I have a large roll of 70mm print film for doing some experiments.
Originally Posted by Photo Engineer
This material is very low in ASA. Making contact prints under the enlarger needs exposure times of 100-120 sec. with open aperture and a 150W enlarger lamp. RA paper is 20 times more sensitive.
The problem is that the normal exposure time at professional printing is very short (1/500s?). So you need a very very bright light source. With long exposure times, the Schwarzschild effect will multiply the required exposure time.
5-10 ISO could be a good value for highspeed printing. But when using a normal lamp, the effective sensitivity would be around 1 ISO or less. I calculated an effective speed of 0.5 to 1 when doing contact prints under the enlarger.
Maybe my material (Kodak 2383) is less sensitive because of age. I tried all kinds of developers, also self mixed ECP-2 (CD2).
The best and cheapest solution was using standard RA-4 developer (<1min dev.time), but C-41 will also work.