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Re: [OM] Teleconverter Theory

Subject: Re: [OM] Teleconverter Theory
From: frieder.faig@xxxxxxxxxxxxxxxxxxxxxxxxx
Date: Sat, 18 May 2002 15:00:41 +0200
On Fri, May 17, 2002 at 10:57:58PM -0700, John Cwiklinski wrote:
> Does someone have a technical reference to teleconverter theory. (I am more 
> interested in speed and light passage effects rather than the 
> magnification). Thanks.


The simple rule is to multiply the F-stop with the magnification-factor.
So a 1.4x-Converter reduces the speed one stop, a 2x-Converter reduces
the effective apperture 2-stops. 
e.g.: F2.8 x 1.4 -> F4 ;  F4 x 2 -> F8

Then ther emight be a additional loss of some percents due to glass and
surface loss, and eventually some loss due to additional vingetting caused
by bad design.
This factors usualy don`t need to be concerned in parctical work with good
converters.

I´ve just made a comparison between F4/200 + 2xA =F8/400mm and a 
F4.5/300 + 1.4xA = F6.3/420mm. I was wondering, wide open, both lenses
achieved 1/500 second altough the 300+1.4 combo should be a half stop
faster. So I´ve made three slides, 200@F4(=8), 300@4.5(=6.3) and 300@5.6(=8).
All with 1/500sec, which was meassured at F8 with the 200mm lens, and at F6.3 
with the  300mm lens (??).Result: the second slide is a bit brighter than the 
other two, 
which are equal in brightness.

Sems at one of these lenses/converters the apperture lever isn`t adjusted 
propperly.

Frieder Faig

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