mysteryscribe said:
Since you have shot so much paper under glass tell me if im right that the paper has a differnt iso rating inside than it does outside.
Regarding paper's 'iso' rating (actually, it's EI, or Exposure Index; if you see me using that term, just know that I mean ISO): I've only done calibrated testing using daylight lighting.
When I did calibration tests today, using my Polaroid Land 800 and Arista RC grade 2 versus Ilford MG-RC-IV, I derived an EI of 2 for the Arista graded paper; the Ilford appears to be a tad slower, in side-by-side testing, using the same scene, lighting and camera settings.
When I calibrate for film speed, I don't get fancy and use a densitometer (heck, I don't even own one - except my flatbed scanner supposedly will do that). I shoot a daylight lit scene, composed of bright, medium and shaded subjects, at a variety of exposures, then review the resulting negatives. The negative that gives the best spread between blown out hightlights and dense shadows - that is, the one that places the medium tones well centered between the extremes of highlights and shadows - is the right exposure.
I'll then compare the metered light reading taken at that time with the exposure settings used to take that negative, and figure out the EI of the paper by setting my Gossen meter to the appropriate settings and see what EI it reads out.
For the Arista grade 2, I got a speed of 2. That is using daylight as the light source.
The problem with comparing the film speed of paper in daylight vs indoors is that as the color temperature (the color spectrum) of the light changes, it effects the overall sensitivity of the paper to the exposed scene.
I've made shaded daylight exposures, using this same camera, in the shade of my north-facing covered porch. There seems to be little or not reciprocity failure between brightly lit daylight and shaded daylight, using Arista grade 2 and my Polariod Land 800. The exposure times between the two sources of light have varied from 1/12 second to 15 seconds, with apertures varying from f/8.8 to f/35. My meter, and the resulting images, tell me that the paper is fairly linear in this broad light range.
So, any variation there may be between daylight and indoor light is not because of the intensity of light, but rather the color of the light. There's just not much blue/UV given off by incandescent lamps. Even indoor flourescent bulbs aren't supposed to give off UV (unless there's a flaw in the flourescent coating).
Therefore, to shoot indoors, one would have to conduct a seperate set of calibration tests, to be absolutely certain. But, it sounds like you've found a handy way of compensating your exposure. I'll have to do more indoor shooting and see what results I get.
~Joe