Posterization after processing

Hello,

I have noticed posterization--usually in the skies--in few of my photos after creating a vignette.

Most often I create the vignette using the rectangular marquee tool with feathering set at 250 px (I set this on or near the border of the photo, then Select>Inverse, Edit>Fill>Black>Blending Mode:Normal>Opacity: 100%. I then create a new layer, change the Blending Mode on the new layer to Overlay and adjust opacity as seems best.

(I know a vignette can be created using Filter>Lens Correction as well.)

I notice the posterization on screen and, to a degree, in prints and this happens in spite of my trying not to overdo contrast.

What am I doing wrong and how can I remedy this? Thanks for your input.

Phil
(P-J)

Further info:

I use an E-330, PSCS3, and iMac G5.

Thanks,

Phil

Sam,

Thanks for the input. Yes, I have been using JPEG. I thoroughly suspect that I have overdone the processing. I'll try it with TIFF and RAW and see what happens.

The vignetting example you included reminds me of many of the portraits that my grandparents owned. Very nostalgic. I'll keep your idea in mind.

Thanks again for responding!

Phil
(P-J)

With a JPEG you are asking a lot of a programme to move the pixels around when they are already so compressed.
Shoot raw then Photoshop.
But you are probably getting banding in the sky, I will let Wiki explain...
http://en.wikipedia.org/wiki/Colour_banding
Colour banding is a problem of inaccurate colour presentation in computer graphics. While in 24 bit colour modes, 8 bits per channel should be enough to render images in the full visible spectrum, in some cases there is a risk of producing abrupt changes between shades of the same colour. For instance, displaying natural gradients (like sunsets, dawns or clear blue skies) can show minor banding.

This again is a problem of not enough info in the file.
But would explain why you don't see it as much in a print.

Simon,

Both you and Sam are spot on in your analysis and Wiki describes exactly what has been occurring.

I didn't grasp just how much information I was in what I thought was a simple image adjustment. I will see how RAW holds up and try to be less heavy handed in the future.

Thank you both for your help.

Phil

The banding will effect the computer monitor more than the print so if its a small amount you will probably not see it in a print.