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Author Topic: Distortion correction software vs. well corrected lens???  (Read 6280 times)

Vladimirovich

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Re: Distortion correction software vs. well corrected lens???
« Reply #20 on: January 12, 2014, 01:05:21 pm »

I see a distorted pattern but where does it come from
as I noted it comes from software correction of geometric distortion when noise ("grain") is "banded" in such "clusters" as a result of the transform applied (example just makes that more visible), if curious then you might want to visit http://forums.adobe.com/community/cameraraw and find the relevant topic with the direct answer from Eric Chan of Adobe Labs... that is not unique to ACR/LR - SilkyPix does the same.
« Last Edit: January 12, 2014, 01:07:07 pm by Vladimirovich »
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Hening Bettermann

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Re: Distortion correction software vs. well corrected lens???
« Reply #21 on: January 12, 2014, 03:14:57 pm »

@ reply #9

Hi Bart,

> There is also a difference how the software correction is implemented. When it is a part of the Raw conversion process, the potential gains can be much better than post-processing of an already rendered RGB file. For example, correcting Lateral Chromatic Aberration, can sometimes be done before demosaicing, which will allow higher resolution and more accurate color conversions.

Could you specify software which does this? And/or give some general hints, what you would recommend for CA correction?

Good light - Hening.

ErikKaffehr

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Re: Distortion correction software vs. well corrected lens???
« Reply #22 on: January 12, 2014, 04:02:42 pm »

Thanks!

I have not seen something similar but I can guess a couple of reasons for it, like shooting low ISO and not using corrections very often.

Good to be aware of the problem!

Best regards
Erik

as I noted it comes from software correction of geometric distortion when noise ("grain") is "banded" in such "clusters" as a result of the transform applied (example just makes that more visible), if curious then you might want to visit http://forums.adobe.com/community/cameraraw and find the relevant topic with the direct answer from Eric Chan of Adobe Labs... that is not unique to ACR/LR - SilkyPix does the same.
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Bart_van_der_Wolf

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Re: Distortion correction software vs. well corrected lens???
« Reply #23 on: January 12, 2014, 06:03:27 pm »

@ reply #9

Hi Bart,

> There is also a difference how the software correction is implemented. When it is a part of the Raw conversion process, the potential gains can be much better than post-processing of an already rendered RGB file. For example, correcting Lateral Chromatic Aberration, can sometimes be done before demosaicing, which will allow higher resolution and more accurate color conversions.

Could you specify software which does this? And/or give some general hints, what you would recommend for CA correction?

Hi Hening,

RawTherapee can do it for Lateral CA, at the Raw conversion stage. It also offers traditional post-demosaicing LaCA correction for the occasion it doesn't work that well at the Raw level (depends of accurate auto-detection and complexity, I assume). I image that DxO might do some distortion correction at a very early stage, but I don't know for sure how that's implemented.

I think most commercial converters take the old-fashioned approach of fixing things after demosaicing, but that may also be connected to the fact that very few converters work in floating point precision (like RawTherapee does). Being able to calculate at sub-pixel accuracy and with huge dynamic range differences takes a toll on memory requirements, but also opens up a lot of potential for better algorithms.

There are some Astrophotography related applications that do  use floating point number brightness calculations e.g. PixInsight, but their tools are somewhat specifically tuned for those kind of images.

Cheers,
Bart
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Hening Bettermann

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Re: Distortion correction software vs. well corrected lens???
« Reply #24 on: January 13, 2014, 12:39:24 pm »

Thank you, Bart.

David Eichler

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Re: Distortion correction software vs. well corrected lens???
« Reply #25 on: January 13, 2014, 09:37:59 pm »

Rectilinear distortion varies with the distance from subject to camera. For something like architectural photography, this can sometimes be a quite significant consideration, depending upon how critical you want to be. No software can completely correct for different amounts of distortion in different parts of the image, in all situations, especially with global corrections.
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Nick-T

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Re: Distortion correction software vs. well corrected lens???
« Reply #26 on: January 14, 2014, 02:36:06 pm »

No software can completely correct for different amounts of distortion in different parts of the image, in all situations, especially with global corrections.

Hasselblad's Phocus software does precisely that. Corrections are based on the design of the lens (Hasselblad having designed the lens) and take into account aperture and camera to subject distance. As an aside the lens corrections when using the HTS are quite amazing as the software knows exactly how much shift/tilt has been employed and corrects accordingly.

Also thanks for the correction above re multi-coatings I was just trying to come up with an example of what can be done in design vs what can be done in post, for example post processing cannot fix a lens that is soft at the edges so this needs to be addressed at design. Distortion however, can be very effectively corrected in post (assuming you have the right data) so having lens corrections gives designers more options as to where to devote their resources.
Hope that makes sense.
Nick-T
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ErikKaffehr

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Re: Distortion correction software vs. well corrected lens???
« Reply #27 on: January 14, 2014, 04:17:53 pm »

Hi,

I am quite impressed by Hasselblad's way of thinking out of the box. I have flexbody myself. Regarding the software corrections, I would say that is OK.

Best regards
Erik

Hasselblad's Phocus software does precisely that. Corrections are based on the design of the lens (Hasselblad having designed the lens) and take into account aperture and camera to subject distance. As an aside the lens corrections when using the HTS are quite amazing as the software knows exactly how much shift/tilt has been employed and corrects accordingly.

Also thanks for the correction above re multi-coatings I was just trying to come up with an example of what can be done in design vs what can be done in post, for example post processing cannot fix a lens that is soft at the edges so this needs to be addressed at design. Distortion however, can be very effectively corrected in post (assuming you have the right data) so having lens corrections gives designers more options as to where to devote their resources.
Hope that makes sense.
Nick-T
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madmanchan

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Re: Distortion correction software vs. well corrected lens???
« Reply #28 on: January 14, 2014, 08:19:33 pm »

Basically what's going on in the image with the pattern artifacts is that an image that an evenly distributed amount of noise is being warped (resampled).  Doing so results in the (previously even) noise becoming unevenly distributed.  This will become especially apparent if you leave the noise present and use strong sharpening settings. 
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David Eichler

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Re: Distortion correction software vs. well corrected lens???
« Reply #29 on: January 14, 2014, 09:27:32 pm »

Hasselblad's Phocus software does precisely that. Corrections are based on the design of the lens (Hasselblad having designed the lens) and take into account aperture and camera to subject distance. As an aside the lens corrections when using the HTS are quite amazing as the software knows exactly how much shift/tilt has been employed and corrects accordingly.

Also thanks for the correction above re multi-coatings I was just trying to come up with an example of what can be done in design vs what can be done in post, for example post processing cannot fix a lens that is soft at the edges so this needs to be addressed at design. Distortion however, can be very effectively corrected in post (assuming you have the right data) so having lens corrections gives designers more options as to where to devote their resources.
Hope that makes sense.
Nick-T

It is impossible for the software to know what all the (possibly infinite) subject-to-camera distances are in a particular image. I repeat, rectilinear distortion varies with the distance of the subject to the camera. So, in the same image, an object that is, say, 5 feet from the camera can exhibit a different amount of distortion from one that is 50 feet away. How much of difference will depend upon the amount of inherent distortion in the lens's design. Furthermore, distortion cannot always be corrected in post, or, if it can, can sometimes require extreme amounts of selective retouching. Just trying to move things around with Photoshop Transform/Warp might correct one problem but create another one in the process by distorting nearby objects that do not exhibit the same amount of distortion as the area you are trying to fix. Architectural photographers are very sensitive to this kind of thing.
« Last Edit: January 14, 2014, 09:34:07 pm by David Eichler »
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Nick-T

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Re: Distortion correction software vs. well corrected lens???
« Reply #30 on: January 14, 2014, 10:16:54 pm »

It is impossible for the software to know what all the (possibly infinite) subject-to-camera distances are in a particular image.

Hi David
This data does actually get passed back to Phocus, not to the nearest millimetre for sure but the focus distance is recorded and used for the corrections.

Nick-T
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David Eichler

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Re: Distortion correction software vs. well corrected lens???
« Reply #31 on: January 14, 2014, 11:19:55 pm »

Hi David
This data does actually get passed back to Phocus, not to the nearest millimetre for sure but the focus distance is recorded and used for the corrections.

Nick-T

Can you cite some authoritative documentation to support your contention (preferably with a link)?

Your argument still does not sufficiently address the issue I am discussing. Simply adjusting the correction somewhat according to the location of the plane of focus is not sufficient to address this issue. The software would need to address variations in the amount of distortion for objects not at the plane of focus.
« Last Edit: January 14, 2014, 11:44:18 pm by David Eichler »
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ondebanks

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Re: Distortion correction software vs. well corrected lens???
« Reply #32 on: January 15, 2014, 04:48:53 am »

It is impossible for the software to know what all the (possibly infinite) subject-to-camera distances are in a particular image.
Hi David
This data does actually get passed back to Phocus, not to the nearest millimetre for sure but the focus distance is recorded and used for the corrections.

Nick-T

Nick - surely you mean *a* distance gets passed back - the distance that the camera or user chose as the subject plane to be focused on. David's right: there's no way that the camera can simultaneously measure *distances* all over the image to foreground, background and "side-ground" details. (It would need a grid of thousands of AF sensors covering the entire focal plane. Or else, a complete redesign to employ the sort of wavefront phase-recording technology used in the Lytro camera and similar prototypes).

Consequently, Hasselblad's corrections for distortion can only be optimal for the nominal focused distance.

I don't know if the situation is any better for a highly-corrected lens without software corrections, though. It too will presumably have variable residual distortion for objects at different distances from the camera.

Ray
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ErikKaffehr

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Re: Distortion correction software vs. well corrected lens???
« Reply #33 on: January 15, 2014, 04:59:54 am »

Hi,

I guess that most lenses having very low distortion are symmetrical designs so they may be less dependent on distance.

In my view it makes sense to make a lens with a small and simple low order distortion and reduce it in post instead of creating a lens with a small but complex high order distortion.

Best regards
Erik



I don't know if the situation is any better for a highly-corrected lens without software corrections, though. It too will presumably have variable residual distortion for objects at different distances from the camera.

Ray
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David Eichler

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Re: Distortion correction software vs. well corrected lens???
« Reply #34 on: January 15, 2014, 06:19:34 pm »

Hi,

I guess that most lenses having very low distortion are symmetrical designs so they may be less dependent on distance.

In my view it makes sense to make a lens with a small and simple low order distortion and reduce it in post instead of creating a lens with a small but complex high order distortion.

Best regards
Erik



I am no expert in optics or lens design, but I believe it is true that symmetrical lens designs will generally tend to have less rectilinear distortion. However, this is not to say that other lens designs cannot have very low levels of such distortion as well. I believe the biggest problem with such distortion occurs with retrofocus designs for wideangle lenses for reflex cameras. Conversely, I believe that symmetrical lens designs tend to have a higher amount of vignetting, compared with non-symmetrical designs at the same focal length, image circle and aperture. With some wideangle lenses of symmetrical or near symmetrical design, such as the 90mm Super Angulon, you need to use a special graduated ND filter to compensate for the high degree of vignetting.

Also, I don't mean to suggest that lenses with a high degree of rectilinear distortion cannot be used successfully (with the appropriate digital distortion correction) for demanding applications, just that you need to be more careful how you use them and recognize the limitations. Furthermore, sometimes the benefits of using a zoom lens will outweigh some limitations.
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Nick-T

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Re: Distortion correction software vs. well corrected lens???
« Reply #35 on: January 16, 2014, 03:15:09 pm »

Nick - surely you mean *a* distance gets passed back -

Yes that's what I meant thanks Ray.
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