TeleConverters and DOF + Sigma's quality ?

Discussion in 'General Technical Discussion' started by Elf_8, Aug 20, 2005.

  1. I'm considering getting a teleconverter for my Sigma 70-200 2.8 USM. Two are offered, the 1.4 and 2.0.
    With the 1.4, the max aperture will be f4 while it'll be f5.6 with the 2.0 TC.

    I have two questions in mind from there. The first is regarding the relative aperture. Say, with the 2.0 TC, the relative aperture will be 5.6, but will the DOF behaviour of the lens be that of the "normal" lens at 5.6, or is it still that of the 2.8 max aperture of the front element ? I understand that 5.6 describes more the loss of light than the real lens aperture.
    Second question is about quality. Anyone here has a clear statement about the use of Sigma TCs ?
    I still have a Nikkor 70-300 ED (4.5-5.6 I think). How would it compare to using the the 1.4 TC on my Sigma 70-200 at max lenght (300 vs 280) ?
     
  2. Christian,

    I use the Sigma 1,4X with my 100-300mm f/4 HSM and have tested the 2X with the same lens. With the first, I noticed a slight loss of sharpness with apertures greater than f/8 (lens + TC aperture) (100% crop test images). With the second, the image quality definitively suffers even at f/11 or f/16 (lens+TC). Also, with 1,4X TC on, my D100 AF has more difficulty to achieve focus in low light situations.

    About DOF, it's an interesting question that I never think about until your post. I will do some testing with a ruler and let you know.
     
  3. Thank you Dang. Banning the 2.0 TC sounds like a universal statement. I have read it almost everywhere. As for the 1.4, I am still wondering how it would compare set on a 70-200 at max length, compared to my g'd ol' 70-300.
     
  4. Christian, you got email
     
  5. dbirdsong

    dbirdsong Guest

    Hi Christian
    I have both Sigma TC and the same lens, Rarely is the 1.4 off my 70-200, I love it.
    I have only recently had the 2x and it is nice, but man you need a lot of light to use it.
    As for DOF, it is about half of what it is without the TC (According to Thom Hogan's website)
    These were taken with the 70-200 and the Sigma 2x
    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    I wish I had purchased it a long time ago.
     
  6. Thank you Dave for this reference to Tom Hogan's site. I will investigate it. You've got great shots with the combo, I see. That dragonfly was caught while flying ???
     
  7. nfoto

    nfoto Guest

    With a 2X TC, you do lose 2 stops, the focal length becomes doubled, and the DOF won't change at all. This is because you are now shooting at effectively two more stops down which adds more DOF, just enough to counterbalance the reduction in DOF cause by a longer focal length.
     
  8. Loud and clear. Really makes sense. Thanks.
     
  9. dbirdsong

    dbirdsong Guest

    Yeah the dragonfly was in flight when I took that picture, it amazed me that it was that sharp. They are so small and with that much lens (that was at 360mm before the 1.5x crop factor) it is hard to even follow them let alone get the lens to lock in focus on them.
    I think my favorites from that day were of the sunflowers with the bee's on them taken at 10' away hand held too.
     
  10. Mike Z

    Mike Z

    May 30, 2005
    Northbrook, IL
    Christian,
    I have the 70-200 with the 1.4 extender. Works very well. If you check out my airshow pictures in the Planes, Trains, etc. forum they were taken with that combination. I'm very happy with it.

    Mike
     
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