Tiffen made a number of 2X teleconverter lenses in the past decade. Since it is difficult to keep track the number of models, this page only includes three that I have access to, namely: an old one circulating in late 1990's and early 2000's and the two newer MegaPlus 2X lenses.
Tiffen had a nice 2X teleconverter in the late 1990's. This Tiffen 2X is plastic and came with a number of plastic step rings for threads 46mm, 49mm and 52mm. See the left image on the top row below. The Panasonic FZ30 and FZ50 will need a 5552 stepdown ring to use this Tiffen as shown in the right image on the top row.
Tiffen's new offerings are two MegaPlus lenses, one with a 37mm rear thread and the other 43mm. Both require stepdown rings to be used with a FZ30 or FZ50. More precisely, to use the MegaPlus 2X 37mm version, either a 5537 stepdown ring or a combination of stepdown rings that can bring 55mm to 37mm would be needed. The images on the second row below shows a Tiffen 2X 37mm and the use of 5549 and 4937 stepdown rings on a FZ30. Similarly, to use the MegaPlus 2X 43mm version, either a 5543 stepdown ring or a combination of stepdown rings that can bring 55mm to 43mm would be needed. The images on the third row below shows a Tiffen 2X 43mm and the use of 5549 and 4943 stepdown rings on a FZ30.
An Old Tiffen 2X  An Old Tiffen 2X on FZ30 
Tiffen MegaPlus 2X 37mm  MegaPlus 2X 37mm on FZ30 
Tiffen MegaPlus 2X 43mm  MegaPlus 2X 43mm on FZ30 
As mentioned earlier, stepdown rings are needed to mount the Tiffens on a Panasonic FZ30 or FZ50. Once the lens is mounted, one can start shooting immediately, and does not have to set the camera to the Telephoto mode.
The following table shows the approximate diameter of the rear glass element of each lens. Since the two MegaPlus lenses have small diameter rear elements, light falloff and even vignetting can be expected.
Lens  Old Tiffen 2X  MegaPlus 2X 37mm  MegaPlus 2X 43mm 
Diameter of Rear Element  32mm  17mm  28mm 
The following images show the light falloff and/or vignetting of all three lenses. Aperture used was f/5.6. However, since these images were not taken at the same time, shutter speed and tonality are different. It is clear that the larger size old Tiffen has some light falloff, and the smallest MegaPlus 2X 37mm has significant vignetting at the corners. The MegaPlus 2X 43mm has significant light falloff and, depending on your point of view of vignetting, does show some vignetting.
Old Tiffen 2X  MegaPlus 2X 37mm  MegaPlus 2X 43mm 
As for light loss, a spot light meter was used to measure through the center of the lens, and the measured difference is about 0.2 EV for all three lenses. This is a typical value that may not affect the exposure parameters significantly.
The minimum working distance is the shortest distance measured from the front of the lens to the subject the lens focuses on. In general, the minimum working distance of a converter lens is proportional to the square of its power being used. This distance is also affected by the minimum focus distance of the camera lens, and hence different cameras will have different minimum working distances even with the same conversion lens. The measured minimum working distance of each lens using a Panasonic FZ30 at 420mm (i.e., the maximum focal length) is shown below. Note that these are approximations rather than exact values.
Lens  Old Tiffen 2X  MegaPlus 2X 37mm  MegaPlus 2X 43mm 
Minimum Working Distance  6.8m (266 in)  5.4m (211 in)  5.3m (208 in) 
The following technical information is incomplete, and the length, width and weight values are actual measures rather than from instruction sheets. Blank entries indicate the values are not available at this moment. Note that the combined focal length is based on the Panasonic FZ30. If you happen to be able to fill the blanks, please send me a email here.
Item  Old Tiffen 2X  MegaPlus 2X 37mm  MegaPlus 2X 43mm 
Number of Lenses 

5 elements  4 elements 
Magnification  2X  2X  2X 
Combined Focal Length  840mm  840mm  840mm 
Rear Thread Size 
46mm, 49mm, 52mm (with supplied rings) 
37mm  43mm 
Front Thread Size  none  55mm  67mm 
Size  42mm (L) × 65mm (W)  37mm (L) × 58mm (W)  58mm (L) × 70mm (W) 
Weight  131 g (46 oz)  123 g (44 oz)  260 g (94 oz) 
These Tiffen lenses may be optimized for lower zoom ratio cameras (e.g., 3X or 4X), because the image quality when they are used with Canon A95 and Nikon Coolpix 9xx/4500 are at least average or above average. They are not as good as the Nikon TCE2 2X; but, even with softer corners, coma, and purple and green fringes, they are still better than many 2X lenses. The old Tiffen 2X was discussed and compared with the Nikon TCE2. Click here for more details. Unfortunately, images quality using a Panasonic FZ30 is usually below average and in some cases even unacceptable. So, if you wish to find a 2X telephoto conversion lens, you'd better look elsewhere, perhaps stacking two good 1.5X lenses. A comparison with other lenses using a Panasonic FZ30 should be available in the near future. Meanwhile, please use the following three sample images as examples. They were taken with the aperturepriority mode at f/8 to minimize various aberrations (e.g., chromatic aberration), although diffraction may kick in. Exposure compensation is set to 2/3 EV to minimize possible overexposure. An image taken with the Olympus C210 1.9X is also included for comparison purpose. All of these images were resized to 600×450 and sharpened a bit with Photoshop USM. From these preliminary testing samples, one may conclude that other than significant vignetting (or light falloff) the cheap Olympus C210 1.9X has an edge over all three Tiffens. The old Tiffen 2X is not very far behind; however, it shows more color fringes than the Olympus C210 1.9X for sure. Cheap Olympus lenses are discussed here.
Old Tiffen 2X  
MegaPlus 2X 37mm  
MegaPlus 2X 43mm  
Olympus C210 1.9X 