Questions on teleconvertors

Hi Guys,

I would like to ask some general and reasonable (in my opinion) questions on Teleconvertors??

How do they actually work--

Is it a single magnifying lens or are there multiple lenses inside the body of the TC?

How important is the quality of the lens glass--

A comparison of different manufacturer's spec sheets on their TC's would indicate that there seems to be an across the board loss of one 'Fstop' using various 1.4 TCs. If this is correct, then am I correct in assuming that the quality of the lenses used in TC's is not of paramount impotance? From this, I am lead to ask the obvious. If a Nikon TC and a Kenko TC both lose one 'Fstop' of light gathering ability, then why would I pay the extra, sometimes double, amount for the Nikon TC ??

Extra glass--

If the extra lens in front of (or behind in this case) can lead to the loss of one 'Fstop', is there any loss of color or sharpness to the final image?

Are there any changes to the DOF or minimum focus distance??

Regards,

Bruce

All 1.4 TC's will lose 1 stop, all 2X will lose two. Basically, it has to do with basic optics: the f-stop is the ratio of the focal length to the diameter of the light gathered by the lens (image diameter at the entrance pupil). No teleconverter can increase the pupil diameter, and since they increase the focal length they increase the f-number (making the lens 'slower').

Different TC's differ in several important ways.

• A better TC will combine elements to have lower optical distortion, that is, be sharper.
  
• Better TC's will have better coatings, reducing possibility of flare.
  
• Better TC's will induce less chromatic abberation.
  
• They couple to AF systems differently. For example, the Nikon TCII series works well with Nikon AF-S lenses, but not AF lenses (where Kenko works better).
  

I have several TC's. I use a Kenko with my AF lenses, and a Nikon with my AF-S lenses. Because their compatibility is different, I've not tried to do a heads-on comparison. Both makes seem to be of reasonably good quality; the Nikon has a bit better "feel" for double the price (along with its flawless AF-S coupling at f5.6 or faster).

Hope this helps.

Chris in Red Stick

Bruce,

It's as the post above says about the optics.

The Nikon TC's are so much superior in optical quality, hence the price (as all things Nikon), or at least they used to be when I used a 2 x TC on a 105 with film.

Regards,

Jim A