Hi Tony,
Fredfishe's summary is accurate. Here is a more specific answer.
Think of drawing with opacity in these terms.
If you are drawing with 50% opacity, each brush stroke allows 50% of the lower level through and 50% of the brush color..
So it ends up getting you another 1/2 way there geometrically i.e. first stroke 50%, next stroke is another 1/2 way there to 75%, then 87.5%, then 93.75%, etc.
With 25% opacity, then 75% is let through from the previous stroke or lower layer and 25% of the brush color so the progress would be 25%, 43.75%, 57.8125, 68.359%, 76.27%, 82.2%, etc
That's the math involved. If you have more questions or want more details just ask.
As for feathering, that is a different situation. This is basically the same as a Gaussian blur. If you set a Gaussian blur to 10 pixels, this is just the first sigma of a Gaussian curve and the blur continues out indefinitely in mathematics yet in Photoshop only to 2.5 sigma. So if you pick a feather of 10 pixels, it will be a Gaussian blur both out in in by 25 pixels.
That is both my understanding and experience yet is room for error on my part.
Hope that helps
John Wheeler [TABLE="width: 65"]
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