Processing with Photoshop – Selective Sharpening

Selective Sharpening

When processing a photograph, with any software, the premier sharpening method is Unsharp Masking. Very effective but it has limitations. It draws the light colours into a star from the background to more concentrate the star, it works well indeed, but if done to any excess it produces black rings round the now brighter and bigger star, Panda Eyes it’s sometimes called, so it can be quite a coarse method. High Pass Filter is better but can give you a similar result. Any “global” sharpening method will produce similar results.

The problem is that you’d like to sharpen the nebulosity or dust lanes in a galaxy but not the stars, and sharpening the whole picture improves the nebulosity and galaxy arms, but ruins the stars.

But there is a solution.

This is in Photoshop, but any layer using software will do it.

With your photo on the screen, go to Layers and make two copies of it, they will be on top of each other, with the background layer at the bottom.

Make the middle one active and sharpen it as heavily as you wish in favour only of the areas you want to sharpen. In a galaxy photo sharpen the dust lanes to their best, ignore what this does to the stars (it will ruin and enlarge them badly). At the same time enhance the colours of the dust lanes or star forming regions in the galaxy, by enhancing the colours throughout, again being concerned with the dust lanes and nebulosity, ignoring the perhaps over colouring of other areas.

You now have a stack of separate layers, the bottom background, the middle excessively sharpened and coloured, and the top layer.

Make the top layer active, and with the erasure tool set to perhaps 25% opacity and flow, with a soft edged brush tool, carefully paint over the areas you want to highlight, dust lanes nebulosity etc.. Be very careful not to brush over the stars. You are actually rubbing out parts of the top layer to expose the middle layer in chosen areas.

If you have the History panel open you can step back to change what you’ve done if you want to change it. Use layer opacity too to smooth the image.

Finally, merge the layers, to produce a single layer.

It takes some practice and some enlarging and reducing the size of the image to feel comfortable with it.


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