# Photoshop General Discussion

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## Why Doesn't Photoshop's Opacity Add Up?

### Nov 17, 2012 6:54 PM

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I've always wondered this but now it's beginning to bug me as I'm experimenting with layer stacking to reduce/modify noise.

If I have 2 layers at 50% opacity and a white layer underneath, those combined layers should equal 100% opacity and so I should not be able to see anything below the lower layer.

Instead what I find is that you have to stack over 15 layers at 50% before they even start to appear opaque (and at 15 they still aren't).

What's the math going on behind here? I would have assumed that, with an 8-bit image, you have 256 levels of transparency so at 50% you've got both layers at 128; 128-128 = 0 or no transparency so why does photoshop still allow something to be visible underneath that? While we're at it, why does 100% look fully opaque but 99% looks dramatically different without a layer underneath? Maybe the math is adding up but 50% opacity is actually more like 5% opaque?

Is there a blending mode I can use that will similate the build up of opacity more accurately? I'm really annoyed with the fact I can't really predict how layers are going to interact based on their opacity; I wish I at least understood the science behind it so I could alter my process to produce the results I want.

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Nov 17, 2012 7:52 PM   in reply to sixthcrusifix

Think about dividing a fraction to get to zero...50% of 100% is 50%, 50% of 50% is not 50% but 50% of the result of the previous blend (more like 25%). Hint, dividing a fraction by anything will zever end in zero.

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Nov 17, 2012 9:30 PM   in reply to sixthcrusifix

sixthcrusifix wrote:

That makes sense - I guess what I'm wanting is a way to have 2 or more layers bound to each other and be able to adjust the blend of them together but always in a way that makes that set have full opacity.

Just set the layer below at 100% opacity and you achieve that.

-Noel

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Nov 18, 2012 10:48 AM   in reply to sixthcrusifix

sixthcrusifix wrote:

For some reason I was thinking that the layer at 100% was having too much influence on the look of the picture.

You were correct. If one layer is 100% opaque and the others are less opaque, then the 100% layer will have more influence than other layers. You're not going to calculate a mean image that way.

I've scored through that because I misunderstood what was meant. I guess it will give a mean of the layers if each layer has an opacity percentage of 100/n, where n is its position in the stack, with 1 being at the bottom.

Ps CS6 and CS5.1 (and maybe earlier) Extended feature image stacks that can calculate a mean image.

With a standard version of Ps, floating point 32-bit mode can be used to calculate a mean image by summing your images and dividing by the number of images. Gamma adjustments in the process are required if your source images do not have a linear colour profile because Ps will convert them to linear on opening into a 32-bit document. Let me know if you want help with that.

Edit: see scored through paragraph.

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Nov 18, 2012 10:50 AM   in reply to conroy

That does not compute on my wetware, Conroy.  Edit:  I see you were editing while I was replying.

If the base layer is 100% opaque, and the layer above it is 99% opaque, which do you think will have more influence in the result?

Is there something unwritten being implied here that I'm being too dumb to pick up on?

Are you talking about stacking?  If you want to average layers, you do it like this:

Nth layer:  (100 / N) %

...

Fourth layer: 25%

Third layer:  33%

Second layer: 50%

Base layer: 100%

-Noel

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Nov 18, 2012 10:54 AM   in reply to Noel Carboni

That's exactly what I described in my edit

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