Question: I have a portrait with a blurred background. I'd like to use this to create a mask or for alpha channel or to erase the background.

So after editing I want to have an image with perfect white or transparency where was the background and keep the face unchanged.

Is it possible?

What I tried yet:

* made the image copy
* converted this copy B/W
* made the copy of this B/W layer
* blured this copy
* subtracted these copies:

[![enter image description here](https://i.stack.imgur.com/3FfUp.png)](https://i.stack.imgur.com/3FfUp.png)

So I hoped that *blurred blur* will be almost the same as *blur* and *blurred sharp* will differ from *sharp* parts of the picture. And the difference will be as an indication of the blureness. But I see that my idea did not worked for some reasons.

Any other thoughts?

Answer: 
A little confused about your question: are you attempting to keep the face and remove the existing background? Are you having difficulty making a decent selection? 

If the above is correct, you'll want to make a selection of the face.There are a multitude of ways to do that in Photoshop. In this case,you'll want to make a selection of the background. If the background is solid, you could use color range. Otherwise you could make a selection of the face and then use inverse.

I have found the select and mask option invaluable in refining my selections so that I don't have a halo or that pesky little color around the edges. There is a definite learning curve but well worth it.

Once you have made the selection hit "delete" on your keyboard and there you have it! You can then make a new document and do what you want with that background - make it white or transparent. 

Hope this helps.
As others pointed out, the best way would be to learn:

1. How to use masks
2. Make and enhance selections in Photoshop

This is **THE** major first step when learning how to edit an image, beyond the classic and global adjustments, such as contrast, luminosity, curves or B&W conversion.

Concerning your image, **Photoshop may have a *magic* tool to help you**:

> 
> Select > Focus Area
> 
> 
> 

It *automatically* select the image areas in focus and can create a selection from it. It is not perfect, but if your background is blurry enough it might do what you want in a single step.

Tutorial on adobe.com: <https://helpx.adobe.com/photoshop/using/select-area-focus.html>