Tamron 2X F AF
(From company literature) The 1.4X teleconverter will increase the focal length of the master lens by 1.4X and make the lens one f-stop slower. The 2X doubles the focal length of the master lens and makes the lens two f-stops slower. In addition to increasing focal length, both teleconverters allow greater magnification in macro since the Minimum Object Distance (MOD) of the master lens does not change when used with the teleconverters.
The 1.4X teleconverter can be used with both AF and MF lenses. AF will be retained if the master lens is f/4.5 or faster. AF will be retained with the 2X teleconverter if the master lens is f/4.0 or faster.
Though using a teleconverter on wide-angle lenses is never really recommended, these converters can be used with lenses that are wider than 90mm and are ideal for all lenses that are not f/2.8 or faster.
Tamron 2X F AF
Tamron 2X F AF User Reviews
9 out of 10 points and recommended by joe88 (37 reviews)very sharp 7/5 element AF-converteraperture information is done to Pentax cam's
this is my favorite converter for macro shootingsreviewed January 30th, 2015 (purchased for $50)
if your basic lens is well corrected, you'll get very sharp shootings,
but I stop down lens for macros or close-ups to f8-11
For tele shootings of birds f.e. I've developped a trick:
an additional macro ring with achromatic focal reductor will
eliminate any CA, stopped down to f8-11, too.
my high recommendation
high contrast, good color redition and really sharp photos
8 out of 10 points and recommended by lightknight (30 reviews)Cheap, gooddoesnt report actual aperture, some CA
For the money this TC is great and it frequently amazes me. I use it exclusively with my Nikkor 80-200 ED F2.8 and D700 for wildlife and some action work. Its important to remember with any TC that if your object lens is not sharp your TC will not magically make it better...it simply multiplies what you give it. For example the 80-200 2.8 is somewhat soft at 200mm irrespective of the aperture , whereas is extremely sharp at 80-135. So put the tamron on set the zoom to 105 and hey presto its real sharp at 200mm equivalent....albeit 2 stops slower.reviewed January 31st, 2010 (purchased for $120)
Have a look at the shot below: this was taken with the TC on and is a 50% crop from the original photo.
where it comes unstuck is with focus speed. I am not sure why but physically it seems to slow the focus motor down but in average conditions its capable of keeping up with a flying Gannet once the D700 locks on:-
By comparison I have used the Nikkor 300mm F4 with a 1.4 Nikkor TC in similar situations and its focus performance overall is inferior.
Its annoying it doesn't report the actual aperture as that screws up the EXIF. the only other issue is some CA but its not really bad.
Note its probably a Kenko clone or perhaps vice versa