A quick and easy way to remove chroma (colour) noise from high ISO images.
It is great nowadays that we have cameras which can use high ISO settings to record images in very low light. What is not so great is the horrible blobby chroma noise which is often produced as a result. Here is a very simple solution to the problem using Lab mode in photoshop. Unfortunately you can't do this in a program that doesn't use Lab mode so apologies for that.
First of all you need to take a photo using a high ISO setting and get it onto your computer. I would personally open it in camera raw to sort out any exposure/white balance/colour issues because there will be some with high ISO images. Usually lowered saturation and possibly a colour shift. When you have prepared your image open it in photoshop then:
1. Convert to Lab (in CS3 Image - Mode - lab Color)
2. Duplicate layer (you don't need to do this but it is useful to compare the altered image to the original)
3. Select the duplicated layer then select the channels tab
4. Select the b channel alone.
What you should see is a soft greyscale version of your image covered in light and dark blotches. This is the chroma noise, seen in your full colour image as blue and yellow blotches. The next step gets rid of the chroma noise.
5. Choose Filter - Blur - Gaussian blur and adjust the radius slider until the light and dark blotches disappear (on a 4MP, 12800 ISO image from my HS20 I used a radius of 27 pixels). The radius you use will depend on the size of the image, amount of noise and the ISO setting used. Don't worry that the whole image blurs to mush. Lab carries all of the detail in the L channel so you won't notice a loss of definition in the final image
6. Choose the Lab composite channel to see the result then check this adjusted layer with the original. You should see that the chroma noise has gone. There might be a slight de-saturation of blue and yellow but I think this is a small price to pay.
7 Flatten then Convert back to RGB (Image - Mode - RGB Color)
If your image needs sharpening, I would advise that you do this before you leave Lab. You can apply the unsharp mask to the L channel so that your sharpening will not bring out any colour artefacts.
Here are some details from the full frame so that you can see the effect easier
You will notice that you can't get rid of the more generalised colour noise and there are still artefacts but you should notice a big improvement in most cases, especially in photos using artificial light. When I have the time I will do a more in depth version of this which has more real world use.
I hope this helps
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