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Re: [OM] Options in distortion correction [was Custom Lens Profiles - CC

Subject: Re: [OM] Options in distortion correction [was Custom Lens Profiles - CC]
From: Jim Nichols <jhnichols@xxxxxxxxxxxxx>
Date: Thu, 1 Sep 2016 19:31:25 -0500
It does seem complicated.

Jim Nichols
Tullahoma, TN USA

On 9/1/2016 6:51 PM, Moose wrote:
On 8/31/2016 2:06 PM, Mike Gordon via olympus wrote:
I assume you realize that you only see the distortion because you are using a RAW converter that does not use the lens provided data to correct distortion as part of the conversion.

Yes, I still find it irritating that if use ACR one is stuck with the corrections as designed into the metadata. Sometimes one might like higher res in corners and less corrected geometric distortion. Should be the photog choice, no?

This is a complicated subject. First illustration covers a lot of ground, which may, I hope, become clear as you follow along. <http://www.moosemystic.net/Gallery/tech/DXO/Dist_Corr.htm>

First, look at it from the camera manufacturers' point of view. My sensor is 4592 x 3448* pixels in size. If I output different sizes from my Raw converter, it's going to cause confusion - and raise questions I'm not sure I want to address. Also, I want my JPEG and Raw outputs to look the same. Many of the lenses I make, and many of those of the other µ4/3 maker who codes lens distortion correction info into its Raw files, require little or no correction, and so are correct at the sensor size.

BUT, BUT, linear distortion correction must change relative sizes of parts of the image.

I can correct working from outside to center, but then the central part is reduced in size, I have arcs of black on the edges - AND - as all digital re-sampling reduces resolution, I make a prime spec worse. People forgive some edge decline in resolution, since that's been going on forever. So, that's not an attractive solution.

Or, I can correct working out from center. But with barrel distortion, that actually makes the image larger, in effect uncompressing what the lens has compressed away from the middle. So what do I do? I don't want to output larger images, as above. And I don't particularly want to throw away image data I've captured. But I know, though hardly any users do, that my nominally xx mm lenses really vary ±5% for 95% of production, with a few even further from nominal value. So if I shave a little off the edges, and a bit more off the corners, nobody is likely to notice. As a bonus, I get to chop off the lenses' optical aberrations in the farthest corners.

For my lenses with moderate optical distortion, I'll correct 100%. For my higher distortion lenses/focal lengths, I'll compromise, under-correcting a little, and throwing away less image. Most of this and that last paragraph are plausible fictions, but do agree quite well with my experience.

The Panny 12-32 mm lens has lots of distortion, optically, and their JPEGs and their version of SilkyPix Raw conversion all slightly undercorrect the distortion, but look pretty good. Oly takes a slightly different approach with the 12-50 mm zoom; they've even more optical distortion @ 12 mm than the little Panny, but they correct it more. (This leaves the odd situation where most tests will show the larger Oly to have less distortion than the Panny. That makes sense to us, as we assume it's part of the price for such a diminutive lens - but in fact, the Panny is optically the better lens, at least in that one characteristic, at that focal length.

Why Adobe chooses to go right along with Panny and Oly, I don't know for sure. Perhaps the price of receiving access to proprietary info that allows exact reproduction of the distortion correction of the manufacturers?

DxO, on the other hand, is creating their own profiles, without a need for compromise. So they take the simple way, outputting larger pixel size converted images.

If you roll over carefully, you will also see that there's still a little "bulginess", and a slight curve in the just above center straight line, in the Silky/ACR conversions, while DxO and PTLens correct completely.

PTLens is even trickier. If applied to the uncorrected image, it stays within the pixel bounds, but both corrects more thoroughly AND retains more of the original image. HUH??? If you pay close attention to the roll-overs, you will see that the Panny/Oly/Adobe correction works from the center out; the very center doesn't change in size at all, and the corrected image gets progressively larger as you go out.

The PTL correction works both ends, from some middle distance from center. The very center gets smaller, so that the farther areas don't have to be enlarged as much. It also works with the size of the image it's given. If I feed it the uncorrected original, surrounded by blank space to make the size of the DxO corrected version, PTL uses all that space to create something very much like the DxO correction, although with little arcs of black along the edges.

DXO allows one to back off and doesn't use the metadata corrections but their own, for good or for bad.

I like that DxO option of partial correction, in theory, although I don't think I've ever used it. OTOH, PTL offers the most flexibility. I can get more of the image captured, corrected, in the original pixel size, or give it more room and get something almost identical to DxO correction, and then have the option of filling in the black arcs using PS Content Aware Fill, or crop slightly.

PTL also, at least with this lens, corrects the corners a tiny better than DxO and with the Oly 12-50, a lot better. <http://www.moosemystic.net/Gallery/tech/DXO/LensCorrect.htm>

And here's another comparison, with Oly Viewer 3 and RawTherapee this time, showing clearly the advantage of DxO and PTL for retaining that WA that you spend those $ for, rather than throwing it away in software. <http://www.moosemystic.net/Gallery/tech/DXO/_A001643_dist_corr.htm>

Distorted by choice, Mike

Larger, Straighter Moose

* That's Panny; Oly 16 MP sensor is 4608 x 3456.


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