On Fri, Dec 08, 2000 at 07:45:15PM +0000, Franklin Berryman wrote:
> I recently acquired a Olympus Close-Up Lens f=40cm. According to the
> instructions, use of this lens changes the focusing range of a 50mm f1.8
> from 45cm-infinity to 19cm-45cm. Is there a formula I can use to figure out
> the effect it will have on the focusing range of other lenses?
The calculation is easyer when you caclulate with "Dioptrien" (what`s the
english term?)
1 dpt = 1m / focal lengh.
Close up lens: f= 40mm -> 1000mm /400 mm = 2.5 dpt
50 mm -lens -> 20 dpt.
150mm-lens -> 6.7 dpt.
When you combine them you have to add the dioptrien to get th new focal length:
40cm + 50mm : 20dpt+2.5 dpt =22.5 dpt -> 1000/22.5 -> 44.5 mm
40 cm + 150mm: 6.7dpt+2.5dpt = 9.2dpt -> 1000/9.2 -> 108 mm
This is how the focal lenght of your lens changes. The focusing barrel stays
the same.
Now you have to lock how your focusing barrel works woth your shortend focal
length.
Formula:
m = z/f
m = Magnification ratio
f = focal lenght
z = barrel extension.
the max magnification of the 50mm Standart lens is m=1:6.8 = 0.14. -> s= f*m
=50mm *0.14=7.3mm
At infinity settinng the converted 44.5mm lens is already 5.5mm extended:
a barrel extension with a 44.5 mm lens: m=(7.3mm+5.5mm)/44.5mm=0.29 ~ 1:3.5.
For the 150mm lens, difficult because zoom work somewhat different, but:
Max. extension: 15mm (max. 1:10 ratio) + 42mm difference of focal length = 57mm.
max. magnification: 57mm / 108mm = 0.52 ~ 1:2.
-> Close up lens is more efficient with long focal lenghts,
(extension rings are more efficient with shorter focal length) lenses.
hope this helps a bit,
Frieder Faig
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