Archived posting to the Leica Users Group, 2000/05/21
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Mark wrote It's a graduated filter that passes in front of a blazing hot Hologon bulb. As you turn the dial the filter is like a wheel which moves to a denser or less dense part of its gradation If you use the same "number" of gradation (Say number 100) often that part will fade and you will have "bumps' in your gradation. ie 100 will be less filtration than 99 or even 98! - ----------------------------------- Actually, dichroic filters are not "graduated" and I believe they do not fade from exposure to light in a properly designed enlarging head. Dichroic filters work by selective optical coatings that only transmit a narrow band of the desired color light. So, a magenta filter is uniform, not graduated, and the amount of magenta transmitted is a function of mechanical movement of the filter into the light path of that hot halogen bulb. I can't imagine anybody could see the difference between 99 or 100 units of magenta, but, the errors in calibration are likely to be caused by the mechanical linkage and the color temp of the bulb fluctuating with age, even when voltage is regulated. Some high end enlargers use(ed) a feedback loop that displayed the actual color values according to an optical measurement or adjusted votlage to meet the expected settings. I get more variations from the developing process than from the old Beseler. (Which, I agree, is a ***** to align.) I have overhauled som Beseler color heads and can only say the less expensive models are mechanical and electrical compromises. I've had components, not the filters, melt down. The voltage regulation in the cheaper models is nothing more than a husky diode that keeps the max voltage at 82 volts unfiltered half wave d.c. My Beseler 45S Dichroic head is high quality in comparison except that the calibration is from the planet Mongo. Negs that print at about 55Y/45M on other enlarger brands require 110/89 on the Beseler. Now if I could just get the enlarger aligned... Bill Lawlor Bill Lawlor The Philips color enlargers used a more elegant method than moving filters. They used three fixed filters and varied the voltage on the three corresponding halogen lamps. A circuit board performed som magic to keep total intensity balanced, I believe. I worked with one in the 70's and it was neat!