Sensitometry to RGB? : LUSENET : Large format photography : One Thread

Hi All:

I am doing some Zone System Testing, and due to lack of a densitometer, I asked my local camera store if they could read my developed negs for density. They were kind enough to do so, but the values they gave me were in RGB.

They are as follows:

Film Base + Fog = R6G7B7

Zone V = R119G119B119

Zone 0 (-5) = R16G16B16

Zone I (-4) = R28G28B28

Zone II (-3) = R46G46B46

Zone III (-2) = R72G72B72

Is there any way to convert these numbers into regular film density values?



-- Richard Coda (, October 25, 2000


The short answer is no.
I hope you didn't pay a lot of money for this service, because without knowing the characteristics of the scanner that these readings were taken from, the data are absolutely useless.
For a start, the tone values have been inverted, so that the clearest part of the film is giving the darkest RGB readings.
Secondly, there is no reference from any of the RGB readings to a real density value.
Thirdly, you don't know if any 'gamma' correction is built into the scanner software.
Fourthly, most scanners set an automatic white and black point when scanning negatives, so even the operator probably doesn't know what density values these relate to. (From the very low values of the so-called 'base+fog' readings, it's almost certain that an artificial black level has been set).
Finally, these values are from an 8 bits/channel display, which only gives a resolution of 255 levels. This isn't nearly enough to accomodate the 1000:1 or greater density range of a normal B&W negative, so an LUT (look-up table) has been used by the software to approximate real brightness/density values to the nearest 8 bit value.
All in all, as I said at the outset, the answer to your question is a resounding 'NO'.

-- Pete Andrews (, October 26, 2000.

This doesn't answer your question. But if you have an enlarger and an incident light meter you might try to find a copy of "How to Use the Zone System for Fine B&W Photography" by John P. Schaefer (1983). It has two chapters on testing film to find your own EI and creating processing times using the above equipment in lue of a densitometer.

-- Beau Schwarz (, October 26, 2000.

George Wallace, the man behind the "Expo-Disc", wrote an article for his website explaining how this can be done using a regular 35mm SLR with a built-in meter and one of his Expo-Discs. I no longer have a URL for it but have a copy stashed on my hard drive, in case anybody is interested in reading it...

-- Jeffrey Goggin (, October 28, 2000.

Try No updates during the last few years though!

-- Hans Berkhout (, October 28, 2000.

Unfortunately, that's not the site I was looking for ... George posted 16 or 17 of his articles on another site, including the one about using a 35mm SLR with an expodisc as a densitometer. Upon reflection, though, I don't think this article will serve Richard's purposes...

-- Jeffrey Goggin (, October 30, 2000.

George's articles, an old copy of them, can be found here. The articles were in a constant state of revision right up until his, May 09, 2001.

In answer to the questions about George's articles, an old copy of them can be found here. The articles were in a constant state of revision right up until his death last week. I hope to post his final work at some point, when the family is able to look for it on his computer.

-- Noel J. Bergman (, May 09, 2001.

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