I bought a used Macbeth TD-504 from eBay and, as expected, it came with no manual. It seem pretty straightforward to use. What bothers me is that I can't get repeatable readings off of my negatives. For example, after I zero the densitometer on a blank frame and read a film test sequence, I will often get readings that vary by .03 units when I do a second pass.(posted 8330 days ago)Is this variance normal or do I have a dying beastie in my hands? I found someone who may be able to send me a copy of the manual but in the meantime, how long should I hold the turret down on the negative for a reading? If I hold it long enough (>5 seconds?) the reading will sometimes fluctuate. Is there a specific technique for lowering the turret onto the negative?