Re: 35 mm tube IOR\'s
There isn't anything crazy about the accuracy difference between 1st & 2nd focal planes.
It exists due to optical design, construction limitations, and manufacturing capabilities.
It pertains to variables, but not fixed powers for the most part.
Think about the internals of both a variable vs fixed.
Deer season -rifle, starts tomorrow. Will pick this up later.
Happy Bullet Holes!