There’s a discussion on Nikkor about Nikon teleconverters.
What’s a teleconverter
Teleconverters are also known as “doublers” (even though they don’t necessary double) or “extenders.” They sit between the lens and the body in order to to increase the effective focal length of the lens.
These are not magnifying glasses! A magnifying glass is actually called a “diopter” and changes your effective focal length down in order to allow close focusing.
Nikon’s latest teleconverters are the TC-14E II, TC-17E II and the TC-20E II. These increase your focal length by 1.4x, 1.7x and 2.0x respectively. But since your real aperture doesn’t change, your effective maximum aperture gets reduced by 1 stop, 1.5 stops and 2 stops respectively. (By the way, I still don’t know what the “II” means, maybe cheaper manufacturing?) I own a TC-20E II.
Using Nikon TCs on unapproved lenses
You can see a compatibility chart here. Why doesn’t it all just work?
The auto-focusing system starts to go wonky at around f/5.6-f/8 and, in general, the less maximum aperture you have, the slower it’s going to focus. Also Nikon designs their teleconverters to “match” their lenses in order to make them slightly better than third parties.
Because of this, they use a tab to prevent you from mounting a teleconverter on their cheaper lenses. There is a trick that nature photographers use to hack their teleconverter to work on those lenses or to be stackable.
Unacceptable quality loss
A teleconverter causes reduced sharpness. How bad is it?
Well look at the MTF chart of the lens in question (70-200mm f/2.8G VR). Basically my TC-20E II teleconverter is going to make the red line (10 lines/mm) into a line somewhere between the red and blue lines. Since contrast is essentially sharpness that should be a good estimate. (Remember, read that line only out to 15mm because it is on a APS-C camera). That’s the compromise in sharpness.
I’m told by many people that this is “unacceptable.”
Remember this photo is shot wide open and is relying on the predictive autofocus capabilities of the D70 in order to keep the subject in focus. View it at 100% and stare into the eyes of the pelican or look at the feathers on his wing.
Now you know why I say, “The 70-200mm f/2.8 + TC-20E combination is still so sharp, sometimes it’s scary!”
It isn’t perfect, but “acceptability” is a relative term.