Shooting in the Adirondacks (20 years ago), dense forest canopy gave even subdued light, extreme macro (Canon 70-210mm, f/4 plus FD50 extension tube) Now that makes sense... extension tubes will cut down a good 2 stops on available light.... You may want to do without extension tubes, for this test, make it as normal (if there is such a thing) a shoot as possible. for purposes of comparison between emulsions, should I constrain shutter speed or f-stop? Not sure about that one MGD.... FWIW.... f/16 @ 1/10 is the same as f/8 @ 1/20 ..... I really don't think that DOF by changing aperture will effect color as both of those settings above are letting in the same amount of light. You're just creating a more shallow or narrower DOF, but then again I could be wrong. shrug

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