"Is there any way to (stop-down) reduce the aperature in a mirror lens in order to expand the depth of field a bit"
In the case of most mirror lenses, no.
There is no way to stop them down for increased depth of field. As others noted, you can reduce the amount of light reaching the camera with a neutral density filter (usually rear mounted or fitted in a drawer in the rear of the lens), but that doesn't change DOF at all.
Two exceptions I'm aware of were the Konica Hexanon 1000mm f8 and 2000mm f11 mirror lenses from the mid-1970s. They were both fitted with a disk of Waterhouse stops that allowed the aperture to be changed. I believe they offered as small as f32 or f45 apertures.
In case you are unfamiliar with them, Waterhouse stops are simply a series of precisely-sized holes, each bored in a piece of sheet metal, that were individually placed behind or within a lens via a slot to effect a different size aperture. Waterhouse stops were used with barrel lenses prior to variable apertures and leaf shutters. If interested, there's more into about Waterhouse stops here. and about making a set if them for a vintage lens here.
At least theoretically, you might be able to make something similar for any lens lacking an aperture.
The two Hexanons have a series of these stops inside, all arranged on a metal disk, so that different size apertures could be dialed into place. They were also somewhat unusual using a bellows focusing mechanism behind the main bulk of the lens. Unfortunately, both lenses are long discontinued, sold for the equivalent of a small car when they were new, and now are quite rare. Best estimates are that a few dozen of the Hexanon 1000mm were made and sold. They remain fairly valuable, a friend of mine just sold a very complete and near mint 1000mm for more than $2000 US.
It is believed that only two of the 2000mm were made and none was ever sold. One was in a camera museum and the other was last seen displayed in the Konica-Minolta headquarters, before the corporation's camera business was sold to Sony (around 2006, if memory serves).
There may be some other mirror lenses that used similar arrangements to allow changes to f-stop and depth of field, but I'm not aware of any.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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