“A Seemingly Impossible Picture”

I've spent most of the day (and a big part of yesterday) sepia-toning family photos for a project my Mom is doing.

I found a shot of my grandparents that doesn't seem to be... possible. How could this picture have been taken, when you can tell from their faces that there was noone else in the room (or perhaps in the world)?

That picture was taken at their fiftieth wedding anniversary party, a few years ago.