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You CAN Correct It In Post, but . . .

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. . . . there is no free lunch.

I hear this about 20 times a day and it's true: "Yes, it has a lot of distortion, but that's easy to correct in post." It's a totally true statement.

Unfortunately, way too often, the complete statement goes like this: "I got it because it has such high resolution. Yes, it has a lot of distortion, but that's easy to correct in post." That's two totally true statements. But when combined, they become false.

Distortion correction is a wonderful tool. But every tool, whether in-camera or in your post-processing program, that modifies an image is a trade off of sorts. There is no way you can shift that many pixels around and not decrease resolution. I've said it dozens of times. Yesterday, unfortunately, someone asked me to tell him exactly how much resolution you actually lose?

Don't you just hate it when someone interrupts your Rant of Absolute Knowledge by asking for facts? What a buzz kill. Now, if I were hanging out on a forum under my anonymous handle of LensGuruGod1 I could have used proper Internet etiquette and replied, "If you weren't an awful photographer you'd already know that. I won't waste my time with you anymore."

Being that I was out there using my real name and all, the comment left me no choice. I had to go do some testing and gather some actual facts. I hate when that happens, especially when I've already spent two days playing with the Canon 24-70 f/4 IS and taking it apart. That has me a bit behind on my real job, so please excuse that this isn't an exhaustive test of 30 different lenses. I do think, though, it's a good example.

The Test

I used the Canon 24-105 f/4 IS for this test. It has a large amount of barrel distortion at 24mm (over 4%). I chose it simply because we already had Imatest set up at 24mm and because I feel our Imatest setup is a bit less accurate wider than 16mm. I mention this only as a preemptive strike because I'm 100% certain some Fanboy is going to be saying, "Roger said the Canon 24-105 has the worst barrel distortion of any lens." It doesn't, not by a long shot, but it has plenty for this test.

We usually run Imatest only on RAW files, but that would make it impossible to correct the image. So I took the RAW file to Photoshop, turned off every single sharpening and modifying tool and converted it to TIFF. Then I loaded the correction profiles for the Canon 5D Mk II and 24-105 and did an auto distortion correction, saving that as a TIFF.

Finally, I compared the corrected and uncorrected versions in Imatest. I'm presenting data from one lens here, but I did another for completeness. It was identical.

Image Correction

Below are the original and corrected shots. Photoshop does a really nice job of correcting the image. I measured the autocorrected version as carefully as I could and barrel distortion had been reduced from 4.2% to 0.5%.

 

Uncorrected image

 

Corrected for distortion

The Imatest Results

On top is the Imatest printout of the uncorrected image. Below is the corrected image. The numbers in the boxes are the MTF50 measured in Line Pairs / Image Height at each location.

 

Uncorrected MTF50

 

MTF50 after distortion correction

 

The numbers are a bit hard to read, so I'll summarize. The center MTF50 dropped from 1068 LP/IH to 939 (88% as sharp) after distortion correction. The far sides from an average of 556 to 477 (86%), while the corners decreased from an average of 539 to 460 (85%). Actually I was a bit surprised, expecting more decrease in the corners and less in the center.

I reran the numbers for MTF20 which decreased in the center from 1552 to 1450  LP/IH (93.5%), on the edges it dropped from 1015 to 838 (83%), and in the corners from 1005 LP / IF to 837 LP / IF (83%) which is more like what I expected.

Since MTF20 probably is a more important measurement of resolution for small prints and online jpgs, this would probably correlate more with what most people would see in an image. Landscape photographers making large prints, though, would be interested in MTF50.

So What Does it Mean?

Not a lot for most people. Distortion correction generally improves the look of a photograph and a small sacrifice in resolution isn't too important with today's cameras and lenses, even in the corners.

But when someone wants to argue that they buy a lens with high distortion because it has higher resolution and distortion is easy to fix in post . . . well, it had better be a lot higher, or it's a fool's argument.

I'll add one other note. It's well known among lens designers (I'm not one, but I read their textbooks and journals) that when designing a lens correcting distortion often reduces resolution. In ancient times (i.e. film days) distortion correction was a first priority. After all, it's really hard to stretch film to correct distortion. In current times, lens designers seem to be more willing to leave the distortion to get higher resolution.

I agree with that - I'd rather have the option of correcting or not correcting myself. But I think it's important for photographers who ARE very interested in the best resolution to realize they'll be giving some of that back when they correct in post. If two lenses have identical resolution in testing but one has more distortion, that one will have less resolution after the distortion is corrected.

 

Roger Cicala

Lensrentals.com

January, 2013

 

41 Responses to “You CAN Correct It In Post, but . . .”

Matt said:

Imatest gives two measurements - MTF and "MTF after ideal USM sharpening". I'd be curious to see if the latter changes as much as the former. You'd expect any pixel moving operation e.g., horizon straightening, to reduce MTF, but in a detail critical image, you'd also sharpen after manipulation.

Lisandra said:

aaaaaat any rate...you constantly answer things that keep me up at night. Resolution is the most important thing to me as of today (despite my favorite format being m4/3s) but I think distortion is more distracting overall than a bit of resolution loss.

Manuela said:

I meant "I am looking forward to further testing", sry for that, I am not English native.

Manuela said:

Roger, thank you very much for this test. I would have asked exact the same question as mph if I were the first here to comment. I am looking for further testing because in many fora people use the argument that distortion can be corrected and the loss of resolution is negligible. As I suspected there is in fact no free lunch.

Gee Free said:

Dear Roger,whenever I read your stuff I am always reminded of the late Geoffrey Crawley who wrote reviews for the British Journal of Photography back in the day. Your no-nonsense, cut-to-the-chase, fanboy-averse approach is so refreshing, welcome and much needed. What a different technical world photography is this digital realm. There is so much potential misinformation and confusion regarding real-world IQ and how to achieve it and how to maintain it.
I got rid of a beloved, late, mint Nikkor 28/3.5 PC because the CA was just so outrageous that even after correction attempts in numerous software there was still what I can only describe as a white shadow where the red/purple fridge used to be. Your little article begins to explain why and points to a reality that what we use when we photograph is not just a sensor but a complete system (lens-sensor-electronics, etc)that often-times may deploy software manipulation (that is essenially what "firmware" is isn't it?)even at the AD stage. Notwithstanding that, if we have a "perfect" sensor that requires no AD tricks to be "perfect" and we use a lens that captures an image full of distortion that must be corrected via software and will thus detract from the "perfectness" of the sensor, then what is the point of lauding the camera manufacturer because of having a sensor whose full potential cannot be realized due to deficiencies in the camera manufacturer's lens line-up.
Where to find information on real, genuine and practical image IQ that takes such into consideration? For example, I have yet to read anything on DxO's site that factors in the destructive effects of correcting for distortions like CA nor have they mentioned Nikon's manipulation of the analogue signal before it is converted to digital. Whether this is still an issue or not a do not know. But I remember reading that astronomical photographers would not use certain Nikon cameras because the noise reduction process used effectively cancelled out some of the stars before they were even recorded!
Lens distortion really matters even in the digital age - and thank you for pointing that out. Where can I find an honest rating system of lenses and systems that will honestly rate based upon real world stuff like you have done in basically telling us to be wary of "fools arguments"? Thank you for your blog and your hard work.

abib said:

That's why I sold my 24-105L and my 17-40mmL for a 14mmL mk 2. and sticking to the much better EFS 17-55mm f2.8 and 70-200mm L f4 i.s or f2.8L mk2.

Eric said:

Fantastic article - thanks for creating such a great blog.

WTV3D said:

So true,
great article.
Thanks

John said:

Very interesting and informative post Roger, thank you. It makes me wonder (don't go testing this, hehe), if the sharper 24-70mm f/2.8 mark II, after correction, will be roughly the same resolution as the uncorrected 24-70mm f/4L. ;-)

Daniel said:

Roger: understood and definitelly thank you for your efforts. Carry on the good work!

Esa Tuunanen said:

Like Volker said barrel distortion makes center of image "bulge" out needing some mutilation of also those pixels to sweep distortion under the rug. So resolution loss also in center should be expected.
And just like he said software distortion correction is literally like pixels in some areas of image being compressed smaller and again streched in other places to reshape areas of image.

So why should anyone think that process is lossless.
You don't think same way from drawing some figure/pattern onto air balloon and then inflating balloon?

Also multiple separate corrections should obviously cause more information loss because of multiple times that results of calculations are rounded up.

Photozone did year ago MTF50 tests for few Samsung NX lenses both with and without distortion correction and also confirmed resolution loss.

Simon, Raw Therapee doesn't do automatic distortion correction if you leave that option disabled.

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