alright, i thought this was a pretty straightforward concept and i knew all there was to know about it but i guess not?
here is an excerpt from the wiki page about this. its what i knew before, at least i think so?
If pictures taken from the same subject distance using the same focal length, are given the same enlargement, both final images will have the same DOF. The pictures from the two formats will differ because of the different angles of view. If the larger format is cropped to the captured area of the smaller format, the final images will have the same angle of view, have been given the same enlargement, and have the same DOF http://en.wikipedia.org/wiki/Depth_of_field
i read the luminous landscapes article and that is very interesting. it says that all lenses at a given aperture and subject framing have the same dof, that sounds wrong but when you read the article it makes sense, its a good rule to knowhttp://www.luminous-landscape.com/tutorials/dof2.shtml
but this is what throws me off, the picture is of 2 dof calcs, the only difference is the camera. dof for the 7d is shallower than that of the 5d when fl and distance are equal?
i guess that may be true in perceived relative terms but in absolute terms i think the wiki page sums it up correctly.