Double Exposure as HDR Technique?

[480sparky]

If you need to underexpose both frames in order to create a 'properly' exposed result, then how will you retain details in the shadows if they're totally black in one or both of them?

 

[Garbz]

To answer this question do a quick thought experiment:

How does a film camera record? We have a chemical substrate that reacts with light. The canister and the camera ensure it's always in the dark unless you "expose" it. The film doesn't know when it has been exposed and react differently after, it's a simple case of light hitting = chemical reaction. Morelight = more chemical reaction.

If we open the shutter for 1 second then we get 1 second worth of chemical reaction.
If we "HDR" the frame and open the shutter for 0.5 seconds and then 2 seconds we simply end up with 2.5 seconds worth of chemical reaction.

The result will unfortunately be a simply overexposed frame.