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

Discussion in 'General Technical Discussion' started by Retief, Jul 25, 2005.

  1. 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.
  2. Chris101


    Feb 2, 2005
    Several things. Different TC's have electrical and mechanical couplings that may not be equal. Also ...

    As light gets dimmer (through decreasing aperture size) the amount of light reaching the autofocus sensor becomes less and the camera has a harder time deciding if something is in focus or not. So the ease of autofocusing depends on the subject. Where an f/2.8 lens would have no trouble focusing on a low contrast target, an f/5.6 lens would.

    Then there is the actual light transmitted, vs f/number. While two lenses may both be f/2.8, one may actually transmit more light than the other. This is known as the len's t/number. One lens's t/number may be quite different than another's even if their f/number is the same.

    Things like the number and spacing of elements, coatings and baffle construction can affect t/number. Only the diameter of the pupil affects the f/number. It is the t/number however that determines how much light reaches the sensor.
  3. 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....

  4. Chris101


    Feb 2, 2005
    The t/number is emperical - you need to test it.

    I doubt that t/number differences alone would account for the Sigma's discrepencies. However, I've heard that some Sigma lenses have a color cast, which suggests that they don't transmit as much light as do Nikkors. It is more likely that the electrical and mechanical couplings play a greater role.

    The fact that Sigma lenses work better with Sigma TCs supports this, as they would likely make their TCs to work best with their own lenses.
  5. 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.
  6. Gale


    Jan 26, 2005
    Viera Fl
    I really love these discussions... Thank you...
  7. The safest approach is to follow the compatiblity charts from Nikon and use their products. Otherwise you have to experiment, and hopefully before you buy. At least this has been my experience.
  8. 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:
  9. Took my TC20E-II out yesterday for the first time in a long time and there was quite a noticeable difference in the speed of focus on my 70-200VR versus the TC14E-II. Really lagged and in fact sometimes just kept going in and out, in and out, even though it was aimed at a fairly high contrast focus point.
    Sorry I ever bought the thing - soft pics and slow focus - might as well just use my CP5700 with the teleconverter and get 420mm with the same speed! *LOL*
  10. 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.
  11. I have the TC-14E II as well and am very pleased. Very sharp, instant focusing, etc. I think I'll try the TC20 out on a buddy's lenses and see what happens.

    Maybe they changed manufacturers of the glass they use - who knows but right now it's only a paperweight on my desk. I had some great shots from yesterday, very close of a large dragonfly and they're useless photos. Damn disappointed... or maybe I'm just spoiled with the incredible sharpness of the 70-200VR with the TC14. If I hadn't bought it from the States, I'd be marching it right back to the store.
  12. 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:

    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.
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