I make interpositives with an enlarger. My use an old Omega D2 4"x5" with an
Aristo coldlight head. In order to get reasonable (>5 second) exposure times
with TMax, I use an Omega dimmer, which is a rheostat (or some similar
device) that in the circuit between the 110v AC source and the light source,
in series with the timer. It can dim the cold light source anywhere between
one and eleven stops through a continuously variable range. So, I can keep
the enlarging lens at an aperture which gives good sharpness and dim the
light source enough to give a long exposure and thus avoid having to use a
shutter with the enlarging lens. I bought the dimmer about 7 years ago, for
$70 or $80. It is (or was) listed in the Aristo catalog. A very handy
device.
One other bit of hardware is very helpful for making interpositives. I have
an inexpensive darkroom light meter, which is a CdS meter optimised for use
under an enlarger. The CdS cell is 2 or 3 mm in diameter, so that I can take
spot readings off the enlarged image. I take a spot reading off the darkest
portion of the image (the densest portion of the negative) and expose so that
this area has a density of 0.3 to 0.4 in the interpositive; this insures that
the highlights in the interpositive are on to the linear portion of TMax's
D/LogE curve and within the scanner's sensitivity range.
Charlie Palmer