How to achieve shallower DOF using multiple exposures?

Question

For static or semi-static scenes, there are many ways to "simulate" certain features actually not feasible on single photo using the technology the photographer has. For example,

• wider field of view by taking multiple shots in adjacent directions and stitching them into a panorama;
• deeper DOF by focus stacking;
• lower ISO by averaging multiple frames;
• higher dynamic range by merging differently exposed frames into a HDR image.

This makes me wonder, is there any way to use multiple exposures for creating shallower DOF at a given field of view, normally achievable with a faster lens or a bigger sensor. How to do that?

_{I know it's kind of stupid to start out with a hammer and then invent how to carry water with it... but I stil wonder.}

Answer

Yes indeed, in fact your first bullet point does that. By stitching a panorama you are simulating a larger sensor.

The effect works best when you use a telephoto lens and create a multi-row panorama with approximately the same aspect ratio as a regular photograph. This is sometimes referred to as the Brenizer method (after the person who popularised the technique) or a "Bokeh Panorama".

To convince yourself this works, thing of the last time you saw a telephoto shot (say 200mm) with a sharp background? Never (unless the BG was within the DOF, unlikely unless you shoot 1/128). By stitching several telephoto shots together you get the field of view of a wide lens, but a blurred background in every shot, hey presto shallow depth of field! Stan gives a very good more detailed description of the technique in the answer to this question:

What is Bokeh panorama (AKA Brenizer method)?

The opposite also works, if you want to simulate the deep depth of field of a small sensor camera use a very wide angle lens (like a 10mm) and crop a tiny bit out from the centre of the frame and you'll get the equivalent of a slight telephoto on a crop body (albeit at a much lower resolution).

Check more discussion of this question.