tele extenders

[Jay Todd]

To those that have used a 2X tele-extender with a telephoto (200 or 
300mm), what do you seem to find to be the compromising element...loss 
of sharpness, contrast, etc?  When I used a 300/4A* with a 2x-AS I was 
always impressed with its results.  But now with another system I'm 
getting a slightly diffused image with a 70-200/2.8 and 2xEF, more so 
than I would have expected.