In article <55oblh$asn@nntp5.u.washington.edu>, romulan@u.washington.edu says...

>I have a question about using doublers (or other teleconverters). HOw do
>you know the max aperature?
>For example, if I use a 2X teleconverter with an 75-300 f/4.5-5.6 zoom or
>a 100mm f/2.8. Is there a conversion formula? Does a doubler take off
>one f-stop taking the 75-300 zoom to an f/5.6-8?

Two stops, alas, which makes most lenses that one would want to double
impractically slow. In addition, best sharpness with the 2X converter may
be around marked f8-f16 (f16-32 effective apertures), with unacceptable
sharpness wide open. Also, 2X's work fairly well with some lenses, poorly
with others (it is hard to predict which lenses will work well, except
that it is often shorter, rather than longer ones that do - unless you
are using the big EDIF teles with the expensive 2X converters designed
for them). AF is impossible, and MF is difficult with 2X converters on
slow lenses. Short-lens 2X's may be most useful on macro lenses for
doubling magnification. The situation is better with the one-stop loss
1.4X converters (but the reward is smaller ;-).
Hope This Helps