Inverse Vignetting?

[P3D Carrano, Allan <allan.carrano@xxxxxxxxxxxxx>]

I've recently started tutoring a 3D newbie, having sold her a 3.5
Realist that I had used several times in the past myself.  I shot
several pictures on a test roll (since the camera hadn't been in use for
awhile) showing the newbie how to use it.  Then I turned it over to her
to finish the roll.

When we got the roll returned, my shots were seen to be properly exposed
and in focus (naturally! :-)), but her shots had a variety of problems.
Camera shake, improper exposure settings, etc. were things I could
explain and correct for her, but there was one problem that I can't
explain:

There were three slides that appeared to be poorly focused in the
central area of the frame, but were sharp (or sharper) in the peripheral
area, a sort of inverse vignetting.  No there was no Vaseline in the
center of the lens.  

What I want to know is, is this a phenomena that is known and something
that I'm just not familiar with, or is this some very unusual behavior?
I intend to experiment, if there is no ready answer from the List, but I
don't care to "re-invent the wheel if someone can enlighten me."

Allan Carrano   


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