That's an interesting statement. I thought every photographer knew ortho films speed changes with sun angle and latitude. Yes, the film is faster at the 50th latitude then at the 45th latitude. Sun angle is directly connected with the amount of red in the light, ortho film is insensitive to red, so the redder the light, the slower the film at that time.
How would I go about establishing a fixed number by experimentation? Do I set different ISO's on my meter, then take pictures, and compare the results against my little gray card by eye?
EI calibration is a big topic, and it depends on how much you want to delve into the Zone System. Here's a good place to start on the LFP site:
http://www.largeformatphotography.in...-meter-cal.pdf
I highly suggest reading Ansel Adams' "The Negative" back-to-front as well. But the extremely watered-down version is yes, set a few different ISOs on your meter (a.k.a., bracket) on the same photo and find out what EI works best for your setup.
Or expose for the mid-tones, develop for the shadows and agitate for the highlights - see Minor White.
Perhaps this sun angle thing is why I don't find BTZS to be accurate for x-ray film. My cold light probably puts out more blue light in testing while the sunlight here has more red.
Sorry, got dragged away by work things..
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Question of correct exposure is vast and outside of the scope of Xray imho.
In general Randy is totally right. Experimentation is key to get things to your liking. Not to someone else's liking. To yours. Xray is fairly forgiving and cheap, so why not just go and play with it.
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