Teleconverters, wide-angle converters, compatibiltiy and AF

Discussion in 'Beyond the Basics' started by epp_b, Sep 12, 2008.

  1. epp_b

    epp_b No longer a newbie, moving up!

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    OK, so I have a few questions about teleconverters first.

    1. All of them claim only AF properly with aperture of f/xx or wider. So, if I used my 55-200 VR (f/4-5.6) with a 2x teleconverter (2-stop penalty), effectively making it f/8-11, would it not AF, even if the subject was bright and contrasted object in broad daylight?

    2. This one's a bit more specific: do new Kenko Nikon F-mount teleconverters work with old Nikon AI lenses?

    3. What do you make of teleconvers that sit on the end of a lens like a filter? Do they actually work? What is the effect on image quality? Do they give sharp results?

    Next question is about wide-angle converters, but along the same lines of the #3: do they actually work? How is the image quality? Are they sharp? Do they cause a lot of distortion?
     
  2. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Typically, cameras need an aperture of F5.6 to be able to auto focus effectively. Teleconverters steal light, a 2X will rob about two stops....so anything smaller than F2.8, will have trouble with a 2X TC.
    Will it work in bright conditions....I'm not sure about that.

    Anything that you put in the light path, in front or behind the lens, will affect the image quality....usually it's a negative effect. The ones that go in front, like a filter, are usually of poor quality and it shows in the images...but that doesn't mean that they can't be fun.
     
  3. Overread

    Overread has a hat around here somewhere Staff Member Supporting Member

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    Most 1.4times teleconverters of good quality (not the ones that go in front) will work with most lenses and give a reasonable increase in range with minimal to no loss of overall quality - a good solid investment (actual quality differs from lens to lens).
    A 2* is a more "fun" converter than serious most of the time - on zooms and lower end lenses the drop in quality and light makes them only any good in bright conditions = however they can yeald good results. They tend to wrok best though on high end prime lenses - where the loss of quality is far less than on a zoom lens (due to construction)
     
  4. Helen B

    Helen B TPF Noob!

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    Teleconverters that go in front of the lens and are of reasonable quality are usually very expensive, and very large. They have the advantage of not changing the f-number, if properly designed. The focal length gets longer, but the diameter of the entrance pupil also gets greater (the f-number is the focal length divided by the diameter of the entrance pupil). You still have to stop down from maximum aperture to decrease the quality loss, however.

    For most lenses the only practical location for a wide angle converter is in front of the lens. As with teleconverters the decent ones are very expensive and they tend to be used only when a wide lens is not a practical or cost-effective option – such as with video cameras with non-interchangeable lenses. Distortion is not a big issue these days because it can be corrected in post, but other aberrations can be a problem.


    Best,
    Helen
     
  5. GeneralBenson

    GeneralBenson TPF Noob!

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    What's this about AF not working well under f/5.6? Is that all the time on anything?
     
  6. icassell

    icassell TPF Noob!

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    I have a Sigma EX 1.4X and a Sigma EX 2X

    My Sigma EX 50-150mm f2.8 autofocuses fine with both of them (the 2X makes it an effective 100-300mm f5.6)

    My Sigma EX 100-300mm f4 autofocuses fine with the 1.4X (effective 140-420 f5.6) but not with the 2X (200-600 f8).

    I've stacked the 2 a couple of times and, needless to say, AF doesn't work in that setting (adds 3 stops to the lens).
     
  7. epp_b

    epp_b No longer a newbie, moving up!

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    Thanks for all the help, guys. I think I'll for a #5 or #6 on Bifurcator's list ;)

    Still, could someone with a filter-like teleconverter post some samples?
     
  8. epp_b

    epp_b No longer a newbie, moving up!

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    Those pictures look pretty acceptable by me! Thanks!
     
  9. epp_b

    epp_b No longer a newbie, moving up!

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    OK, I'm looking at this one:

    [​IMG][​IMG]

    I can't seem to find too many reviews of it, but Adorama says that it uses 3 elements in three groups and that it's multi-coated. I haven't been able to find very many reviews of it, but the ones I have found say that it's decent at the longest focal length of a lens and causes vignetting at the shorter focal lengths.

    What do you all think?
     
  10. Bifurcator

    Bifurcator TPF Noob!

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  11. epp_b

    epp_b No longer a newbie, moving up!

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    All of the samples seem to be taken with P&S cameras. I suspect the larger sensors in a DSLR would yield different results.
     
  12. tsaraleksi

    tsaraleksi TPF Noob!

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    The basic truth is that you cannot use tele-converters with a great deal of success on anything other than very high grade optics. The TC degrades image quality because it only uses a small part of the lens. Glass that is of excellent quality can handle the degradation without it being noticeable. Some examples of lenses that this is true for are the 70-200 2.8 lenses made by Canon and Nikon, and the big professional prime lenses such as the 300 2.8, 400 2.8, 500 4, etc. On a lens like the 55-200, the image quality you'll get when using a TC will be very poor, and because you'll be limiting the amount of light you get into the lens, your AF will either be quite poor or non-existant.

    Everything I was saying is when using a 1.4x TC. A 2x TC is really something that should only be used on the high grade prime lenses, any other lens will look dreadful through the TC. I think you'd be better off cropping your images in post than using a 2x TC on the lens you're talking about.
     

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