ACR: Vertical distortion fix adds its own squish distortion

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  • Updated 6 years ago
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When I fix vertical distortion - from shooting at a low vantage point - with Camera Raw, it squishes the image vertically. Circles become squished ovals, squares become squished rectangles.

I can fix it with Scale, but that seems silly. Is this a bug? Or just something I have to tweak manually? Or ?
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Max Rockbin

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Posted 7 years ago

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Eric Chan, Camera Raw Engineer

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I'm not following you. You say that squares become squished rectangles when adjusting vertical distortion, yet you also say that Scale fixes the problem. But, Scale is a uniform scale, so it can't change shapes.

Maybe you can post an example?
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Max Rockbin

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When I said I could fix the problem with "Scale" I meant that I could pull the top little square on the scale box to stretch the image back to the correct proportions (after also enlarging the canvas to accommodate the stretch).

I can't seem to find a good example since I don't save intermediate stages of corrections, but most typically I'll notice an electrical outlet looks wrongly proportioned after the correction. Or a door knob looks kind of oval.
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Jochen Kratschmer

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I observed the same issue and already discussed that with Eric via Mail. Attached are two versions of the same image. The degree of correction of the keystoning is about the same too. Sliding the keystoning correction tool in LR rotates the image around it's horizontal (or vertical) center line. When used with buildings there is no distortion that really shows at first sight. But if people are in the picture they get ”dwarfed". C1's tool appears to use the upper/lower or left/right edge of the image as the axis of rotation. However it is resulting in much less distorted proportions.

The solution to the distortions could be separate scaling tools inside "manual transform" that work selectively only on verticals or horizontals with a default coupling to the corresponding transformation sliders for "vertical" and "horizontal" compensating the (in this case vertical) compression. And the scaling should be uncouplable as well if you really want to play.

But I think it still would not look "right" because the amount of stretching/compression has to increase with the distance to the "base" edge of the image (where it should be zero). You would have to work that effect into the scaling tool as well maybe using the intercept theorem. It looks to me like a mathematical representation of the original Scheimpflug way of correcting keystoning then.

When using a shift lens the image gets stretched towards the direction of the shifting. When doing perspective correction the Scheimpflug way ( in the darkroom with a tilted image on the negative not only the negative itself (or sensor in this case) needs to be tilted but also the lens plane until the negative plane, the lens plane and the image (photopaper) plane meet in one straight line or one point when seen from the side. This too results in non-linear vertical stretching.

I found this page on the web:
This example compares the effect using a tilt and shift lens compared to Photoshop's perspective correction. The image example clearly illustrates my point: The corrected image's proportions look compressed.

Another one appears to analyze (and correct) the issue much more thoroughly: