In article , wrote:
Thank you for understanding. It occurs to me that if the tool knows how to select at 128, it ought to know how to select at 64, 32, etc. All we need is to understand how the tools select at 128, and take it from there. Better yet, Adobe can build the selection option in.
Actually, it doesn’t "select at 128" per se. It draws the dotted "marching ants" marquee at 128, because that’s the point where something is 50% selected.
What it does is make a selection based on luminosity. Something that’s completely light is completely selected. Something that’s completely dark is completely unselected. Something that’s 50% light is 50% selected; something that’s 30% light is 70% selected; and so on. The selection is made smoothly and continuously; it does not center around any value. All selections draw their dotted lines at the 50% mark; you believe, because of the way the "marching ants" marquee looks, that everything above 128 is selected and everything below 128 is not, but that is not actually what has happened.
To make a selection around any arbitrary luminosity level:
1. Select the luminosity of the image (Control-Alt-~).
2. Enter Quick Mask mode.
3. Image->Adjust->Levels
4. Move the hilight (white) and shadow (black) triangles together, and set them both on the leuminosity level you wish to select around.
5. Exit Quick Mask mode.
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