Originally Posted by
Kiwi7475
This is interesting because at least per the respective specs, the difference is not huge, I mean, Foma starts rolling off at 680nm and Rpx at 700nm, both about as sharply.
Of course by using a red filter you effectively amplify this because you're only letting red go through (I assume you mean a #25, which usually passes above ~580nm, of course the number depends on the brand, etc.). So if the incident light was spectrally uniformly distributed, then Rpx would get 20% more "light" in.
It's just interesting how much difference there is in the trees. I am not an expert in spectrometry but I thought chlorophyll absorbs red light (creating a low right around the ~680 nm in fact), but probably it also depends on the type and state of vegetation...
All this assumes that film linearity, development process and exposure have no significant say in the difference.
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