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Ed, "I mentioned that I had under exposed (about a stop) and then opened up that dark blue sky."

 

I was going to ask that very question from you, which you answered.

 

It think if you "opened up" that dark blue sky in PP, that is the source of the noise. Whenever I try to lighten anything in PS not matter the ISO, I get noise. So  generally I'll overexpose areas of a  scene such as the sky, if necessary, and then tone it down in PP, to prevent noise. I think you have a lot more latitude darkening over-exposed areas to the proper desired tone, rather than lightening them.  

My method anyway, and if I find sky noise for whatever reason, then I use the previous mentioned PP techniques to get rid of it- like luminance control. I've even used dust and spot removal in the sky, but that can look "too good" if over done. I like the Gausian Blur tip mentioned above too, which I'll try.

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I agree with you agreeing with me, Mr. Miller.  :)  It's one of the first things I learned when I moved into digital: if you underexpose and then open the shadows in post you get noise. Particularly with an underexposed deep blue sky. I was off balance and having a bad day.  :wacko:  Maybe being down there I was remembering 9/11.

Edited by Ed Rooney
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The super saturated blue in the sky is causing the problem. You will have this problem with any saturated colour, not only blue. You know that when you try to brighten dark shadows you get noise. A super saturated blue sky is bright only in the blue channel. The red channel is very dark. Therefore when you try to globally lighten the sky you lighten the very dark red channel as well, and get noise in the red channel.

 

A medium DARK blue super saturated sky will probably read 200 in the blue channel, 50 in the green channel, and 5 in the very dark  red channel. When you lighten the saturated blue globally you might get 250, 100, 55. The red channel has gone from a dark noisy 5 to a brighter noisy 55 and this causes the noise to become obvious.

 

Your software may have already tried to lighten the corners of your image because of underexposure in the corners caused by your lens. Particularly if it is a kit lens or you used a good lens at a wide aperture. This only exacerbates the red noise problem in the corner.

 

The solution may be to lighten the Blue and Green channel but not the Red. At the same time you may want to darken the red channel so that all of the red noise from 20-0 zeros out. This will change the overall colour, but is the new colour still pleasing ? Do the none sky colours read ok?

 

Another solution may be to run a median filter on the entire red channel, if the red channel contains no important information.

 

Another solution may be to select the sky and run the median filter on the sky part of the red channel.

 

If the noise is only in the corners, select the corners and run the median filter on the corners of the red channel.

 

When you lighten a saturated colour, be sure you do not push any channel beyond 255. If you push beyond 255 then you lose information in the highlights and you get banding.

 

This is one of the arguments against using a polarizer to saturate a blue sky. It will add noise to the red channel.

 

 

 

 

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Interesting information, Bill. Thanks. I'll take a look at the original files using your info. Basically, though, I will be careful not to underexpose blue skies . . . or anything.

Edited by Ed Rooney
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Ed:

 

The sky problem is mainly the high natural saturation, and not underexposure.

 

Any highly saturated colour will have noise problems in the other colour channels. I usually overexpose by 2/3 stop over the exposure set by the camera. I then pull back the exposure to normal, in post processing. At the time of exposure, I keep an eye on the 3 individual colour channels in the camera histogram to make sure I am not overexposing the most saturated colour channel.

 

If you overexpose blue super saturated skies in camera, you run the risk of blowing the blue channel. IE blue channel at 255. This gives false colour, (for true colour the blue channel should have been 275 but the raw file is not capable of that), and  no room to maneuver in post processing.

 

This often happens with the colour yellow. Even at normal exposure, yellow has very high values in the red and green channels and very low values in the blue channel.

 

There are two factors. The level of exposure for all channels combined (lightness), and the relationship of the exposure between the 3 channels (saturation and colour hue). You have to keep both factors in mind for a correct exposure you can work with in post processing.

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Ed  I'm not completely sure if this is in anyway useful to you but on top of the obvious noise reduction facility in LR5, what I was doing tonight involved 4 black and white 20 by 22 inch (approx) prints for a commercial sports outlet. A bit of 'grain' in B&W is always good but these were just too noisy. So in Elements 12, I exported in Topaz Denoise 5 and then using a mask, just brushed over the parts I didn't want de-noised as it were. You can tell I'm no expert, but that seemed to work well.

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It wasn't me who gave you that red ticket, Bill.  :o  Maybe it was an over-saturated blue sky?  

 

Those are big prints, Gervais . . . I can't even remember where I put my photo printer.  :blink:

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Nice info, Bill. I never knew that, but it makes a lot of sense. That is, one color may be saturated and exposed perfectly, yet the other two channels are essentially 'underexposed', and thus prone to noise.

 

Still, I am not sure why you would choose to overexpose a sky only to darken in PP? Seems like whatever existing, albeit minimal noise in the other two channels would not be a serious issue with the proper initial exposure.  Unless that's necessary to maintain proper exposure  with the main subject matter, then I see the preference to overexpose the sky and darken it PP.

 

Different exposures and PP stacking might be an alternative for stationary subjects.

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I would overexpose the entire image, without raising the levels of the saturated color channel over 255, in the hope of raising the dark not saturated color channels above the low level where noise would occur. Then there is no noise because all channels are higher. When I pull the overexposure back to normal in post processing, I am working with a raw file with no noise in any channel. Pulling the overexposure back to normal does not introduce noise. Therefore I get a noise free saturated image.

 

This is called expose the histogram to the right, and there is a more complete explanation here:

 

http://www.luminous-landscape.com/tutorials/expose-right.shtml

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In my case the sky was indeed a bit underexposed in the affected images. Also, I was using the 18-55mm kit lens for convenience (still waiting for the 16-70mm Zeiss to become available) which has quite a bit of vignetting and distortion. It looks like the lens profile in Lightroom increases the noise and weird patterns in the blue skies further.

 

I managed to get acceptable quality by adding a touch more Colour NR and Luminance NR in Lightroom. A first small batch passed QC this week.

 

Thanks for all the interesting thoughts and comments, certainly helpful!

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