Mask Gray in Photoshop

Question

I want to make everything that is 50% Gray in Photoshop trasnaprent. Black and White should be 100%, but all in the middle transparent.

Let's compare these to images to demonstrate:

Start:

Goal:

Yes the checkerboard is part of the image in order to illustrate which effect I want to accomplish

I would like to do it mathematically exact, rather than faking it.

Answer 1

I've made a video showing this very thing for another question, here it is at the relevant starting point: https://youtu.be/F6IhXVxBSQM?t=5m36s

Going off your image:

• Middle Gray is 128, 128, 128
• 1 stop Lighter is 191, 191, 191
• 1 stop Darker is 64, 64, 64

So adjust the Blend If slider accordingly:

If you want it to look more like your "Goal" though it's not actually only removing 50% than adjust the sliders to your liking.