Thanks for the email Bart,
I will have a go with the equations.
Thanks James
>
> These formulas are the basic ingredients:
> The direct connection between focal length (f), distance from the object
> to the optical centre (o) and image
> distance from the optical centre (i) is given in:
> 1/f = 1/i + 1/o
> Normally, the distance scale (d) on a lens is measured from the object to
> the film plane:
> d = i+o
> Now we should know that the magnification ratio (N) can be defined as:
> N = i/o
>
> Some solid handwork shows how we can express o as the result from f and d:
> o = 1/2 * f * (d/f - SQRT(SQR(d/f) - 4d/f))
> We also learnt that i = d - o , so:
> i = d - ( 1/2 * f * (d/f - SQRT(SQR(d/f) - 4d/f)) )
>
> Now, the idea was to express N as a finction of f and d:
> N = i / o =
> N = (d - ( 1/2 * f * (d/f - SQRT(SQR(d/f) - 4d/f)) )) / (1/2 * f * (d/f -
> SQRT(SQR(d/f) - 4d/f)) )
>
> Nope, I haven't checked this. Try it, a 90mm (f) lens should do 1:1 (N) at
> a working distance (d) of 360 mm.
> You do the math :)
>
> One additional nastiness is that many lenses (esp. zoom lenses) use
> floting alements while focusing. Efectively,
> this means that the focal length of the lens depends on the distance you
> focus on. This effect is not present when
> you use a bellow of course.
>
> Or you could put a ruler in your composition and see how many mm's on the
> subject side fit into the 36 mm's that
> are resembeled by your focusing screen (allright, multiplied by 98% or
> whatissit).
tried that! its a pain for wide angles!
>
> Or follow Chuck's advice, and forget about the math and figure it out in
> an emperical way. :)
trying that too!
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