LX7 awful edge resolution due to distortion correction

Started Apr 30, 2013 | Discussions thread
Detail Man
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Re: Distortion is the best compromise
In reply to technic, May 2, 2013

technic wrote:

I don't have experience with this combination, but in general PTLens also does a good job for distortion correction, resulting in much better corner sharpness than LR/CR or in-camera processing. Probably those more complicated distortion corrections cannot be done in-camera at the time.

Not familiar with what PTLens does either (the profile-data is collected by individual users from characteristics of their own individual units). If it uses the same process that the Panasonic image-file meta-data uses, I think that it is a fairly simple correction algorithm that proceeds from a single point located at the center of the image-frame, the identity for which is shown here:

http://en.wikipedia.org/wiki/Distortion_%28optics%29#Software_correction

From attempting to replicate what DxO Optics Pro does using RAW Therapee 4.0.9.x's rectlinear distortion correction tool, I am convinced that DxO corrections utilize more than a single reference-point located at the center of image-frame. I think DxO uses multiple reference-points.

Sean Nelson wrote:

technic wrote:

It's not just the 'stretching' from distortion correction...

You know that "stretching" in the corners is normal to get a rectilinear image at very wide angles, right?   It doesn't matter whether this is done in software or by the optical design - you can't have straight lines appearing straight and also have a very wide field of view without corner stretching.   Without that stretching you get lesser or greater degrees of barrel distortion which, if you go to the extreme, gives you a fisheye lens.

Wrong, it makes a HUGE difference if the stretching is done by optics or by software; at least as long as we have relatively low resolution sensors.

I'm not worried about distortion related to optical projection, or (not) having straight lines. I'm worried about the loss of sharpness in the corners, and especially the very asymmetric loss which often looks ugly/unnatural to me (pixels become elongated ellipses towards the corners).

I think that Sean is talking about what given amount of correction-processing must occur in software given whatever amount of optical rectilinear distortion exists as the result of particular lens design.

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