In article <4ngb1s$9c9@solaris.cc.vt.edu>, wachira@birch.ee.vt.edu says...

>If I put 2X converter with 70-210mm f/4 lens, the focal length will be
>140-400, right? Question is what is the "effective" aperture when the
>aperture number is set to 4. Is it 8? If it is the case, 2x converter
>will not work on slow telezoom lenses--like a f/5.6!

You are right, and it is even worse than that - f5.6-8 (f11-16
working aperture) is the widest useable aperture for acceptable performance with most GOOD 2x/zoom combinations (I did not say
"good" performance...), which makes the (potentially) most
useful converter/lens combination pretty useless, alas. 2X's
are probably most useful for flash macro work, and for
lengthening the horribly expensive big fast teles. 1.4X's
offer better optical performance and less speed loss, but
(sigh!), only 1.4X magnification....
Hope This Helps