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Believe that for both 24bit and 48bit images when 1:1 cloning at a 50% transparency slider setting, the transparency is actually closer to 90% or 10% depending upon your point of view... in the illustration, it was expected that the cloning into either the black or white image would be a 50% gray:
50percent B to W 800px.jpg (61.26 KiB) Viewed 1587 times
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Changing the Amount to 50% with Transparency at 0% seems to produce the expected 50% gray.
The clone and other tools work by applying the brush repeatedly as you drag the cursor. Depending on the Spacing and the rate you move the cursor and the speed of your computer, brush applications will overlap so the effective transparency will be lower than what is set in the slider. If you just click once, you will see the transparency is actually 50% for a single application of the brush. To get a higher effective transparency, you need a higher setting.