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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