Kodak Sensitometry Workbook
Step 1 in the standard method is to be sure that the film has been properly developed.
This is done quite easily. Find a point on the curve that is 0.10 density units above D-min and label it A. Make a note of the log exposure at this point. Count over 1.30 log E units, and mark this point. Draw a line from this point up to the curve, make a note of the density at this point, and label it point B. The film is properly developed if the density at this point is 0.80 (± 0.05) more than the density at point A.
For example, if D-min is 0.18, then the density at point B should be between 1.03 and 1.13. Point A is 0.18 + 0.10 = 0.28. Thus, 0.80 (± 0.05) units above point A is 1.08 ± 0.05 = 1.03 and 1.13.
The problem I think I'm having is that my info is not in "log E" units... I am working off of the steps on the step wedge.
I think I could measure the density of the steps on the step wedge, and then run inverse logs on them to yield equivalent units to the Lux I'm looking for???
But that seems awful ... er.. contrived.
When I look at my graphs, assume that the steps are one half stop off of each other, consider that one full stop in exposure is equal to .30 change in density.. .. then it seems that my graphs tell me that my development in not "correct" .. . i.e. to get a .8 change in density above D-min I must go up almost five stops on the step wedge. When it should be something closer to two and a half steps.. ??? (but that then would seem to give me ridiculously steep curves, which are too contrasty.. no?) . . .
ugh... I can imagine why people don't do any of this jazz... it's either ALL nitty gritty, or it all unravels into ever more fine tuned questions.
Correction to the broken link to the Kodak article I keep referring to:
Thank you! Nice find.
Originally Posted by Pastiche
I'd be delighted to talk about the reading if you get into it...
Originally Posted by Anon Ymous
I think the article is rich with things that (for the relative newb like me) merit discussion.
Go ahead. Even if I don't understand something, chances are that someone else will have an answer.
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One of my first "?" is -
Originally Posted by Anon Ymous
how do I convert my light meter readings into Lux? or better, into milliLux as used referred to in the article.
The next thing to figure out is how to translate the step tablet density readings (straight from the Stouffer wedge) into exposures? (this is so that I can have a graph that does not have "steps" on it, but actual log Exposure units...
In order to perform some of the analyses the manual talks about I'd need to be able to count log exposure units. Right now, all I have (I believe) are density measurements of the Stouffer Tablet and of my test negs, plus the meter readings from when I made the exposures.
Lastly, when I made the measurements for the exposure I used my light meter as an incident meter. I set it to f/1, and fiddled with the aperture and height of the enlarger lens till it gave me a one second exposure.
I'm still not totally certain HOW that aperture setting was "right" ... if indeed it was right... but it was intuitive. I guess part of my mind thought - if the aperture is "1" then the lens is delivering all the light that falls on the lens to the film plane (this is probably poor reasoning, maybe the right action, but for the wrong reasons).
Because I was making a contact print, the only aperture to monkey with was at "the light source". . .. and not at the "film plane" at all.
Er.. back to Kodak's article on sensitometry:
I'd like to be able to read my curve and know, according to Kodak, that the film was "properly" developed. To do that I have to count an actual LogE change of 1.30 With my current system I can't do that b/c I don't know what the exposure is, in milliLux, that gets to the film behind each step of the Stouffer tablet.
Once I fiddled with development till it was technically correct, THEN it would be time to sort out "speed point".
In order to convert light meter readings to lux, you'll need to know the EV that the meter reads. EV0 is 2,5lux. For every 1 unit increase of EV, you'll need to double the lux value, thus EV1 is 5lux, EV2 is 10lux etc. EV-1 is 1,25lux. EV is exposure value. If speed is set at 100ISO, then EV0 is 1 second exposure with an f/1 aperture. Of course, that would be equal to 2 seconds with f/1,4 and so on. Now, if you want to convert lux to millilux, just multiply by 1000. Milli is a thousandth, like millimeter is 1/1000 of a meter.
Regarding the readings you get from your densitometer, are you sure they're not logarithmic?
(edit - I see you posted while I was off looking these up Thanks!)
not liking to simply whine
I found that an exposure of 1s at f/1 gives me a EV of 0 (zero). This info here.
I then found an EV to Lux (and vice versa) conversion calculator. Here.
OK.. so now I know that the amount of light falling on the step tablet was 2.5 Lux or 25,000 millilux.
Taking the log of 25,000 (here) gives me 4.4 (rounding up one tenth of one percent)
Everything so far is peachy, but then on page seven of the Kodak manual on sensitometry there may be a first "hiccup"...
In the section describing "figuring exposure" on page seven, the model used has a filter that takes out some of the exposure. In my process there was no such filter (well, the glass of the contact frame, which I have not properly measured... but which, until I have reason to suspect I need to measure, I'll consider to have no effect on exposure).
Now that I have exposure units I can work with, I'll have to go measure the steps on my Stouffer tablet to know how much each step reduces exposure.
I'd then subtract that much exposure from the total incident light, and THAT would tell me where on the logE scale I should plot each one of my density measurements.
Geez Louise... this might all end up as actual curves after all.
I'm with you everywhere but one place:
Originally Posted by Anon Ymous
How would the EV change if the ISO the meter was set to was "50"....
It's twice the amount of light... so would it simply double the EV? and that would mean an EV of -1 ?
And as to the readings of the densitometer... I'm fairly confident the readings are logs. I'm not certain, but fairly confident. Or, in other words - I'm fairly happy to assume that they are for the sake of seeing how the numbers fit the calculations the Kodak article asks of the reader to perform. So far, so good. Whatever language the Kodak article was written in (mathematically), my densitometer seems to speak it.
After looking around some more it seems that (this is an invite for anyone to correct me) EV 0 is 1s at f/1 for any film speed.
So EV 0 is still 2.5 Lux, which is still 25,000 millilux. The log of which is 4.4
OK... that's clear. Next step, get density values for the step wedge, and subtract them from the Log E numbers.