[OM] teleconverters and depth of field question

Subject: [OM] teleconverters and depth of field question
From: Dr Peter Gilbert <peterg@xxxxxxxxxxxxx>
Date: Wed, 19 Aug 1998 15:35:29 +1000
hi all

read with interest the recent thread on the super telephotos, and as I am
looking at purchasing a Zuiko 1.4x TC, I thought I would raise a couple of
related questions to the group.

Firstly, I know that a 1.4x TC will effectively reduce the light falling on
the film by 1 stop ( and a 2x TC by 2 stops). Many people say "it reduces
the aperture" but is this technically correct? Does it increase your depth
of field to that which would be obtained at the smaller aperture?

ie a 250 F2 used at F4 has y amount of depth of field when focussed at say
40 feet

If I add a 2x TC to get effectively a 500F4, and leave it set at F4 on the
lens, do I still get y depth of field? Or do I get the depth of field as if
I am using the lens set at F8?

Or is the depth of field at (effectively) F8 on a 500mm lens the same as
dof at F4 on a 250mm.

I'm sure those of you with more knowledge of optics etc might be able to
enlighten us as to how this all works.


I too use a non-Zuiko fast telephoto (see John Petrush's e-mail - he has
the Tokina 300 F2.8 ATX). I have a Sigma 300mm F2.8 which I picked up new
in OM mount for an unbelievably good price earlier this year (still waiting
for a lotto win to afford a 350 F2.8). Does anyone on the list have any
first hand experience of the Zuiko 1.4x TC on the Sigma 300 F2.8 lens ie
will it physically fit, image quality etc? I am wary of spending the $$$$
on this 1.4x if it may not even fit the main lens I would like to use it

thanks in advance


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