adding an aperature stop to a fixed lens

[Steve]

This may sound like a strange question, but is it possible to add an external
aperture stop to a fixed lens to increase it's depth of field? If so, how would
you do it?

The reason for this question is I have a very old Imacon Precision II film
scanner. I want to scan mounted slides with it, but when they made this
scanner, they used an 8W high CRI fluorescent tube and had to get a fast lens
because the light source was limited. This resulted in a shallow depth of
field, and you can't get good edge-edge sharpness with a mounted slide (because
of the curvature of the film).

I can make a new light source using high CRI LEDs, and stop down the lens, but
I'm not sure how to do it so the lens sharpness isn't reduced.

Thanks in advance.

Steve

[Phil Hobbs]

Post by Steve
This may sound like a strange question, but is it possible to add an external
aperture stop to a fixed lens to increase it's depth of field? If so, how would
you do it?
The reason for this question is I have a very old Imacon Precision II film
scanner. I want to scan mounted slides with it, but when they made this
scanner, they used an 8W high CRI fluorescent tube and had to get a fast lens
because the light source was limited. This resulted in a shallow depth of
field, and you can't get good edge-edge sharpness with a mounted slide (because
of the curvature of the film).
I can make a new light source using high CRI LEDs, and stop down the lens, but
I'm not sure how to do it so the lens sharpness isn't reduced.
Thanks in advance.
Steve

Once the aperture gets small enough that diffraction is important,
there's a tradeoff between DOF and resolution. On the plus side, you
win DOF quadratically as you relax the resolution.

1970s-era Kodak slide projectors cheat by matching the field curvature
of the projection lens to the average shape of a free-mounted slide.
(Fairly brilliant actually.) European projectors such as Leitz and
Rollei expected you to use glass-mounted slides, and so had a flat field.

So if you have only a few slides to scan, you could remount the slides.
There's some danger of spoiling them, of course, and you have to worry
about Newton's rings from the multiple reflections.

Aperture stops need to go at the pupil of the lens. Some lenses have
pupils outside the outer glass surfaces, but most don't, so this is
probably pretty hard.

On the other hand, it might be worth experimenting with an iris
diaphragm right against side of the lens with the longer conjugate and
see if there's a useful tradeoff of DOF vs. vignetting. A bit of black
Sharpie on the outside surface is a reasonable poor-man's version--it
comes off easily with a bit of methanol and won't leave a residue.

Cheers

Phil Hobbs