On Fri, May 27, 2005 at 01:31:24PM +0100, Steve Jolly wrote:
> Mark Roberts wrote:
> >>>That's it: With K/M lenses the movement of the lever is directly
> >>>proportional to aperture diameter, with "A" and later lenses it's
> >>>directly proportional to aperture *area*.
> >>
> >> If that's true then it is quadratic, not logarithmic, no?
> >
> >Since the area varies with the square of the radius that'd be a "yes"
> >;-)
>
> Define A = area of aperture; R = radius of aperture, D_A = displacement
> of lever on A-series lenses and D_K = displacement of lever on K-series
> lenses:
>
> D_A prop A
> D_K prop R
>
> but
>
> A prop R^2
>
> so D_K prop sqrt(A)
>
> ie the displacement of the lever on a K- (or M-) series lens is
> proportional to the square root of the f-number.
>
> S
D_A prop A is true in all cases - it's specified to work that way
D_K prop R is true, empirically, for most lenses, simply because
that's how the simplest mechanism works. But there's
no requirement for it to be true, and I wouldn't be
totally surprised to find a few lenses that violated
this relationship.
