Blotchy blue skies

[teekay]

In many images which include blue sky I have a problem. On close examination the color is not a uniform blue but a blotchy mixture of magenta/blue/red. I am viewing these in 32-bit color on my monitor and it comes out the same when printed.

See an example at a temporary Web page at:

http://www.netidea.com/~teekay/temp/sample.jpg

Questions: Is this due to chromatic aberration peculiar to the G1 and, if so, has the problem been corrected on the G2? If not CA, then what is the cause?

I have yet to find a satisfactory way to correct this when postprocessing. Any tips? I use PSP 7.0.

[dbookbinder]

I get the same thing with my G1. It's unlikely to be chromatic aberration, which as I understand it is a prism effect due to the different wavelengths of light being refracted at different angles by the lens. I had always assumed it was due mainly to the software trying to make non-primary colors at different illuminations and saturations out of Red, Green, and Blue or (in a print) Cyan, Yellow, Magenta, and Black. Does anyone have a different explanation?

- David

[shaunyc]

I'm not sure if I have a definitive answer, but I do have some observations from my own shots that might be helpful.

Firstly, exposure. I was outside the other day right after a storm, and there was a great rainbow. In order to maintain some saturation in the rainbow colours I underexposed by a stop. When I got home I noticed a much more pronounced mottling of the colours you observed in the sky on the underexposed shots than on the normallly exposed ones.

Second, haziness. Looking through my shots the ones with clear blue skies have much less mottling than the ones that are taken on overcast or hazy days.

Third, saturation. In photoshop, even a slight increase in saturation on some of my shots brings out unreasonable amounts of mottling in the sky. This suggests that the colours were there all along, but that they were close enough as to be indistinguishable from one another. Perhaps one idea is therefore to make sure that any in-camera saturation is kept to a minimum. RAW mode is best here, but there is a low saturation setting for JPEG users.

I wonder if a skylight filter would help any...?

Next time I'm out on a hazy day I'll experiment and see if I can get anything more conclusive.

Shaun

[teekay]

Re your posting, Shaun, I'm going to try and find the answer by running some test shots next time we see blue sky here - it's not in the forecast yet :-(

I tend to underexpose and then lighten up most of the image later if needed. This may be part of the problem.

I usually shoot in JPEG at highest size and resolution, with saturation set to normal, so I'll run the tests in both JPEG and RAW and changing all these variables and then report back.

I have had nopbody say yet whether or not they are finding the same thing with the G2.

[cornovius]

Load your image into a paint program like photoshop or psp that lets you examine each of the red green and blue channels individually. You will probably find a lot more noise in the blue channel.

This can be reduced by using the fastest ISO setting (50) - take a couple of shots of the same thing, one at ISO 50 and one at ISO 400 and then look at the blue chanel of each.

If you have a good photo that shows some of this color noise, try going to the blue chanel and doing a blur on it (gaussian blur or dust and scratches, 3 or 4 pixels) then an unsharp mask (low values but a wide radius like 7 or 8). This will make the blue channel blurrier but with a lot less noise. When you lok at the rgb image again these is a lot less color noise but it still looks as sharp, this is because the balck-and-white detail mainly comes from the green channel and some from the red channeland little from the blue channel.

[teekay]

There's a shortage of blue sky here right now for making test shots but I have done some tests to see if JPEG or RAW made a difference. RAW was very slightly better but I had to blow images up 4x to see the difference.

I also tried different WB settings but all that did was color the blotchiness slightly differently.

Best solution I have so far for those sky images that show this mottled effect is to slightly reduce the red and increase the blue in PSP and then use "blur more" after selecting the sky area. This fixes it.

What I'd really like to know is whether the G2 is better in avoiding this effect in the first place. Anyone with a G2 got any comments?

[dbookbinder]

You are more likely to get a response from a G2 owner if you post the question at the G2 site. Or, probably better, try the dpreview Canon forum, as both G1 and G2 (and a lot of other Canon owners) frequent that site, and comparisons between different cameras would be easier to find.

- David