How can I create a layer that separates a photo into black, white, and a specified number of grays?

[rbanks88]

I want to use this to simplify the tones to help with evaluating eye travel. Most often I would use black, white and two or three intermediate grey tones.

I tried the posterize adjustment layer, but it appeared to have other things going on, actually creating more grey levels then I had set in the properties. 

Maybe something using threshold settings? I've looked around for an action that would create this layer, but haven't found anything. Of course, knowing the proper steps I could just record them as an action.

Thanks,
Russell

[D Fosse]

Posterize doesn't use more levels than you specify - but a photograph is usually much more tonally complex in reality than how you perceive it. The brain is very good at abstracting.

To get rid of overly busy areas you could try noise reduction at a very high radius value. If you do this on a duplicate layer, you can mask out areas where you want to maintain the original detail.

Really hard to say without seeing it.

[rbanks88]

It would look something like this. The very lightest areas are white, slightly darker values a light gray, medium values about 50%

black, darker values a dark gray, and then the darkest values black. So, the tones might be 100% black, 75% black, then 50%, 25%, and zero% (or white).

[George_F]

I wonder if you could use Blend-If here? 

Try a black solid color layer, a white solid color layer, and then maybe a few different shades of grey.  And then use Blend-If to isolate the range of tones you are looking for.

[rbanks88]

I've been playing around with Blend-if, without much success so far. However, I had not yet got to the point of making layers with different shades of grey, along with the black-and-white ones. I'll give it a try.

[George_F]

I used 5 layers when I tried it, and it seemed to produce the effect you described.  At least I think so.

One white layer, one black layer, and then 75% grey, 50% grey, and 25% grey.  

I hope it works out!

[AlanGilbertson]

As @D Fosse says, Posterize will give you what you specify. Set to 4 levels, you'll get grey values of 0, 85, 170, and 255. It can look as if there are more for the same reason the famous chess board illusion works, or this smoky one from a couple of years ago, but if you open the Info panel and drag the cursor around, you'll see that only those four values exist.

[rbanks88]

Thanks for clarifying!

[AlanGilbertson]

The follow-on from that is that if ou need to split the image into layers based on those grey levels, all you need is the Magic Wand tool set to a low tolerance (16, say) with Contiguous unchecked. Then you target the base layer and use Ctrl/Cmd+J to create a new layer with a copy of that part of the base.

[Trevor.Dennis]

Cutout allows up to 8 different colours or shades of grey, but does not force full black and full white.  That would be easy to fix with curves, levels or Camera RAW

[Semaphoric]

Another way to do it would be to use a Gradient Map adjustment layer, with a Gradient that puts stops "on top" of each other to give hard boundaries. These are super fiddly to set up.

You can see this one has a stop that's not quite right.

[Trevor.Dennis]

Now that's just showing off John. 😉

[rbanks88]

Lots of great ideas here. I'll play around with them and see what works best for my needs.

Thank you all for your help.  I do appreciate this community!