Depth of field is when an object is in focus and the rest of the scene out of focus(blurry).

(3d studio max)
1.Open your scene in 3d studio max.

2.go in RENDERING > render > render elements > add... > Z-depth > ok.

3.Render your scene. this will give you 2 images. the render(good) and the zdepth(black and white). save them both.

(photoshop)
4. open the render in photoshop. double click the background layer and click OK. (this unlocks it).it's now called layer 0. This is just to give you more control over your original image.

5. open the zdepth in photoshop.
6. in the zdepth image, select > ALL. then Edit > copy. (you can now close the zdepth image).

7. in the render image, go to the channels tab(same window as the layers usually).
8. click the arrow "|>" on the channels tab and click "new channel".

9. name it alpha1. and leave the rest like it is. shouldn't matter for now.
and paste the copied zdepth into this new alpha1 channel. Edit > paste.
10. make sure the RGB channel is active and the alpha is not. you can deselect w/e now.
11. go back to the layer tab.
* copy your render layer as a duplicate and hide it. If you ever want to add a diffrent DOF, you can use this layer.
12. Filters > Blurs > lens blur...
13. Depth Map > source > alpha1. it should allready look like its working.
14. while still in the lensblur window, click on the focus point of the image(preview).
-this will make the area where you click in focus and the rest blured out.
-anywhere works so you could do a more than 1 DOF with the same images. try more settings for the best look.
-the more radius, the more blur there is. when it's nice, click OK.