color in corners of image....night time shots.

[DGC photo]

I have recently started doing star trail images, and have found in some images a deep purple color in the corners where the image is generally real dark or early black. Only time it seems to show up is on images I may bump up the brightness in Photoshop, I use CS5. Think, night time star shot, f 2.8, 1600 to 3200 ISO, 25 second shots, sometimes 30 seconds. When viewing in camera raw, it shows up more when clicking on lens profile correction tab. I am hoping this can be corrected in CS5 post processing, or is there a setting in my 5dm3 camera I should be changing? By the way, I am shooting in RAW. Never is this a problem with any other kind of shot during the day.
Thanks in advance for any feedback or help.

[PhotomanJohn]

DGC photo - My guess is that the corners of the images are not really black or dark grey due to some chromatic aberration in the lens. I suspect that Photoshop is properly displaying the color when you increase the brightness. Turning on lens correction essentially is doing the same thing in the corners to compensate for the vignetting your are probably getting operating the lens at 2.8.

A simple way to check this is to turn on the Info panel in Photoshop and hover the cursor over the corners of the unbrightened image and look at the values the the three color channels. If the color is actually black or very dark gray, the values for each channel would be small and nearly the same value for each one. I suspect you will see that the red and blue channels have slightly higher values than the green channel does. You can then brighten the image and see if the relationship between the three channels remains the same showing the software is doing what is it supposed to.

I haven't seen your photos but one brute force method to correct this is to sample the color farther away from the edge that you think is correct then paint that color over the affected areas. This may not be satisfactory if there are stars in those areas that you want to preserve. Another simple method would be to simply crop out those areas if that would still yield an acceptable image.

If it is actually a lens problem you might try it again with a smaller aperture which might help the issue.

Hopefully someone else out there has had this experience before and can be of further assistance.

John

[DGC photo]

DGC photo - My guess is that the corners of the images are not really black or dark grey due to some chromatic aberration in the lens. I suspect that Photoshop is properly displaying the color when you increase the brightness. Turning on lens correction essentially is doing the same thing in the corners to compensate for the vignetting your are probably getting operating the lens at 2.8.

A simple way to check this is to turn on the Info panel in Photoshop and hover the cursor over the corners of the unbrightened image and look at the values the the three color channels. If the color is actually black or very dark gray, the values for each channel would be small and nearly the same value for each one. I suspect you will see that the red and blue channels have slightly higher values than the green channel does. You can then brighten the image and see if the relationship between the three channels remains the same showing the software is doing what is it supposed to.

I haven't seen your photos but one brute force method to correct this is to sample the color farther away from the edge that you think is correct then paint that color over the affected areas. This may not be satisfactory if there are stars in those areas that you want to preserve. Another simple method would be to simply crop out those areas if that would still yield an acceptable image.

If it is actually a lens problem you might try it again with a smaller aperture which might help the issue.

Hopefully someone else out there has had this experience before and can be of further assistance.

John

Thanks so much for the help, I will try those things you mention.