accuracy of Polaroid tests for determining exposuregreenspun.com : LUSENET : Large format photography : One Thread
I've been photographing interiors on EPN after getting the exposure right on Polaroid 54. I make 3 identical exposures. Well, in almost every case, I end up having to push the EPN a half or full stop to get the same look that the Polaroid had. (And push-processing introduces a magenta bias.) Can anyone explain what is happening here?
-- Stewart Ethier (firstname.lastname@example.org), July 16, 1999
ASA (ISO) ratings are not rigid scientific standards..rather subjective guidelines. (this is why zone system calibrations require that you 'set your own asa'). Also different film emulsions respond differently to different light sources..and will actually have different ratings with different light sources. I test on Type 55 so I can use the neg for a critical focus check. Type 55 has a daylight (or strobe ) rating of 50, but it is rated @ 32 under tungsten light. So I will establish a base exposure based on this. When I shoot film I will translate this to the speed of the emulsion I'm using LESS the total filter factor of the filter pack that my color meter suggests. I don't recall off hand which emulsion EPN is..but lets say its the tungsten 100 film. A good looking test,in an interior with tungsten .. with Type 55 (rated @ 32) results from, lets say, an exposure of f22 @ 15 sec. I'm using a filter pack on the camera with a total filter factor of 1 stop (to keep the math simple). My starting exposure for the 100 speed final film would be f22 at about 22-25 sec. I'll keep the same f stop so the depth of field dosen't change. I'll then do brackets (by changing the time) in 1/4 to 1/3 stop increments which will be biased towards the over or under depending (mostly) on how dark or light the overall scene is..and what kind of contrast ratios I'm looking at. Depending..I'll usually do 4, maybe 5 brackets. Film is cheaper than coming back to the location. I never push process film shot under mixed light (unless I'm looking for funky results. Polaroid is (unfortunately) not always 100% consistent, either so I always cover myself. The emulsion I choose for interiors depends not only on the overall color temp of the scene..but also the light level. With the exception of Fuji Astia, daylight transparancy materials have lousy resciprosity...and I often use a tungsten emulsion with a heavy daylight camera filtration (and lots of UV filtration).
Windows??..depends. They can either go blue for a nice effect, Or (if you want natural looking outsides) you can gell them to whatever tungsten temp you're working at, you can gell the interior available light sources to match the daylight temp of the windows, and match with camera filtration.... or light the interior with strobes or HMI's ( maybe gelled a bit to match the window color temp)
-- C Matter (email@example.com), July 16, 1999.
EPN is daylight balanced, ISO100 film. I dont know of any tungsten film from Kodak in a ISO100 speed.
-- Ron Shaw (firstname.lastname@example.org), July 16, 1999.
Kodak 100T; it is a relatively new film. What is happening here is that you are using two different films from two different manufacturers. You have unwitttingly been performing your own tests to see how the two films relate to each other in terms of sensitivity. Make notes and next time try either exposing for that extra half stop of sped when you make your real exposures or make your tests a half stop under. Also try using color polaroid (Type 59) and see how that film correlates to your transparency film.
-- Ellis Vener (email@example.com), July 16, 1999.
You didn't mention whether artificial light is being used or daylight. If light levels are low, reciprocity characteristics come into play. Those adjustments for reciprocity differ between films. Indoors with ambient lighting I might meter a chrome film for 1 second and would not adjust for reciprocity at that speed. When 1 second is metered and I'm using polaroid, I expose for 4 seconds. The factor grows as light levels wane.
-- Mike Long (firstname.lastname@example.org), July 17, 1999.
Be careful that you don't assume that reciprocisty characteristics for two different films are the same...they're not. I establish the base exposure with Type 55, visually. Then I carry a tech sheet (from inside the box) of the emulsion no that I'm using which will give me specifics as to how that particular emulsion no responds to various exposure lengths, both in terms of exposure compensation and color shifts...with reccomended filtration. This will vary a bit from emulsion batch to emulsion batch. It's of course necessary (in theory, at least) to ensure that all of the film I'm using for a particular job has the same emulsion no. In practice...especially in mixed daylight, flourescent and tungsten (of various color temps)...well
-- C MATTER (email@example.com), July 23, 1999.