Kodak has CI, Ilford has G, And Tetenal has Beta. I know roughly how to calculate the Kodak and Ilford, but what is the formula for Tetenal's Beta? And before somebody tells me to look and Tetenal's web site for it I already have and cannot find it...

-- Russell Brooks (russell@ebrooks.org), December 07, 2001

Answers

Greetings,

The Gamma, or Contrast Index, should be the ratio of exposure to density, per given film development. I'm not familiar with Tetnal's Beta, but assume it's the same thing. If it is, then you would graph the density vs exposure (using Log units) and find the slope; the slope is the contrast index or gamma.

While Tetnal's WEB site may not be of help, JOBO-USA is the US destributor and they have very good technical support. If I remember correctly their WEB site is JOBO-USA.COM and you can find an e-mail address for their technical support (I think it's tech@jobo-usa.com, or something like that.)

Regards,

-- Pete Caluori (pcaluori@hotmail.com), December 07, 2001.

Beta has nothing to do with Tetenal, but is a standardized expression of the the relation of the amount of light and the "blackening" of the emulsion. The term has been used in sensitometry for a long time. That's how you find it: On a standard graph of the film you determine "point A", 0.1 log. densities above base fog. This determines -- as everyone knows -- the filmspeed. Then you determine "point B" on the graph, which received 30 times the exposure of "point A". You now can draw a triangle and the tanges of angle alpha is refered to as "beta". It tells you how steep the Line AB runs. Most film manufacturers used to state developing times for beta 0.7 and beta 0.55. The first was considered the optimum for diffusor type enlargers, the second the optimum for condensor type enlargers. With a contrast ratio of 1:30 you would get a negative that would print on a "normal" paper grade. 1:30 typically would be a landscape scene without direct sunlight. This was a far superior system then to state a gamma of around 0.6-0.65 which is most commonly done today, which will not give an adequate contrast negative for optimum quality. This all may sound a bit complicated in the description, on the standard graph it will become evident. Tetenal has excellent literature which explains sensitometry.

-- Volker Schier (Volker.Schier@fen-net.de), December 11, 2001.

Ok, that sounds like an explaination. So, if .1 density is zone I then 30 times more exposure is roughly zone VI, right? So the formula would be (zone VI density - 0.1) / 1.5 ? Or am I doing something wrong with the math? Thanks again...

-- Russell Brooks (russell@ebrooks.org), December 12, 2001.

Someone came back to me in email and confirmed the formula.
beta = (zone VI density - 0.1) / 1.5

I guess I have been underdeveloping my negatives a bit. Now I have to go back and test...

-- Russell Brooks (russell@ebrooks.org), February 25, 2002.