I look at quite a few images online as I am an amateur photographer.
One thing I've noticed is that there is a pretty wide range of
abilities in being able to discern the differences between images.

Often people will compare two cameras on the same image.  And when
they do some people can see pretty significant differences and others
seem to be unable to see any difference at all.

In order to help the debates along a little I would like to find a way
to objectively or at least quasi-objectively be able to test the
ability to discern differences between two images.

Here are two that are currently being discussed on www.dpreview.com in
the sigma forum:
http://forums.dpreview.com/forums/readflat.asp?forum=1027&thread=29963586

The images are at:
http://www.pbase.com/franklin/image/105711528
http://www.pbase.com/franklin/image/105709356

These two images are from cameras with vastly different capabilities
and one would expect there to be differences simply base on the sensor
size, pixel pitch and number of pixels, yet some seem to be unable to
see the difference.  Aside from the blue sky and objects in the very
far background from around the grocery cart next to a car in the
parking lot and further back, the remainder of the pictures shows much
better granularity, edge sharpness and clarity in the latter image
than in the former.  It's not unexpected given the two cameras.  What
puzzles me is why some people seem unable to see these differences.

Is there somewhere online a test that would help objectively assess
the abilities of the participants in the convsersations so that they
could realize their abilities or lack thereof?

thanks,

Mike