I just bought a used digital color densitometer (a Cosar 75/76) to use for zone system testing. How do I use it for assessing black & white negative densities? There are settings to measure densities for red, green, blue, and red-green-blue combined. No setting for B & W. In addition there are separate settings for E6 and C41 processes, each of which covers the same color choices. What do I use? Can anyone advise?

And the same question applies to assessing B & W reflection densities.

Thanks

-- Alan Shapiro (ashapiro@yorku.ca), September 25, 2000

Answers

Use the combined/neutral setting.

In special circumstances, such as getting rather brown negs from Tech Pan in some developers or if you use a pyro developer, using the blue filter setting can give you a more useful reading of printing density.

-- John Hicks (jbh@magicnet.net), September 26, 2000.

Thank you John. Perhaps I could ask a follow up question. My densitometer did not come with calibration strips. I have been calibrating each color and the combined red-green-blue using the same black & white negative with known density. Is that sufficient or should I seek out a color standard and calibrate each of the colors separately?

Thanks again.

Thanks.

-- Alan Shapiro (ashapiro@yorku.ca), September 28, 2000.

> calibration

Neither did any of mine. I started out looking for a step wedge and what I could find was ridiculously expensive. Maybe not ridiculously for the product, but far more than I wanted to pay.

At any rate, I talked with Fred Newman at Darkroom Innovations / The View Camera Store and obtained one from him. It's a step wedge he contact-printed onto Tech Pan, then read (and notated) with his freshly-calibrated densitometer. I don't recall the price, but it was reasonable and imho it's sufficiently accurate. Over several years I've occasionally gone through what amounted to a "reality check" with Ctein and another friend who has a densitometer and the numbers from my readings are usually within 1/3 stop of theirs.

I set each channel to read zero with no target, then read the densest block and set each channel to match Fred's reading for that block. I usually then check a midrange block just to be sure that it's reading is close to Fred's; since none of these instruments are "perfect" it's never an exact match but qualifies as being mighty close.

Then to read a test strip I zero it on an unexposed piece of film leader, then proceed to read the test exposures. That saves having to subtract the base and fog density from each reading when plotting.

The only color filter I ever use is blue, and that's only when a neg has a significant brown or yellow-green tint.

Your densitometer may operate differently, especially in the calibration procedure.

-- John Hicks (jbh@magicnet.net), September 28, 2000.

BTW, Fred can be found at:

http://www.darkroom-innovations.com/

-- John Hicks (jbh@magicnet.net), September 28, 2000.