In article <19970227152601.KAA25833@ladder02.news.aol.com>, kytsch@aol.com says...

>the other day, in the shop where i work, we stuck two 2x teleconverters on
>an fm2 with a nikkor 300, effectively giving as a 1200mm f/16. it was a
>hoot, albeit a bit on the dark side through the viewfinder.
>at any rate, does anyone have any practical experience with these
>teleconverters? they actually have built in macro capabilities,
>essentially making them teleconverters and extension tubes in one. [..]

The Macro Vivitar 2X is actually quite good, and a bargain - especially
considering that the best use for 2X's may be for macro work (at small
stops). For tele work, the resultant aperture with the two-stop loss
plus the f11 lens optimum aperture for use with the short-lens type
2X's (f22 effective) makes the combination nearly useless, alas. The
very expensive long-lens type converters, combined with the big, fast,
expensive teles, can provide good image quality at relatively wide
apertures (and can sometimes be combined with a long-lens type 1.4X
for even greater length with good quality at smaller stops). Doubling
the 2X Vivitars on a 300mm lens is very unlikely to result in good
sharpness even under the best of conditions...
Hope This Helps