Sensitometry problemgreenspun.com : LUSENET : B&W Photo - Film & Processing : One Thread
Here's a good one for all you students of sensitometry. I am trying to jury-rig a sensitometer, as I don't have access to a real one. In my last job, my employer had an EG&G flash sensitometer; alas, we have parted company.
As a light source I am planning to use my trusty Metz 45CT-1 flash in manual mode. This will give me an exposure time of about 1/300 sec, which is within the range where no reciprocity correction will be required for the two films I am planning to evaluate. Unfortunately, it will also provide way more illumination than I need to expose my film, through a Kodak 10-inch step tablet.
Okay, here is my dilemma: I require about 0.008 MCS of light in the toe region of the film to expose it properly. If I measure the light outpit of the flash with a Minolta flash meter, is there a formula by which I can express 0.008 MCS as an f-stop/shutter speed combination? If I can do the calculation, I can make the required neutral density filters so that the film won't be grossly overexposed.
I should add that I know full well that this will not give me anything in the way of true film speeds. However, I can compare two films, one of which I have exposure/development combinations which work for me.
-- Terrence Brennan (firstname.lastname@example.org), October 01, 2001
My computations tell me that you will be looking for:
Shutter = 15 minuits Fstop = 1.4
MCS = Meter Candle Second (about 10 MCS = 1 Foot Candle Second)
Direct sunlight is about 8000 foot candles, thus about 80,000 MCS
You are looking for 0.008 MCS or about 1/10,000,000 of the sunlight for one second.
Full sunlight at ISO 100 reads about f 16 and 1/100 second.
Reducing this figure I get about 15 min exposure with a f 1.4 lens.
As far as using your meter as a flash meter you will need to look for.. gulp f 0.05 .. something your meter will not tell you.
You say that you want to determine your exposure on the toe of the film, where the film is just starting to respond. I would suggest you meter on the shoulder of the film, and use the step tablet to reduce the exposure for the toe. If the step tablet has say 20 steps each of 1/2 stop then it has 10 stops. This would mean that you could use 2^10 or 1000 times as much light. Then you would be requiring about 8 MCS and that would only be 10,000 times less than sunlight. This means that when your flash meter is set for ISO 100 speed flim you should see the flash meter say about f1.4 to expose the sholder.
Hope this helps
-- Ken Heflinger (email@example.com), October 03, 2001.