ATM Aberrations

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

Roger Ceragioli's note a while back got me back to working on
a long-standing problem.  I have been attempting to increase
the speed of one of my designs.  For this, I only decreased
the desired focal length.  However, I have encountered an
effect I did not expect.  I hope I can get some understanding
of it with the help of more experienced designers.

I have found that I had to modify one glass type (about 5%
difference in Abbe number) to increase the speed.

It appears, to me, the differences here are the thicknesses,
being based upon aperture, are larger relative to the focal
length and that the off-axis aberrations are emphasized more,
as the physical field size dimension is larger.

I do not recall seeing a direct reference to this effect in
any of my reading.  Any help would be appreciated.

                    Rick S.