01-28-2015, 09:02 PM
The f number is defined as 1/2u where u is the marginal ray angle. It is only defined as efl/edp (effective focal length / entrance pupil diameter) at infinity where first order approximations are valid.
The entrance pupil diameter is not a particularly useful specification because it says almost nothing about the system. Look up the lagrange invariant and conservation of etendue.
field*fno is what drives design difficulty. if field*fno is the same across different format sizes, the lenses are equivalents.
The entrance pupil diameter is not a particularly useful specification because it says almost nothing about the system. Look up the lagrange invariant and conservation of etendue.
field*fno is what drives design difficulty. if field*fno is the same across different format sizes, the lenses are equivalents.