As I haven't bit the bullet and bought a 600 mm lens yet, I occasionally use a Nikon TC-20E II teleconverter on my 200-400 VR f/4 lens. I realize that per Nikon's recommendations this combination should not work properly with autofocus, although I have found that it does seem to work OK with reasonable light.
My question concerns the sharpness of the photos taken, which leave a little to be desired, relative to photos taken with the same lens, but without the teleconverter. Is a degradation in sharpness to be expected with any teleconverter or is it a reflection of autofocus issues using this particular teleconverter (i.e., although the lens appears to lock in a focus, perhaps it's slightly off)? If I were to go to a TC-14E II teleconverter, should I expect sharpness similar to that of just the lens itself?
Generally speaking yes.
2X image quality will be less than a straight lens and less than a 1.4X. The 1.4X IQ can be great and can be very hard to tell the difference between the 1.4X and the straight lens sometimes.
See the teleconverter/extender link below for more info, although the thread concerns Canon the same general principals apply.
Kelly,
With my TC20E I usually stop down to F11-F14 seems to be the sweet spot and increases the DOF.
My TC20E produces better images(sometimes awesome) than my TC14E. I probably get a little sloppy with the 1.4
The teleconverters magnify the image by 2X and 1.4X. They also magnify any flaws in the optics they are attached to, but more than anything they magnify poor long lens technique, resulting in soft images.
By the way the Kenko Pro 300 series 1.4X is pretty dang good.
Nikon USA Service has advised two acquainttances NOT to use Kenko TC's with the D300.
Both were having power issues / lockup and Nikon stated it was likely the TC's, although one of them still experiences the isues with no TC's...go figure ?
Teleconverter opinions?
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