Resolving red subjects

[Poisson Du Jour]

There are several things that could be going on. All of the above are good points but you have to remember that Red/Green/Blue all focus at different planes due to the way they interact with glass. Coatings are applied to correct for this, and bad, missing or improper (old) coatings don't do a very good job.
PE

Coatings correct aberration??
If I recall correctly the composition and structure of the glass in a lens i.e. aspheric/LD/ULD/CaF2 (fluorite) greatly reduces (but can never exliminate) colour aberration: the falling of R/G/B rays at different points; I don't know where coating comes into the formula. I agree the problem with red is more peculiar to the film emulsion than the optic; it never worked for me on Velvia, only Kodachrome 200.

[Photo Engineer]

Fish of the day;

Please consider that different films have different thicknesses.

PE