using a light meter as a densitometer - how?

grain elevator · May 1, 2020

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!

Chan Tran · May 1, 2020

Yes! 1 stop is equal to 0.301029995664 density.

grain elevator · May 1, 2020

Thank you!

BrianShaw · May 1, 2020

Wouldn’t one need a rather stable light source too?

grain elevator · May 1, 2020

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.

138S · May 1, 2020

grain elevator said:
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!

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.

grain elevator · May 1, 2020

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.

ic-racer · May 1, 2020

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.

Bill Burk · May 1, 2020

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.

john_s · May 1, 2020

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.

Chan Tran · May 1, 2020

Just curious I did a test using my Minolta flash meter VI and the calibration film that came with my densitometer. It seems OK.

ic-racer · May 2, 2020

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.

using a light meter as a densitometer - how?

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