Teleconverters and F-Stops, why is there a....

difference in how well they will AF with lenses of the same f-stop?

I have 3 lenses that I use my TC's on:

1. Nikon 70-200 f2.8 AFS VR
2. Sigma 120-300 f2.8 HSM
3. Sigma 500 f4.5 HSM

I have the following TC's:

1. Nikon TC-14E
2. Nikon TC-20E
3. Sigma 1.4

All three lenses work really well, no really noticeable focus difference, with the Nikon TC-14E attached.

The Nikon 70-200 works great with the TC-20E as well.

The Sigma 120-300 almost works with the Nikon TC-20E, it "gets close" after some hunting, it works great with the Sigma 1.4.

The Sigma 500 does not work with either the Sigma 1.4 or the Nikon TC-20E.

It has always been my understanding that the limiting factor is the "cumulative" f-stop after adding the TC. So, for example, on the 70-200 the TC-14E yields an f4 (+1 stop) and the TC-20 an f5.6 (+2 stop). Given this, what other factors might be inolved that cause the TC-20E to not work on the 120-300 f2.8?

On the 500, adding the TC-14E yields f4.5 + 1 stop, f6.3 or so I'd guess, so I understand why the TC-20E would not work as that would be f9. But why would the Sigma 1.4 TC not work?

I will, in the near future, be picking up the TC-17E which I expect to work fine on all 3 lenses, but I am wondering about what other factors are in play here that I am not aware of.

Thanks in advance for the education.

 

Thanks, Chris, this makes sense, but a couiple of "follow-up" questions.

1. How does one find this "t/number"?
2. Could the t/number be THAT different for the Sigma 120-300 that it "acts" like an f8 to the TC-20E when it is "really" only an f5.6? That seems like a rather huge difference.

I suspected that the coupling issue was the deal with Sigma 1.4 extender, although it is interesting to me that it works just fine with the 120-300 f2.8. Must be something different on the 500, as they list that as MF with this TC although it works fine with the TC-14.

Sure would be nice if all these "little bits" didn't factor in....

Thanks,

 

It is probably a combination of all those factors that make the TC-20 not work well.

Actually, in my case, the Nikon TC's actually behave better across both lenses than the Sigma TC. Remember, that TC won't work on the 500mm at all where the Nikon TC-14E does.

 

In general this is good advice, it just puzzles me that the Nikon TC-14E works on my Sigma 500 when the Sigma 1.4 doesn't. "Working" in this case is defined by maintaining AF. And to be fair, it does say MF only for the Sigma TC on the Sigma lens page. But then why does the Nikon TC work so well? From The King and I, 'Tis a Puzzlement..... :lol:

 

Sandi, I often here this regarding the TC-20E. I think I am just incredibly lucky, as is Jim Thiel, in that we both have TC-20's that do a darned good job. I have posted my GBH head-shot here before which was shot with a stacked TC-14E+TC-20E and no one yet has commented that it is "soft", maybe you are just all too polite. Now, as to focus, that combo is Manual Only. But my TC-20 works very, very well on my 70-200VR. As I recall Jim had the same experience. Slower than the TC-14? Yes, but is certainly very functional. If you have the opportunity it might be interesting to try with a different TC. If we were in closer proximity, I'd be happy, maybe you can find another Cafe member in your neck of the woods? Then again, mine is a very old TC-20E, not the II version, but that in and of itself should not make a difference.

 

Sandi, I just ordered a TC-17E, and I sure hope it lives up to expectations  .

Here is a link to the stacked TC image:
http://deweydrive.com/gallery/albums/album23/DSC2227Frame.jpg

You can see the same bird but with only the TC-14 prior in the gallery for reference. If you get out to my part of the world, even West Coast of Canada, let me know and we can meet up for a test.