Today I learned a bit more about "chemical focus" and I am going to try to make it more understandable here.
Chemical focus is just a name even if it sounds your film or plate are going to catch fire or something, it's a convention to select focus on the blue image, makes sense if you need contrast. See the puyo lens is made to keep some chromatic aberration, there are two types of aberration, lateral and longitudinal, since the lens is corrected for spherical aberration it makes sense to consider the maker just kept the longitudinal, the image will be separated in different colors with different planes of focus, one after the other, "a la queue leu leu" (excuse my french), so if you focus on blues the reds will be blurry, it's a question of choice! If like me you experienced infra red photography you must be used to defocus a bit when you shoot infared film, some SLR lens even have a scale for this but on apochromatic lens there is no need for such a chemical focus.
Apparently Darlot/Puyo had a demi anachromat (like the P//S).
I can't wait to shoot color, it should produce color halos but no rainbow fringes (that would be a lateral chromatic aberration)
I hope this was helpful and anyone correct me if I am wrong!
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