AA Sharpening - how early?

Started Feb 23, 2013 | Discussions thread
Astrophotographer 10 Veteran Member • Posts: 9,255
Re: AA Sharpening - how early?


Image processing in astrophotography handles noise first and artifacts second then sharpening early in the processing then selective sharpening later on.

If you sharpen an image it tends to emphasise noise.

Dim areas of an image are usually where the noise is. So if you intend pushing shadows you will bring up noise that will be exagerrated heavily by any sharpening process.

So my workflow would be the same as the previous post - noise and artifact control at the start then minor sharpening then colour processing then a selective sharpening once you are happy with correcting any colour bias, colour tweaking.

My image processing philosphy is:

1. Respect the light

2. Any image processing should be natural and like you never did anything.

3. Its better to get excellent data than to try to process a poor image into something decent. That is spend more time getting the shot right with an excellent image. Image processing then is totally easy. If you have to work processing hard then you messed up the capture stage. A well captured image is easy to process to a great result.

Ideal image processing looks like you did nothing. If the final image shows evidence of processing then you failed and were too heavy handed. I tend to pull back a processing step because of this.

Be subtle, modern image processing software is very powerful and highly manipulative. Its easy to slam an image into CSI Miami colours (which is fine if done for that effect).

Oversharpened images is a rookie error and looks terrible. Easy does it with sharpening and the minimum filter in Photoshop is one that should be used in particular, very sparingly. I generally use high pass selective sharpening/contrast enhancement using layers in Photoshop. I also use Deconvolution in the early stages of image processing (astro photos). Again sparingly.

There is a technique of multiple deconvolution layers blended together that can bring out amazing details in images in select areas of interest in the image. Again this is done early in the image.

Also with sharpening you are really wanting to sharpen the luminance part of the image not the red,green,blue colour part. That can actually be slightly blurred. You can separate an image this way by changing it to Lab mode in Photoshop. It splits the image into luminance and channel a and b (colour). So you could sharpen a bit more without simply amping colour noise this way.

A final rule. Finish the processing and leave it. Come back to it later or the next day and see if you still like it. You'd be surprised how often you go "yuck what was I thinking?" You regain your perspective and you can be too "in" an image after a while when processing it too long.


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