Steve, I think I understand what you are asking and your query rings a
bell. I think, but I'm not positive, that the Minox sub-miniature people
have discussed that very issue. If you can find a special-interest site
Minox people, you might ask there.

Are you restricted to a particular format?

You mean a film that has great underesposure latitude? Tri-X is pretty good.

underexposure means overdevelopment.
but the quality will drop..

If you take the time to learn how to use it you'll find that the Kodak Tmax
films are very forgiving about being misexposed. That means it deals well with
both over and underexposure.

I wouldn't call it a properly adjusted print, but I've made a recognizable
print that just lacked shadow details using Fuji Neopan 1600 exposed at
12,800 (3 stops underexposed) and developed normally in Xtol, diluted 1:3,
scanned on a Polaroid SprintScan and then printed from a digital file after
adjusting the curves in Photoshop.

  What exactly do you want to do?  If you want thin negatives because
they are somewhat sharper you need to reduce _development_ not
exposure. Actually, exposure must be increased somewhat to compensate.
The resulting negatives will be low in contrast and maximum density
but the image will be confined more to the surface of the emulsion
resulting is better sharpness.
 If you are asking about _pushable_ film the answer is that no film
has much underexposure latitude. Tri-X roll and 35mm film is a
traditional pushable film. You must use a developer which maximises
film speed. Good developers for pushing any film are Xtol, T-Max and
T-Max RS, Ilford Microphen and DDX (liquid concentrate Microphen) and
other Phenidone developers.
 The reason that negatives which are developed for a limited time are
sharper than those developed more is that the development extends
further into the emulsion as time is extended. When the exposure is
made the light is scattered by the emulsion (called irradiation) so
the image at deeper levels is somewhat spread out compared to the
image at the surface. Thin emulsion films eliminate much of this image
spread. Thin emulsion films can be developed more fully without loss
of sharpness.
 Many modern films have a single thin coating. T-Max films are an
example. In fact most films made by the major manufacturers have much
thinner emulsions than those of about pre-1960, even those with double
coating.
 If I haven't addressed your question please post again.

--
Richard Knoppow
dickburk@ix.netcom.com