Yesterday I wrote about the causes of noise in digital images. Over the coming days I am going to look at how to reduce noise in digital images. I don’t like saying remove noise because I don’t believe you can completely remove noise from an image as there will always be some noise. How much depends on how you have shot the image.
I am going to use two images to show the effects of the noise reduction.
The first image is a picture of the Wool Market in Krakow. This is a night time image and was shot handheld (why I used a high ISO).
The second image was also taken on the same trip to Krakow and was shot at 200 ISO (my standard ISO when shooting out and about unless it is really sunny).
Both images were shot using the same camera (Canon G10) and suffer from noise issues. The noise in the second image is not as obvious but it is still present and on a print will be visible Where as with the first image the noise is visually obvious. With both images work is needed but I will say, with the Wool Market the noise will never completely be removed so the outcome I am looking for is to make the distortion less intense.
The today’s technique is using one of Photoshop’s own filter. The Reduce Noise Filter.
First I opened the image in Photoshop and opened the Reduce Noise Filter by going; Filters-Noise-Reduce Noise.
In basic mode you can adjust the strength of the filter. This filter will reduce both colour and luminance noise. You use the strength slider adjust the amount of the reduction (to the luminance noise) and the preserve detail slider will help preserve the edge luminance. The luminance noise reduction will be stronger if you set the preserve detail slider to 0. This is because even though you are trying to remove noise by using the preserve details slider you can inadvertently add noise to the image.
Next is the colour noise slider, which allows you to reduce colour noise separately. When reducing noise using this filter, I would suggest reducing colour noise first and then seeing how much luminance noise is left. This is because colour noise can be easily mistaken as luminance noise.
Once all the noise has been removed I added some sharpening using the sharpen details slider. I would advice caution when using this slider because too much sharpening can add more artefacts into the image.
Using the advanced mode you are able to target the noise more accurately by adjusting the noise in each colour channel separately This is extremely useful because in the basic mode you adjust all channels with the same strength. When doing so, if the blue channel contains more noise you may over do the adjustment on the other two channels. In adjusting each channel individually you can protect details in each channel while removing noise more aggressively in another channel.
Normally the red and green channel contains less noise than the blue channel, so I adjusted the blue channel first. The adjustments are strength and preserve detail that work in the same way as in the basic mode.
You can see that noise is less saturated and has made the sky slightly more blue but it is still quite a noisy image.
With the second image I could not really see a lot of difference between the two images.
In using the reduce noise tool you can, by accident make the image less sharp by adding an extra step into the process you can protect/preserve edge detail, keeping the image sharpe.
First you have to reduce the noise using the Noise Reduction Filter as above.
Then go Image- Calculations
Using calculations creates a new alpha channel that is a blend of the red and green channels, using the Pin Light Blending mode using the settings above.
Using this alpha channel (you can find out more about channels here) you can make a selection of it using the dashed circle at the bottom of the channels palette or you can just use ctrl/cmd and click on the channel. Create a new layer mask and the selection will be copied into the mask.
With the layer mask selected, next go Filter-Stylize-Find Edges and then still with the layer mask selected Image – Adjustments -Curves to lighten the mask. The idea is that the darker shades of grey will hide the Filter adjustment while the lighter shades of grey will allow the filter to show through.
The sharpness in the right half of the image automatically pops when compared to the original image. Also the noise in the sky sticks out more as well. The sky is more natural looking after the edit though.
In the Sukiennice image the method still didn’t show that much difference except that the letters on the window seemed more defined, but not much.
It could be said that this is the prescribed method in Photoshop as it is in the filter menu. I find the problem with the filter is that it is still too generic even with the advanced mode you still lose a lot of sharpness. Creating the layer mask can help preserve the details in the image but it doesn’t matter if you have still not reduced the noise enough.
Tomorrow I will look at using channels and the surface blur filter to reduce noise.
If you have any comments or any thoughts of your own about the reduce noise filter. Feel free to use the comment box below.
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