>W.J.MARKERINK wrote:
>>FWIW, there are projection lenses that have an aperture, to compensate >>bulging, but they also have better performance closed down. Of course, >>you need at least a 250W projector. [...]

>In the good ol' times where projector lenses had apertures of 6 to 7
>or less there were no problems with bulging slides. Nowadays, with
>apertures of 2 to 3 the DOF has become so small that glass mounts
>(with all their drawbacks) are a must.

It is usually practical (assuming you can stand the brightness loss)
to add a black paper "doughnut" inside the rear of the projection
lens to stop it down about one stop for a bit of additional DOF and
sharpness improvement.
Hope This Helps