On Mon, Oct 30, 2000 at 08:09:43AM -0600, Franklin A. Berryman wrote:
> Thanks for the very lucid explanation. So a 90mm f2.0 Zuiko which goes to 1:2
> has 45mm extension (45mm/90mm), with a 25mm extension tube its go to 1:1.3
> (45mm+25mm/90mm), and with a 65-116 variable extension tube, it goes from
> 1.2:1 (65mm+45mm/90mm) to 1.8:1 (116mm+45mm/90mm). Now, how do I figure out
> the magnification ratios for the regular lenses from 135mm up. The sales info
> only give close focusing distances?
>
This is not too easy to calculate. It is necessary to iterate
or you`ve to solve quadratic equation.
In a book I found the following instructions to calculate
the extension for a lens for a given focusing Distance (FD):
1.) The needed formulas are based on this two equations:
FD = z + 2*f + zs [1]
f^2 = z * zs [2]
Explanation:
FD - focusing distance: Film plane -> Object
f - Focal length of lens
zs - extension of the lens, The value you want.
z - distance object -> 1x focal length before lens ´regard as just a value´
1. Iteration assume zs -> 0 :
Calculation of z with [1]: z= FD - 2*f is the first value for z called z_1
for now.
Calculation of zs with [2]: zs_1 = f^2/z_1
2. Iteration calculate a more accurate z value with zs_1:
z_2 = FD - 2*f -zs_1
zs_2 = f^2/z_2
3 .... and so on.....
With evera iteration you`ll get a more accurate value for the lens extension.
With this value you can determine the magnification ratio.....
m = zs / f
Hmm, now here a calculation for 200mm and FD =2500mm :
1. Iteration:
z_1 = 2500 - 2* 200 = 2100
zs_1 = 200^2 /2100 = 19.047...
2. Iteration:
z_2 = 2500 - 2*200- 19.047 = 2080.95...
zs_2 = 200^2 / 2080.95 = 19.22....
I think this is accurate enough.... you can try another Iteration when you want.
Now the magnification would be:
m = 19.22 / 200 = 0.096..
This calculation is not too nice, but works.
Frieder Faig
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