Hidden Power Support - - Chromatic Aberration

jbruceb · #1 03-20-2004, 06:20 PM

Richard, as a new tool suggestion:
I've read a lot about this unsightly purple fringing, but no one ever suggests how to get rid of it on a print. Everything that I have tried gets rid of the purple, but leaves behind a fuzzy, grey edge that looks almost as bad.
What would be nice is a tool to quickly eliminate the purple, yet leave a sharp edge. I notice some cameras (like my Canon S230) are more prone to this than others.
Is such a tool possible?

Richard_Lynch · #2 03-21-2004, 05:14 AM

Please post an example. I realize some camera / lense combinations are more prone to this...my current equipment is not.

jbruceb · #3 03-21-2004, 10:39 AM

Thanks Richard. Attached is "park" which I think that I have fooled with somewhat. You can still see some purple amidst the leaves at the upper right. This might not be as obvious in this reduced attachment as it is in the original file.
I have had shots much worse than this, but I disposed of them.

Richard_Lynch · #4 03-21-2004, 01:52 PM

Do you have the original?

jbruceb · #5 03-21-2004, 07:14 PM

I don't have the original of that one, but I am attaching an acceptable downsized image (less than 100K) of another shot that is original. Look at the edge of the tree trunk and some of the branches.

Susan S. · #6 03-21-2004, 09:08 PM

Quote:

Originally Posted by jbruceb

I don't have the original of that one, but I am attaching an acceptable downsized image (less than 100K) of another shot that is original. Look at the edge of the tree trunk and some of the branches.

That's actually not too bad! My G3 can produce it fairly horribly at apertures of 4.0 or wider and with any degree of over exposure on high contrast edges....the colour varies from vivid magenta to blue depending on the lighting conditions. It's easier to fix by making sure you don't overexpose and then bringing up the shadow detail in the postprocessing - the blooming aspect of the overexposed high contrast areas bleeding through into the dark areas are what makes just getting rid of the colour not the perfect solution, leaving the fuzzy grey edge. I can't see an easy way to automate via an action or other tool. I've used targeted HSB adjustment layers to get rid of the colour - luckily those color tones tend to be fairly rare in the real world so you don't often need to select the area precisely.
Target the blues and magentas, desaturating the colour range, using the +, - eyedroppers to narrow the range so that image detail is not affecte, then move the lighten slider to the left to darken the blooming
I've atttached one of my samples (not the worst, but I don't tend to keep them too often!) plus a correction. you need to view this at 200 per cent to see it in its full glory....
susan S

jbruceb · #7 03-22-2004, 07:48 AM

Great before and after example. The good bit of work that you put in is what I'm trying to avoid by asking Richard to come up with a more "automatic" solution. From what I have read, Canon cameras seem especially vulnerable to this weakness.

Richard_Lynch · #8 03-22-2004, 09:12 AM

Automating the solution would require predictability. I am not seeing a sufficient amount of predictable behavior here in a way that suggests I would be able to automate the process -- or make a fix that I feel is viable and desireable.

These problems should only happen when using wide apertures, and could possibly be attributed to specific lens/camera combinations. What I was hoping was that the problem occurs in specific, definable, and isolatable image areas. I am not seeing that in these examples...and could possibly only base the selection on saturation and maybe a manual selection to determine color range.

Any more pronounced examples to work from?

jbruceb · #9 03-22-2004, 09:57 AM

Thanks for taking the time, Richard. Attached is one more "tree" picture that I have. For me, I usually notice this when a dark edge is against a bright sky. I was thinking that something like the Red-Eye Brush could quickly fix the problem. Unfortunately, my little Canon S230 does not offer aperture priority, where I might get this a little bit under control.

Susan S. · #10 03-22-2004, 02:54 PM

Actually the "good bit of work" took about thirty seconds! - and if you have several shots with similar problems it's an easy matter to copy the correction layer from one image to another. The colour and even the nature of the problem does vary from shot to shot though so it' s best to hand tune the correction. In my view, automation would be hard - the colour and size of the distortion also varies quite a lot from camera to camera as well as between images from the same camera.
On my camera (and Canons are by no means the worst as far as I can tell) it can get worse than that, but it was a 100 per cent crop - it doesn't usually show up too badly on printed or on screen images. It seems to be the big downside of trying to push more pixels on the little sensors in these prosumer cameras - the 5 and 8mpixel cameras from all retailers using the small chips seem to be worse than my 4mp G3

jbruceb · #11 03-22-2004, 03:42 PM

You are right Susan; the variation shot-to-shot calls for individual treatment.
Your method of using a HSB adjustment layer is pretty good, but I had to be careful with the sliders so as not to wipe out a pretty blue sky. For a while there I was wandering around with a sponge and a burn tool.
Thanks for taking an interest!