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CA, Vignetting, Distortion--- Questions?
- Thread starter AP7
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Bobo Hodls
Forum Pro
CA when present must be the first thing corrected for my sensibility. It makes quite the difference when developing and the final sharpening is applied. Vignetting - I often apply some; concerns depend on the amount any particular lens introduces, and the objective. Distortion - depends on the subject being presented. IMO, of course. I like to turn to PTlens to address all three issues.
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...Bob, NYC
'Well, sometimes the magic works. . . Sometimes, it doesn't.' - Little Big Man
http://www.bobtullis.com
It really depends on the extent of the aberration. CA can take many forms, some of which are more difficult to deal with than others. Geometric distortions when they are in the form of simple barreling or pin cushioning can be easy to correct, but some lenses, especially some wide angle lens, have quite complex ‘wave’ or ‘sombrero’ distortions. When you have complex geometric distortion, then it is best to use a program that uses corrections specific to that lens. PTLens and DPP use a database of information for specific lenses at different apertures and focal lengths, Photoshop only has general purpose correction tools, unless you use third party plugins,
Again it depends on the extent, and in many cases on the specific shot parameters. For example, minor light fall off at the edges is easy to correct, but if your shot was at a fairly high ISO value, then lightning the edges will cause a significant increase in noise levels, in the same way that using software to brighten an underexposed high ISO shot will increase noise.
Correcting geometric distortions doesn’t degrade image quality too much, but will result in some loss of resolution in those areas affected by the correction algorithms. The side effects of CA correction depend on how the algorithms are implemented. Some shift red/green or cyan/magenta pixels to attempt a better convergence, some simply desaturate a narrow range of color values (often used in the case of purple fringing), while others use a combination of techniques.
Brian
you already got some comments.
Think about this: with vignetting you only change the intensity levels in the individual pixels, nothing else. By enhancing the intensity in the corners you might get some more noise in dark areas there.
with CA, you shift the position of e.g. red and blue pixels with respect to each other. Depending on how it's implemented, you might get a bit more interpolation artefacts e.g. in the red channel.
Distortion correction is most dramatic. In all the image areas that need to be adjusted you need to interpolate. In same parts you might have to stretch the image, so you have to fill gaps with interpolated information. Or you compress those parts that are "bigger". Often you end up to have to crop after corrections, so you loose some of the field of view.
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Life is short, time to zoom in ©
Ben_Egbert
Forum Pro
I dislike all of these, and the best way to eliminate them is with a good lens. When I use a well corrected lens, it is just night and day over one that is not.
When you correct these in post processing, you always lose something. DPP has fixed corrections for many lenses, call up an image and let DPP fix it. You get a message that some IQ will be lost and it will be very noticeable. The image even shrinks in the process so you lose angle of view.
CA must be fixed and nobody defends it as artistic. Some say distortion is not critical in landscapes, but once you have seen good images, this statement no longer holds at least for me.
In deference to my friend Bob, I have never cared for vignetting either. I always see it as art that started out as an acceptance of a defect, like grainy film.
Other items to add to this list is edge and corner sharpness, and flare.
I have found that a good lens can be used at much wider apertures. Bt f8, my 24TSE is as good as it gets and it is very good. My 17-40 needs f16 just to get a reasonable sharp corner and still has all the other issues.
--
When you can't focus, nothing else matters
Once you can, everything else does.
http://ben-egbert.smugmug.com/
Ben
When you correct these in post processing, you always lose something. DPP has fixed corrections for many lenses, call up an image and let DPP fix it. You get a message that some IQ will be lost and it will be very noticeable. The image even shrinks in the process so you lose angle of view.
CA must be fixed and nobody defends it as artistic. Some say distortion is not critical in landscapes, but once you have seen good images, this statement no longer holds at least for me.
In deference to my friend Bob, I have never cared for vignetting either. I always see it as art that started out as an acceptance of a defect, like grainy film.
Other items to add to this list is edge and corner sharpness, and flare.
I have found that a good lens can be used at much wider apertures. Bt f8, my 24TSE is as good as it gets and it is very good. My 17-40 needs f16 just to get a reasonable sharp corner and still has all the other issues.
--
When you can't focus, nothing else matters
Once you can, everything else does.
http://ben-egbert.smugmug.com/
Ben
Bobo Hodls
Forum Pro
My path in that regard was simply to buy the best lens I could rationalize affording, and work with it until you become intimate with it's strengths and weaknesses. Basically, use that to get the experience to answer that very question [g].
I started with a prime and a general range zoom, something like $350 and $500 respectively. Grew with them for a year or two before "needing" something better. But that was "at the beginning". Not sure where you are in the game. It's a process of learning what suits YOU as the photographer while you learn the nuances of lens characteristics.
There are so many considerations - it's probably best to set your budget (but always count on having to spend just a bit more, though [g]), figure out what type of photography you're most passionate about, and what might be a secondary one. I'm sure many would have good experience to advise specific lenses to consider.
--
...Bob, NYC
'Well, sometimes the magic works. . . Sometimes, it doesn't.' - Little Big Man
http://www.bobtullis.com
Peter Kwok
Veteran Member
Since CA, Vignetting, & Distortion can be corrected by software, your priority should be the resolution, sharpness, contrast and build quality of the lens.
This is the new trend of lens design. For example, Canon's new 15-85IS is very sharp for a zoom lens with more than 5x range. This is achieved by trading vignette and distortion for resolution.
This trend started with P&S. With their in-camera processing and correction, you would not notice this from the JPG files. But you can see it in Panasonic 4/3 & the Canon S90 RAW files.
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Peter Kwok
http://www.pbase.com/peterkwok
WYSIWYG - If you don't like what you get, try to see differently.
Well, after the explanations above, you should see yourselves actually.
Vignetting can be corrected without doing anything to resolution, it's probably the least to worry, and it usually disappears when stopping down, so will only be an issue in some shots and can even be artistic as Ben said.
CA often doesn't improve much with stopping down, so it's always there at the edges. It deteriorates resolution, and in contrasty situations you get these horribly coloured edges. It can be fixed relatively easily, though not perfectly, even in jpg files in e.g. Photoshop, without much loss of resolution.
Distortion is the worst, you do loose when correcting this, and some is not easily correctable unless you have specalized software. But it depends if you care about it, and how much it's visible. It depends on the lens, usually it's the wide angle/ultrawide angle that suffers from barrel distortion. If you take architecture shots, it can be annoying if the straight lines are curved.
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Life is short, time to zoom in ©
gdanmitchell
Veteran Member
Vignetting is very easy to fix. Distortion (by which I assume you mean barrel/pincushion distortion) is usually pretty easy to fix in post, though with some lenses, notably zooms, it can be a bit trickier. CA is usually pretty easy to fix, too, though I suppose it might be "harder" since the correction values vary from lens to lens, FL to FL, and so on.
As to which degrades the photo more, that is a very relative question. In an architectural shot "distortion" could be the worst problem.
Dan
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G Dan Mitchell - SF Bay Area, California, USA
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Jan Madsen
Senior Member
You have to differentiate between lateral CA and longitudinal CA. Good luck correcting LoCA (of which purple fringing is a sub-type), it can in practice be almost impossible to remove. LaCA can offen be removed quite effective, but better start out with an image without CA, or at least low level CA.
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- Jan
quite correct, though in the usual lens test sites, usually only lateral CA is measured, (so, that's the major measure you have to compare different lenses against each other for CA), and that's the one that can be corrected. So, yes, one shouldn't consider a lens that has lots of purple fringing, because that can't be corrected.
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Life is short, time to zoom in ©
Mike K
Veteran Member
It is my subjective observation that the lenses with a reputation for sharpness always have very low CA. Whether it is lateral or longitudinal, I believe that low CA will lead to a sharper image, especially at wider f stops. Thus I think that CA is the parameter of the 3 mentioned which should be minimized in the design of the lens. For example the Zeiss 21 has long held a reputation as the "best" WA lens made, and it certainly has some waveform distortion and vignetting at f2.8-4.0. I find the waveform distortion can be readily corrected with PT lens, an inexpensive and easy to use software tool, without perceptible loss of detail. Vignetting is readily addressed with the same software as well as PS. However, to get another WA lens (say the 16-35 II) to look as sharp and dramatic as the Zeiss 21 using tools in PP is very difficult indeed.
Mike K
Mike K
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