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03-21-2018, 06:53 PM   #1
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Cropping distortion

This is definitely a novice question. I have a new K-70 that I've been taking to a local wildlife refuge. Much of the wildlife I shoot with the kit 18-135 lens, as well as my smc Pentax M 200mm prime, is too far away for a close up exposure. When I crop the image on my computer (Corel After Shot) I get purple or green color distortions around some of the objects in the image. What is this distortion called and is there anyway to deal with it in post processing? I know the solution is to obtain optics which have the reach I need but I don't have the budget at the moment.

03-21-2018, 07:05 PM - 1 Like   #2
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Chromatic aberration is what it is called and it shows in high contrast areas of pictures and occurs even in new lenses although not as must as older designs. Stopping down the aperture will help and it can be easily corrected in photoshop. Try to avoid extreme contrast to start with, not sure what 200mm lens you have but if it's F/4 then try shooting at F/5.6 or F/8.

Last edited by bschriver11; 03-21-2018 at 07:13 PM.
03-21-2018, 07:20 PM   #3
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QuoteOriginally posted by bschriver11 Quote
Chromatic aberration is what it is called and it shows in high contrast areas of pictures and occurs even in new lenses although not as must as older designs. Stopping down the aperture will help and it can be easily corrected in photoshop. Try to avoid extreme contrast to start with, not sure what 200mm lens you have but if it's F/4 then try shooting at F/5.6 or F/8.
Is it present as a result of my cropping of the image or is it also present in the native image? And thanks for pointing me in the right direction for further research. You can see the green distortion in this picture.
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03-21-2018, 07:35 PM - 1 Like   #4
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It is there irrespective of cropping. It just looks wider when you crop.

In Lightroom for example, one click generally makes it go away. Don't know about Corel. Read the documentation about chromatic aberration.

Chromatic aberration is caused by different wavelengths (colours) bending by slightly different amounts when passing through a lens. It's physics. Lens manufacturers expend a lot of effort trying to correct it, but it's always potentially there.

03-21-2018, 07:59 PM   #5
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QuoteOriginally posted by mikecnorthwest Quote
Is it present as a result of my cropping of the image or is it also present in the native image? And thanks for pointing me in the right direction for further research. You can see the green distortion in this picture.
The green halo is probably chromatic distortion and is more apparent on the crop image because of enlargement. I said probably because I have no personal experience with AfterShot and because green is an unusual color for axial/longitudinal CA (so-called bokeh CA or LoCA). Usually users see blue-violet or reddish. Assuming you were shooting with the Pentax-M 200/4, my experience with my copy is that the lens is not particularly prone. That being said the third example photo of the review linked below shows fairly obvious green LoCA. Many tools offer CA or fringe correction that you may give a try. Alternatively, shooting at a narrower aperture might also work.

SMC Pentax-M 200mm F4 Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database

BTW, good job nailing focus on that Red-winged Blackbird.


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03-22-2018, 06:26 AM - 1 Like   #6
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Try doing as little as possible to the image and then crop. Compare the results. Sometimes processing steps make the color fringing more visible. This comparison may let you verify it. Also if you crop first you may be able to see what processing manipulations are influencing the final outcome and make changes to reduce this.
03-22-2018, 08:37 AM - 1 Like   #7
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If chromatic aberration correction (or whatever it's called in AfterShot) does not remove the fringing, try decreasing green saturation in your image. The bird has no green so it won't hurt the overall image.
03-22-2018, 10:18 AM   #8
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I can't locate it right now... but I have an image that is largely B&W in terms of subject. When I went to process and gave it a high contrast look it really screwed up the branches in the background. Even in B&W it developed this crazy onion ring bokeh that was not there before the processing. I re-did the processing using a different method to gain a similar look and the problem went away. My point earlier is that you should crop into the area - and check it pre-processing. If the effect isn't there it is likely due to the processing and can be minimized if you work carefully. At a minimum - if you fix it prior to other processing steps you may be able to do so with less loss. Normally I would process first and then crop but this is the exception case.

03-22-2018, 12:13 PM   #9
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I had an "updated" version of Adope Photoshop Elements that came free with some equipment I bought, which I found would introduce both wrong colors and wrong exposures on prints. My earlier version, the original PSE 1 did not do that, and it was free coming with my old scanner. Now I still use both the old scanner and the PSE 1. PSE is now up to version 18, but I don't have it or know how good it is.

Last edited by mikesbike; 03-22-2018 at 12:23 PM.
03-24-2018, 03:23 AM   #10
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QuoteOriginally posted by mikesbike Quote
I had an "updated" version of Adope Photoshop Elements that came free with some equipment I bought, which I found would introduce both wrong colors and wrong exposures on prints. My earlier version, the original PSE 1 did not do that, and it was free coming with my old scanner. Now I still use both the old scanner and the PSE 1. PSE is now up to version 18, but I don't have it or know how good it is.
Did you check the colour settings in the latest version of PSE ? Did you have a printer profile ? Getting prints to look the same as you see on your screen is not a simple process. Have a look here at some good tutorials on colour management Tutorials on Color Management & Printing
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