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using a light meter as a densitometer - how?

grain elevator

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I've exhausted my google fu. I recall having read the answer somewhere on this forum, but couldn't find it again. Essentially my problem is just that I'm bad at math. I'm trying to roughly dial in a new film/dev combo, and I currently don't have access to the darkroom where I could just do it with test prints. I do have a light meter here and I just want to know the target value by which a zone VIII frame held in front of the meter cell should darken the meter reading. Is my math correct that a density of 1.2 equals about 4 stops? Thank you!
 
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Wouldn’t one need a rather stable light source too?
 
Why? It only takes a second to move the negative in front of the meter or away from there. Sure, the light shouldn't change during this time. I plan to aim the meter at a wall illuminated by my normal room light, same way I expose the test negs. It's all not very precise, but this is for roll film where each negative will have a different contrast range anyway, and they'll all be developed together, so the goal isn't high precision.
 

Apendix 2, in BEYOND THE ZONE SYSTEM book, has detailed explanations and drawings to make a densitomenter adapter for a Pentax spot meter.


Today you have a way better choice, just scan your film alongside an Stouffer T2115 density wedge, and compare gray levels in Photoshop or GIMP. Disable any image enhancing feature in the flatbed scanner, a cheap one will work perfectly for that. A low res scan is enough for that, but better is it is 16 bits/channel, scan all histogram, auto exposure mode may clip highlights and extreme shadows, so adjust scaned levels taking all histogram. With that you have total precision.
 
That's cool and all, but I was looking for a way to do it without any extra gadgets, and this does all I need.
 
If you have a spot meter and a light table, just meter the negative, then meter a blank frame and record the difference in stops. Every 1/3 stop is 0.1 log D. Or if your meter measures 1/2 stops then every 1/2 stop is 0.15 log D.
 
You got all good answers. As for stable light... I would use a plain light bulb, not fluorescent or LED because some light meters will think it’s a flash.
 
I found that reflections from ambient light gave me variable inconsistent numbers, using a Pentax spotmeter. Try doing it in a dark place with a single light source. A light table would be good if you have one. They are quite cheap.
 
Just curious I did a test using my Minolta flash meter VI and the calibration film that came with my densitometer. It seems OK.
 
I put the meter opening tight against the film when I used to do this. My densitometer works the exact same way and has a rubber o-ring to press against the film from above over its light-table base.
 
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