How to reduce posterization in dark BG?

[gail bisson]

I am struggling to find a way to reduce the posterization in dark backgrounds in properly exposed subjects . I did not touch the exposure slider on this image.
I process the RAW in DPP, following Arash's guide. This shot has ISO 800 so I set chrominance Noise Reduction to 8. I then move image to CS5 and select the BG and reduce the noise by moving the slider to 8 or 9 or 10. The noise is reduced but then the posterization creeps in. I have tried reducing the amount of NR or using Gaussian Blur or surface blur after NR and it makes it worse.
I have tried using Tonal Contrast in NIK Efex by moving the sliders for shadows and midtones into the negative and that helps a bit but I still see some posterization.Finally, if I use the levels slider on the left side of curve by only 1 or 2 points it really worsens the edges of posterization .
What can I do to eliminate this?
And does any of this NR and or posterization matter for prints ?
Thanks for any light you might shed on this,
Gail

[arash_hazeghi]

If you are using CS5's built-in NR on the background it will posterize it. That's why I never use or recommend Photpshop's native NR, even in CS6 it is garbage.

if you have a very good printer that can seperate subtle tones it might show posterization in prints, depends on the type of paper too. do a test print...

sometimes posterization only happens as a result of JPEG compression for internet posting, that has no remedy either but it shouldn't matter much so long as TIFF is OK.

[Don Nelson]

Gail
Conversion to 8 bit sRGB for web posting can often be the culprit for posterization, especially when the background is nearly the same shade. (there are few colors represented in 8 bit than the normal 16 bit colorspaces we use for editing).

A printing guru showed me how to avoid it in print (all the print rasterizers ...sometimes called a RIP...work in 8 bits as well even if you send them a 16 bit file). Solution sounds opposite of everything we've been taught during editing -- but its to use the Photoshop Add Noise capability. you add just enough noise to prevent posterization, but the noise itself does not show up on the print (and you may recall that we oversharpen files prior to printing to keep them from printing unsharp.

Regards
Don

[Don Lacy]

Gail, Not sure what camera you are using but if it's a newer 1D camera you really should not be having this issue at all in fact at iso 800 you should not need any or very little NR. My guess its something in your work flow thats is causing the problem if you want email me the raw file at lacydon@comcast.net and I will work it and see what I can do with it.

[arash_hazeghi]

you always need noise reduction on the background if you want it to be smooth, even with a 1DX or a Nikon D4.

[Don Lacy]

you always need noise reduction on the background if you want it to be smooth, even with a 1DX or a Nikon D4.

If you use selective sharpening through out your work flow even for capture sharpening you can get away with a very light amount NR on your BG. Arash your processing skills and knowledge are top notch how often have you encounter this issue with a properly exposed Raw file.

[John Chardine]

Hi Gail- Beautiful image by the way, and the posterization is quite subtle on my display anyway.

Emil Martinec from the Univ. of Chicago wrote a very good article entitled "The consequences of noise", back in 2008. That's five years ago but I still find it very useful to refer back to. Here is the link to part 1:

http://theory.uchicago.edu/~ejm/pix/....html#bitdepth

In one section of the article here:

http://theory.uchicago.edu/~ejm/pix/.../noise-p3.html

he discussed the interplay between noise and posterization and shows that in certain circumstances, noise can hide posterization in a process called "dithering". It makes sense that if you remove the noise you could reveal the underlying posterization. I have seen this myself in very plain parts of an image such as a blue sky after I have run aggressive NR, so it isn't restricted to dark backgrounds.

Here's a an earlier related thread from BPN:

http://www.birdphotographers.net/for...ation-Question

PS Don's post above is on the same subject

[arash_hazeghi]

Hi Don.

I use advanced NR on the background so I don't encounter this issue in my TIFF files even at very high ISO where you need stronger NR.

[Gary Kinard]

It creeps in on me with CS6 on occasions. As a hobby I really try to keep it simple, but it is starting to bug me to no end. I also de-noise most all of my backgrounds. No matter what ISO settings.

[Diane Miller]

Is the image being brought into PS from DPP as 16 bits? I haven't seen this problem when I reduce noise, but 16 bits gives you much more leeway to lighten dark areas before hitting posterization. Maybe the same applies to NR.

The other issue may be in what is considered proper exposure. If exposure is to the right (only as far as it is possible to recover highlights well in RAW conversion) a dark BG may be a little lighter than in a middle exposure and thus have more tonal values. Maybe then it would hold up better to de-noise procedures?

If you divide the histogram into some number of stops (the exact number doesn't matter) there is a dramatic decrease in the number of tonal steps going from the right-most division to the left.

[shane shacaluga]

Hi, can you explain what is meant by posterization?

Is that the blurring on the edge of the main subject that appears when you use PS NR on a backround layer?

[Don Lacy]

Shane , Here is a good tutorial on the subjecthttp://www.cambridgeincolour.com/tutorials/posterization.htm

[shane shacaluga]

Ahh now I understand

Facebook does that all the time to my photos!!!

Thanks for the link ;)