DOF @ a focal length

[T3D Peter Abrahams <telscope@xxxxxxxxxx>]

"But getting back to the original question...how out of focus images can be
made sharper without stopping down"

A photographer/technician was quoted:
" two lenses with identical focal lengths can have different DOF at the
same aperture"

I own a 'solid cat' camera lens, a catadioptric 800mm f11, US patent
3,547,525, designed by Juan Rayces and made by Perkin Elmer.  There was
also a ~600mm Vivitar model.  Physically, the lens measures about 70mm
front to back.  It has a corrector of rare earth glass, a mangin primary
that is over 50mm thick, a mangin secondary,  and two field lenses.  I
haven't used it much, but I found it had an extremely shallow depth of
field, even when shooting at a distance, and I asked around to learn how
this could be.  An optical engineer told me that was unlikely, since DOF
was determined by focal length and aperture only; and indeed the books seem
to bear this out.  

Another qualified individual told me that a lens with a secondary
obstruction can have a DOF that is effectively reduced, because the out of
focus airy discs become rings, and therefore broaden more quickly.  
Then, as my last post quotes: "the slow rise and decay time of leaf
shutters produces a more attractive and clearer out of focus image and more
apparent depth of field than a... focal plane shutter"

I still don't find these qualifications in the books.  I'm wondering what
might be meant by 'apparent depth of field'.  Does anyone have a handle on
how 2 lenses of the same focal length and aperture can have different DOF?
The answer would likely give a clue to the question of how to make out of
focus images sharper without stopping down.
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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