## How do I use a densitometer to get my EI and dev time?greenspun.com : LUSENET : B&W Photo - Film & Processing : One Thread |

I's shooting 35mm film and using the Leitz V35 (a diffusion enlarger). So what do I do? Look for the first zone I exposure that gives 0.1 over fog base? Ok, then I look for a matching pair where I get 1.0 in the highlights (zone VIII)? How do I calculate the gamma then? Is the gamma where zone V comes in at? Why does one book say look for 0.06 density and other books say for 0.1?

I guess I need a little help.

Right now I've got TMAX where it gives me: zone I = 0.1, zone V = 0.7, zone VIII = 1.0.

-- Russell Brooks (russell@ebrooks.org), July 28, 2001

I think you shoudn't worry about the difference between the 0.06 and 0.1 zone I density mentioned in your two books. 0.1 is alright though, I think you should extend your development time to increase your Dmax (since you are printing with a diffusion enlarger). I would anyway do some test prints before changing anything, the goal is to get your negs to be printable on YOUR enlarger and YOUR way of working. Don't worry too much about the rest...

-- George Papantoniou (papanton@hol.gr), July 29, 2001.

Thanks, but I would like to know how to calcuate this gamma value I hear so much about. Yes, I know in the end I have to be happy with the results, etc, etc. But indulge me in letting me learn how I make this gamma calculation.

Also what are you saying about developing for a higher dmax? I am developing for 1.0 in the highlights right now. Are you saying I should go for something higher? And I always thought that you look at this gamma thing to find if you have the right contrast for diffusion... Something like .7 gamma for diffusion and .6 gamma for condensor.

Can anybody explain how to calculate this gamma for me? thanks...

-- Russell Brooks (russell@ebrooks.org), July 30, 2001.

gamma as discussed in photography is a measure of the rate of change of the film's density with respect to exposure, if you look at a film's density curves (you can find some for tmax on kodak's web site) gamma is the slope of the line (it's a 1st derivative if you know a little about calculus) on the graphs.... note also this shape depends on developmentgood luck

joe

-- Joe Holcombe (joe1013_@excite.com), July 30, 2001.

Come and have a look at my website, I have data on TMX and TMY on Zone System calibration.http://i.am/very.lonely

-- Lonely Boy (lonelyb@yahoo.com), July 31, 2001.

>> Thanks, but I would like to know how to calcuate this gamma value I hear so much about. <<Hi Russell. Gamma is actually the slope of the "straight part" of the characteristic curve. You don't have enough data points to know if it's "straight" or not, so you can NOT actually confirm gamma. However, I'll describe how to figure it. BTW, there is no guarantee that a given film will have a straight line portion, which has something to do with Kodak's invention of a concept called "contrast index" quite a few years ago.

I'm not a zone system guy, so bear with me on definitions. I presume you consider that a Zone I exposure produces only a very small film density above base + fog density (note: 0.06 or 0.10 are both fairly small densities). Second, I presume that each additional zone number doubles the previous exposure. Everything else I say is based on these assumptions, so if these are wrong, disregard any further.

Assuming the above, here's what we can do:

To calculate slope of the characteristic curve, you need to have compatible units for both axes of a graph. Traditionally, in sensitometry you would use density of the negative for the vertical axis and the base 10 log exposure for the horizontal axis. You already have density values for your negative. Don't get scared about the other term; It's just technical mumbo-jumbo whereby each f/stop exposure change is equivalent to a 0.30 change on the log exposure axis where more exposure makes the number get larger. It can be a relative scale for your purposes, meaning that you can just make up a number for a starting point.

I would suggest you just call your lowest density patch (Zone I exposure) an exposure value of 0. If you do so, the doubled exposure (Zone II) has a log exposure value of 0.30; doubling exposure again (Zone III) is log exposure 0.60, etc, etc. So Zone V is equivalent to log exposure 1.20 and Zone VIII is like log E = 2.10.

Now, IF Zone I to Zone V were a straight line response, the gamma would be: change_in_density / change_in_log_exposure. Your density changed from 0.10 to 0.70 = 0.60. At the same time, your log exposure changed from 0.0 to 1.20 (Zone I to Zone VIII). As Joe H indicated, gamma (provided there is a straight line response) is density change divided by log E change = 0.60/1.20 = 0.50 for a gamma value.

For your second set of data points, Zone V (log E =1.20) to Zone VIII (log E = 2.10), you have density change of 1.00 - 0.70 = 0.30. This divided by the log E change of 2.10 - 1.20 = 0.90 gives 0.30/0.90 = 0.33 for the gamma value.

Now, clearly the the two "gamma values" of 0.50 and 0.33 ARE NOT THE SAME; this proves that they are NOT on the same straight line and cannot both be legitimate gamma values. It sounds very much like either A) your response curve (slope) is "shouldering off" on the top end or B) your zone exposure system (or my interpretation of f/stop film exposure changes per zone) is not legitimate. I really doubt option "A" unless you are really doing what I would consider underdevelopment.

I hope that this is more helpful than the other responses; feel free to email if necessary.

-- Bill C (bcarriel@cpicorp.com), July 31, 2001.

Russel, just to add to Bill's wonderfully clear and concise explanation (thanks from myself as well, Bill) I've readnetdensity recommendations for Zone VIII of 1.3 to 1.35 for diffusion enlargers and 1.2 to 1.25 for condenser enlargers.

-- Bong Munoz (bong@techie.com), August 01, 2001.

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