Alley in Pisa, Italy

by Lars Kotthoff

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Given that software can pretty trivially fix pincushion and barrel distortion patterns if you're shooting in RAW (Lightroom did the right thing just by knowing what lens I used with the given EXIF information), does it really matter if a lens has these types of distortion (so long as the are not so significant that the post processing does not lead to other significant artifacts of course)?

Reasoning for asking this:
I've found the distortion with the D7000's kit lens to be really bad -- but optically it's fine on pretty much every other front. Lightroom makes this go away, but I want to be aware of possible issues this might cause while I'm actually shooting...

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Fixing the distortion in post-processing almost unavoidably reduces sharpness at least a little (and sometimes pretty badly). –  Jerry Coffin Jun 27 '11 at 5:54
    
and or adds other distortion... Try correcting a shot from a 10mm circular fisheye and you'll find that a lot of things get seriously distorted to correct those curved straights for example. –  jwenting Jun 27 '11 at 6:03
    
@jwenting: Well, a fisheye is a bit different than a little barrel distortion from a slightly off lens design. :) –  Billy ONeal Jun 27 '11 at 6:18
    
it is, but this is a follow on from his other question where he asks for generic lens correction functionality to match Lightroom's in camera –  jwenting Jun 27 '11 at 7:36
    
Why do you think RAW is easier to correct than jpeg? –  ysap Jun 27 '11 at 7:47

3 Answers 3

up vote 8 down vote accepted

No, it's not all that bad. On most shots, you won't be able to tell by eye if the curvilinear distortions have been corrected for or not - you'll need straight lines adjacently parallel to each other or to edge to tell. In nature, you won't have those lines. In portraiture, it will exaggerate some parts and diminish others, but perspective distortion and lighting has much more effect on result.

The reason you've heard so much about them is that you've read many lens reviews, and distortion is rather easy to measure and write about.

Possible downsides of shooting distorted and correcting in post:

  • you'll spend more time setting up and verifying your workflow;
  • in your heart, you might want to do as much as possible in camera (many cameras can correct JPEGs for manufacturer's brand lenses);
  • some contests have quite strict rules against manipulation in digital post-processing;
  • as Jerry Coffin already commented, slight loss of sharpness;
  • as you mentioned in your question, it's easy to correct if you're shooting RAW and your software knows the lens. These conditions might not be true for various reasons.
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plus increased battery drain from the CPU and memory consumption of the correction process/algorithm. Up until recently it would take minutes on a highend PC per image, and cameras don't have nearly that amount of computing (or electric) power at their disposal. –  jwenting Jun 27 '11 at 8:12
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@jwenting: WTF? In Lightroom I check a box, and it's done in less than a second. It must've been a quite long time ago that this process took minutes. –  eWolf Jun 27 '11 at 8:19
    
I think @eWolf is being rather kind by simply saying "quite a long time ago". Personal computer CPU's have been rediculously fast for a very long time now. Correcting severe distortion on my three year old battery-hog laptop with Lightroom happens instantaneously, and done countless times does not demonstrate any measurable consumption of battery life. I also believe there are cameras these days (olympus?) that automatically correct lens distortions in-camera, based on a profile stored in the currently attached lens. CPU power is beyond cheap and readily available these days... –  jrista Jun 28 '11 at 3:01

Yes, distortion is the second worst type of aberration. The worst is softness.

That is why when Nikon introduced their 18-200mm lens, instead of making it extremely soft like all others of the same focal-range, they made it sharper but left in a lot more severe distortion.

Distortion is particularly bad because it is impossible to correct without losing sharpness.

Correcting distortion also affects composition, so some things that were in the frame can end up cropped-out of the frame as the distortion correction involves stretching parts of the image and the results get cropped to keep the frame rectangular.

The reason distortion is better than general softness is that you at least have the option to not correct it and - of course - not make everything soft at the same time. There are plenty of subjects for which distortion is not easily noticeable.

Chromatic aberrations are annoying but generally avoidable and affect only small parts of the image. Correcting them has a very localized effect.

Vignetting is extremely annoying but, unless it is severe (1+ stops), can be correct quite easily without much detriment to image quality. You will get increased noise in the extreme corners but that is usually it. Of course, when it is above +1 stop, it can cause areas to be under-exposed while measurably withing the camera's dynamic-range. Personally, I find that 1/4 EV or less of vignetting is not noticeable on most subjects.

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Do you have any examples of the loss of sharpness induced by distortion followed by correction? It seems like the effects should be minimal except in the extreme corners. I've never used a superzoom lens so I don't know how bad they are, but the few times I've done barrel distortion correction I've had difficulty identifying which was the before and after image! –  Matt Grum Jun 27 '11 at 19:01
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Also I would rate field curvature and longitudinal chromatic aberration (along with maybe coma and purple fringing) far ahead of barrel distortion as the worst types of aberration as they are impossible to remove automatically and thus require laborious correction by hand. –  Matt Grum Jun 27 '11 at 19:03
    
Field curvature can be seen as a form of spatially-dependent softness, so yes, it is worst than distortion. Purple fringing is usually avoidable and highly localized, so I prefer it than distortion though. –  Itai Jun 27 '11 at 21:16
    
This answer seems much more "personal opinion" than really factual. I don't think its quite as simple as saying "This is worst, this is second worst, this is third worst". Different types of photography interact with lens aberrations in different ways. In some cases, such as photographing a brick wall, distortion will probably be the worst, where as CA might be worst in a landscape shot. In the case of the brick wall, correcting distortion and incurring some slight softening would seem FAR preferable to not, and keeping a "sharp" photo. –  jrista Jun 28 '11 at 2:53
    
As for softness being the worst lens aberration, I think that is highly dependent on how you use the photos. If you print them in a magazine, its highly doubtful that even the softest lens is going to produce "bad" photos, even without post-process sharpening. On the flip side, if you usually blow your photos up several hundred percent their original size, softness would very likely be the "worst" lens aberration. Vignetting can be annoying...however in many cases, it can also bring a very strong artistic element to the table. I would say the only truly "bad" aberration you mentioned is CA. –  jrista Jun 28 '11 at 2:55

As the post above says, it depends on what you shoot. I do allot of architectural and travel photography: Except where I want to use a distortion for artistic effect, I dislike any pincushion or barrel effects. Five years ago when I mainly shot landscapes it could also be a problem when the horizon was off centre, it would curve up or down. However for nature and close up shots it should be less of an issue and you should look for the lens sharpness, fine detail or speed instead.

Although you say you can easily correct distortion in you computer, it's always best to get it right in the camera first time. You should only resort to the computer when it is impossible or impractical to do better at the time.

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Do keep in mind that it's always going through the computer in some capacity to make the JPEG, but I see what you're saying :) –  Billy ONeal Jun 27 '11 at 10:22

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