Grayscale noise when adjusting the blue mix

[DaveCurtis]

When converting a color image with a blue sky to Grayscale and then adjusting the blue on the grayscale mix to say -50 or -60 to darken the sky, I notice an enormous gain in noise in the sky. The image was shot at ISO 100 with a ID Mrk2, so has no noticable noise to start with.

Is this normal when adjust the blue grayscale mix? What would cause the excess noise? I have reproduced this with several images with blue skies.

Thanks Dave
www.magiclight.co.nz

[jdyke]

I have noticed the same thing and I shoot with a 20D.  I think that it may have something to do with noise tends to be mostly in the blue channel.  There may be more info in one of the Lightroom forums over at Adobe, I'll let you know what I find.

Jon

[francois]

I've seen it on a 1D2 and a 5D even at low ISO. Jon is right regarding noise in the blue channel. I use a noise reduction software in Photoshop to deal with it.

[timhurst]

On a 1Ds2 I find the colour controls pretty much unusable apart from very slight tweaks. The image breaks up with noise so easily which is a shame. Similar edits are much more robust in PS.

[tomrock]

When converting a color image with a blue sky to Grayscale and then adjusting the blue on the grayscale mix to say -50 or -60 to darken the sky, I notice an enormous gain in noise in the sky.
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Could you open the image in PS and adjust the blue channel just to see what happens? Do you get a similar increase in noise? This would help you decide if it's a LR issue or a blue channel issue.

[John Sheehy]

When converting a color image with a blue sky to Grayscale and then adjusting the blue on the grayscale mix to say -50 or -60 to darken the sky, I notice an enormous gain in noise in the sky. The image was shot at ISO 100 with a ID Mrk2, so has no noticable noise to start with.

Is this normal when adjust the blue grayscale mix? What would cause the excess noise? I have reproduced this with several images with blue skies.
[a href=\"index.php?act=findpost&pid=113368\"][{POST_SNAPBACK}][/a]

When you cut the blue channel, the sky is much more dependent on the red channel, which has a very low exposure in the sky, especially deep blue sky.

The quick fix is to mask the sky and blur the red channel, but the more ideal thing to do is to get your exposure up, or use a red filter when you shoot.

Digital cameras are the least sensitive to red light, and in daylight WB, the red channel has the most noise in white/grey subjects, especially in blue subjects.

[BFoto]

Could you open the image in PS and adjust the blue channel just to see what happens? Do you get a similar increase in noise? This would help you decide if it's a LR issue or a blue channel issue.
[a href=\"index.php?act=findpost&pid=113391\"][{POST_SNAPBACK}][/a]

I get the same thing on a 5D, sunny day at ISO 100 Tv 250 as well. Any ISO over 400 is useless in LR.

I should not have to open a 100 ISO file in PS. Very annoying.

[61Dynamic]

Depending on the image content this happens with any of the sliders. What is happening is the difference between the slider you are adjusting and one of the adjacent sliders becomes too great. The solution is to move the adjacent sliders as well. In your case try moving the aqua and purple sliders along with the blue.

I never came across this issue in the betas. This is a new issue with v1.0 and the addition of the new sliders.

[DaveCurtis]

Yes, using the adjacent sliders does help however the result is still extremely noisy. I processed the same image with Photoshop's channel mixer with no increase in noise at all.

I hope the next release of Lightroom sorts this out as the B&W workflow is great.

Dave
www.magiclight.co.nz