Determining exposure indexgreenspun.com : LUSENET : B&W Photo - Film & Processing : One Thread
Yesterday I read a very interesting article from a different site on how to calibrate black and white negatives to find a new E.I.
I have tried emailing the author and publisher, but I haven't received any response back yet from him, so I'm thinking maybe I could ask here?
This particular lesson describes to use hot lights as the main light source for measuring an 18% gray card. I have a novatron set and was wondering if incident readings could be made on all 4 corners, and center, of the gray card to determine the light exposure? The reason is, I'm using my hassy to do the tests with, using Plus X pan pro, and there is no reflective meter in the prismfinder. Can incident be substituted for a reflective meter under this circumstance? I was thinking of performing the test outdoors, but that could lead to inconsistencies in case of a cloud cover or change in the daylight itself.
In this lesson, it was also recommending the use of a terry cloth that should resemble in tone with the gray card, I don't understand why I need this if I have a gray card standard?
The test is performed by making an exposure with the cap on for film+fog, then the gray card on frame one, then after this, the stepwise exposures showing a portion of the gray card and a black background. I have also realized that this test requires 13 frames to be used for the test, hassy has only 12 frames and I don't want to waste a second roll for only one extra exposure. Could I use the film rebates as means for determining film+fog instead? One site seemed ill-advised and against this method because of the film edges being darker than an actual blank frame.
If anyone would know if what I mentioned above is possible with the metering and other stuff would be doable, please let me know.
Thanks for all your time.
-- Rob Danylieko (rj is firstname.lastname@example.org), January 28, 2002
Watch out for hot light sources. Unless your regular shooting is under similar hot lights, or if you know your light meter very well, I recommend to use sunlight or fluorescent light sources.
This is because hot light has higher spectral intensity in longer wavelengths, and most photodiodes and photoresistors are most sensitive to infrared region if no compensation filter is placed. The overall response pattern varies with each meter design. With many light meters, it is very easy to underexpose (or overread) with hot light sources.
With 120 format, no part of the film should be exposed to the ambient light when loading/unloading the film, so I see little reason to avoid using the leader/trailer as the base+fog reference.
My scanner has a lamp house for transparent material, and can load and scan 9 6x6cm frames or 24 35mm frames at a time, and my scanner driver allows me to void automatic adjustments. With some reference density steps, I suppose I can use it as a densitometer and make decent measurement fairly quickly. (I haven't started doing it yet.)
-- Ryuji Suzuki (email@example.com), January 28, 2002.
Thanks Ryu for the information.
I had an idea about using novatron strobes and taking incident readings from them, since I don't have a reflective meter in the Hassy. Is this possible? The Novatrons are 1000ws and I could use the modeling lights to see if the gray card is fully covered with the lights.
Thanks for your time.
-- Rob Danylieko (rj is firstname.lastname@example.org), January 28, 2002.
Answering only part of your question re: recommendation of terry cloth with gray tone - I believe this is to find out if your Zone III exposure of the terry cloth (i.e. 2 stops below meter reading of Zone V) results in a proper Print III value when exposed on your chosen paper at the reference standard printing time. That is, the print should show full detail & should represent the "full detail in shadow" that is the accepted definition of a Zone III exposure. However, it is not essential that the terry cloth be gray, as the brighter/darker ambient light conditions might make a lighter/darker terry cloth necessary to accommodate the available shutter & speed combinations to achieve the total number of tones you want to test for.
-- john l bagtas (email@example.com), January 28, 2002.
Dear Rob, If you are going to use an 18% gray card, flash, and an incident meter, you can do the test accurately. However use of the incident meter REQIRES a 18% gray card. The indident meter should , theorectically, match a reflective flash meter of the same gray card. If you use a terry cloth that doesn't reflect 18% grey use of the incident meter won't match the reflective meter reading and won't be accurate. CAVEAT! One important variable you need to test for daylight shooting is the accuracy [and perhaps consistency] of your shutter speeds. The flash test wont do that. I suggest doing both. My bet is that you will get one EI for flash and one EI for out doors unless your shutter speeds are very accurate. John Elder
-- John Elder (firstname.lastname@example.org), January 28, 2002.
Dear Rob, One other piece of advice. When focusing, focus on infinity! Fill the frame with the gray card. Focusing on infinity will eliminate any "bellows facter" skewing the results. I have done the test many times for many films and formats. Feel free to contact me directly.
-- John Elder (email@example.com), January 28, 2002.