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Why are tele-converters only compatible with certain lenses?

Discussion in 'General Equipment Chat & Advice' started by Len Scap, Aug 16, 2019 at 12:00 PM.

  1. Len Scap

    Len Scap Member

    As a hobbyist photographer, a TC seems a good compromise in getting more 'reach' - I don't have thousands of pounds to pay for lenses I won't use very often.

    However, I notice that they all seem to be listed as only being compatible with certain lenses (I'm using m43 by the way) I just wondered if this is compatible in the sense that it literally will not work with another lens, or if it's compatible as in it's optically optimised for certain lenses?

    Obviously the mounts will mean the TC would physically fit any m43 lens, so I presume it's about optical optimisation, but would love some info from someone who actually knows!
  2. El_Sid

    El_Sid Well-Known Member

    OEM converters such as those made by Canon & Nikon have optical elements that protrude beyond the lens mount and as such can only be used with lenses that have a sufficiently recessed rear element (eg telephoto lenses). Non recessed rear elements would impact with the converter's optics leading to dire consequences. I've not seen an Oly or Panny m43 converter but the same possibly applies to those as well.

    Third party converters such as those from Kenko/Teleplus don't have this issue, all their optics remain well inside the lens mount face and as such can theoretically be used with any lens. I say theoretically because most cameras will only AF reliably when the maximum aperture of the lens is greater than f5.6 (or f8 with some more advanced models). If the converter takes the effective aperture beyond these limits then AF either becomes slow and/or erratic, won't find focus or in some combinations is switched off automatically if the maximum aperture is too small. This situation can effectively render a lens incompatible with a converter even if it can physically be mounted.

    Modern converters and lenses are generally better optically than they once were but there may still be occasional optical mismatches between a lens and converter that render the combination relatively useless - again one might reasonably describe this is incompatibility...
  3. Len Scap

    Len Scap Member

    Thank you - I hadn't thought of the protruding elements.

    I thought about getting one because I came across a Tamron Adaptall 2x I bought from a charity shop when sorting out a cupbaord last night - it doesn't have protruding elements, and it's the only one I've ever handled. (I'm going to have a play with it - I've never actually used it, and according to the sticker on it I invested a whole £4.99 in it :) With my 103a 80-210 on m43 I suppose I have a hard-to-focus f8-ish 360-840 - albeit one made up of optical elements getting on for 40 years old :)
  4. El_Sid

    El_Sid Well-Known Member

    Never seen a Tamron converter that I can recall though I do have a couple of Vivitars from the same era. Be interesting to see what you get, my Vivitars were never that great TBH.
  5. Len Scap

    Len Scap Member

  6. Andrew Flannigan

    Andrew Flannigan Well-Known Member

    I have one and it works OK with the SP 300mm/f5.6 lens. Not so good with the 135mm f2.5. That's about par for a teleconverter.
  7. nimbus

    nimbus Well-Known Member

    Well give it a go, just don't expect wonders!
  8. SXH

    SXH Well-Known Member

    And if anyone complains, just claim it's the Lomo aesthetic...
    nimbus likes this.
  9. Chester AP

    Chester AP Well-Known Member

    I used a 2X TC with a 200 mm F 4.5 lens many years ago: the central definition wasn't bad, but the real problem was focusing because the viewfinder image was so dim. If your camera body has one of the new electronic viewfinders that brighten the image the focusing may be less of a problem. For £4.99 you can afford to experiment.

    You don't say what the resolution of your camera body sensor is, but you may do better just cropping the image you get without the TC.
    For example, if you have a 24 megapixel sensor, cropping it to a quarter of its area (to simulate a 2X TC) will still give a 6 mp image that may be more than adequate for your needs. Will you want to do a large print, or merely see it on a monitor?
  10. Len Scap

    Len Scap Member

    I'm using a OM-D E-M10 (the mk1) which is 16mp - it's fairly easy to see what you are doing with regard to focus, but the image never seems to actually *be* in focus!

    Anyway, I had a bit of a go with it in a snatched hour this afternoon - this is the view with my 20mm standard-ish on m43 lens:

    and this is the same view with £4.99 of 2x TC on a Tamron 103a 80-210 zoom from the early 80s - with the m43 crop factor this is roughly equivalent to 840mm

    Not exactly razor sharp. but not bad for £4.99 (I was given the lens)

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