Question - Opacity slider in PS

[Michael H. Cothran]

Can someone please shed some light (pun intended) on this.
Say I have a black mask on a Layer. I paint an area on the mask with a white brush @ 50% opacity.
What I should have left in that area is black reduced by 50%, or in essence, mid gray. Correct?

Now, I make a second pass over the same area with the same brush @ 50% opacity again. Here's where my brain gets real fuddled.

Each time that I make another pass over this same area, am I reducing what's left by another 50%?
This is what I think, but am not sure.

It means that I could make an infinite amount of passes over the same area @ 50% opacity, but never arrive at a pure white in that area - always leaving half of what was there. The only way to achieve a pure white in a black mask is, at some point, to set the opacity to 100%. True?

Thanks for clearing up this non-earth-shattering-but-still-important-to-know issue.

[louoates]

That is true. For me the progression was 50%, 75%, 88%, 94%, and 97%. I would have carried this exercise out further but my brain hurts.
Please tell me there was a point to this topic.

[Michael H. Cothran]

My apologies for causing your brain to hurt, but thank you for the response. The "point" was that I never knew for sure. On the other hand, it could have been that two swipes @ 50% would have given a clear mask, meaning any other passes would have been futile.