I've recently obtained a Vivitar 2x Macro Focusing Tele Converter, known as the V2XMFTC for short. It is remarkably difficult finding hard info on this device, so I thought I'd gather some here and solicit updates from the rest of you. The results I'll put together in a "definitive" article.
The V2XMFTC uses a combination of optics (a multi-coated 7-element configuration) plus an extension tube to double the focal length of any lens attached to it. It's solid metal and heavier than you'd expect. Though with caps the unit is 6cm tall, the working portion is only half that, about 30mm. The rest of the device is taken up with the K-mounts.
The magnification ratio achieved for an extension tube is calculated as extension length / lens focal length. According to the dual-coloured ring markings on the V2XMFTC a 50mm lens results in a maximum of 1:1 magnification at a minimum distance of 45cm. Apparently, then, this device works as though it was an extension tube of length 50mm. The effective aperture will then be double the set aperture number. For example, f/1.4 becomes f/2.8, or a 2 stop loss in light.
In short, the V2XMFTC converts a 50mm f/1.4 lens into a 100mm f/2.8 1:1 macro.
There are two distinct versions of the V2XMFTC. Contrary to some rumors, you cannot tell the difference based on the colour of the ring markings. Both types can be found with either blue or green numbers. Instead, look to the text before "JAPAN". If it says "P/K" it lacks the coupling pins for aperture info. If it says "P/K-A R-P/K" lenses attached may be operated in "A" mode. In either case you must focus manually.
Supposedly the same device is available with the brand names Panagor, Albinar and Lester Dine, but not in the "A" configuration. All were manufactured by Kiron.
Working as a 2x teleconverter, the V2XMFTC has all the problems of such devices. It accentuates any flaws in the lens and introduces some of its own. For one you will get wicked purple fringing and chromatic aberration in any back-lit shot. The consensus is that 2x is too much magnification to expect of a teleconverter. It is better to stick to 1.4x if you require good image quality. Or use 1.7x only in the case of the Pentax-F 1.7X AF Adapter, which goes for big money as a result.
However the advantage of the V2XMFTC is in its macro focusing. It converts any lens to a macro lens, and might be handier than carrying two separate lenses. Jens Roesner compares the Vivi plus Pentax A 50mm 1.4 favorably to the D-FA 100mm 2.8 Macro on
this informative page.
I decided to do some testing of my own, using three lenses I happen to like, the FA43mm, FA77mm and Cosina 100mm. I'll post some results once you've pointed out any errors in what I've written so far.