In article <01bc6604$56b5f940$0f0886c2@849899070>, Pasw_cs@euronet.nl says...

>Is there a limited to the aperture an extender can handle?
>For example:
>A 1.2 50 mm lens with an extender is still a pretty light strong lense.
>But, because the back end lens of such an objective is large, will the
>extender catch all the light?

No - most converters will pass only a bit more light than provided by an
f2 lens, so metering will be incorrect with lenses faster than about f1.8,
and the additional speed of faster lenses will not be available when they
are used with converters. Of course, there is also the problem of sharpness,
since even the best lenses on the best 2X converters (with the best matches)
rarely perform very well at apertures wider than about f5.6 (f11 effective).
An f1.2 lens on a 2X would be quite soft at the effective f2.4, if it were
possible to use it that way... With 1.4X converters, there is some
improvement (though not in maximum lens speed useable...), since they
can be good by f4 (f5.6 effective).
"Hope This Helps", "Bob Neuman", David Ruether
http://www.fcinet.com/ruether