chart, right is the same corner of the 6000 x 4000 image after correcting for barrel distortion.

The shift you see in the pattern illustrates pretty well the actual resolution that gets lost.

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400% :

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Any tips on how to do this more accurately concerning what the test software calls "7%" are welcome.