On Thu, Feb 07, 2002 at 10:42:38AM -0000, David Lee wrote:
> No, your treatment is only correct for a single thin lens, where the two
> principal planes are in the same place. For a thick or compound lens, the
> image plane is 2xf from the rear principal plane and the subject plane 2xf
> from the front principal plane. (You can consider the two principal planes
> as the effective position of an equivalent thin lens when viewed from in
> front and behind). Depending upon lens design, the separation between the
> two principal planes can be either positive or negative, giving a working
> distance greater or smaller than your calculation suggests.
>
> For example at 1:1 the 50mm/1.4 lens has a working distance of about 51mm
> compared to about 69mm for the 50mm/3.5 macro despite only 1mm difference in
> the lengths of the lenses.
For exact calculation, I´d also like to know this data from Olymus. May be
Olympus didn`t
publish this values because they redeigned some lenses several times, which
might has impact
on this values.
>
> Correcting your arithmetic gives a 1:1 working distance of only 165mm (6.5
> in) for the 85mm lens. However taking the minimum focus distance (850mm) and
> minimum field size (250x170mm) from a
table on the (presently defunct) e-SIF site gives a helicoid extension of 12mm
for this lens - which sounds reasonable - and a magnification of x0.14. This
leads to a front principal plane 59mm IN FRONT of the lens barrel and a
whopping 229mm (9 in) of working distance when extended to 1:1.
Hmm, my table says: minimum field size of 290 x 190 mm @ 850mm focus distance.
-> Helicoid extension 10.7mm; I´ve meassured 12mm Extension - what is wrong?
- Oh it is a floating element design - does the focal length change? -
is it really 85mm?
I can focus my 85mm (5E/4G) to 820mm, where I can see 270x181 mm in the OM-1
viewfinder.
This gives a magnification about 0.125. My guess for the principle planes is a
distance of -6mm
to make the values consistant with the data from Olympus.
You can see the results of my calculations at:
http://studweb.studserv.uni-stuttgart.de/studweb/users/mas/mas12462/Optik/Makro/Makro_85.html
Remember: I´m guessing too. But a 1:1 distance about 6in seems reasonable to me.
Frieder Faig
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