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.
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.
On the other hand my Olympus Lens Group brochure gives the same minimum field
size as the 100mm/2.8 (290x190mm) - I suspect a misprint - which leads to a
helicoid extension of 10.6mm, front principal plane 22mm behind the front of
the lens barrel and a 1:1 working distance of only 149mm.
Hence my original question - where is the front principal plane of the 85mm/2.0
lens really located? Easily measured by reversing the lens off-camera and
measuring the image to lens barrel distance for a projected image of a subject
at infinity. The front principal plane is then one focal length behind this
image.
Cheers
David
> Date: Wed, 6 Feb 2002 08:48:43 -0500
> From: Tom@xxxxxxxxxxxxx
> Subject: Re: [OM] Zuiko 85mm f2.0 data
>
> At 1:1, the distance from the focal plane to the subject is 4 x focal
> length, or 340mm Since the front of the lens is 90mm from the focal plane
> when focused at infinity, and another 85mm at 1:1, 340-85-90=175mm, or
> about 7 inches from the subject to the front lens surface.
>
> The 100/2.8 should have a slightly larger working distance, perhaps 1.2" /
> 30mm more?
>
> Tom
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