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Effect of teleconverter on bokeh

Discussion in 'Adapted Lenses' started by Jcarrith, Nov 4, 2012.

  1. Jcarrith

    Jcarrith Mu-43 Regular

    131
    Apr 26, 2012
    Colorado
    I have recently acquired a Tamron SP 01F teleconverter and have used it on a number of Tamron macro lenses including the 90mm and 35-80mm. While the sharpness is acceptable I am concerned about degredation of the OOF area. My photos with the 01f seem to have a "jittery" OOF appearance. Has anyone else observed this or could it just be this tele unit.


    Tamron 90mm without teleconverter
    Bee_90_bokeh.

    Tamron 90mm With teleconverter
    Butterfly_902X_bokeh.

    Tamron 35-80mm without teleconverter
    Fly_35_80_bokeh.

    Tamron 35-80mm with teleconverter
    Apache_Plume_2.
     
  2. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    A TC magnifies everything, including aberrations, halos, etc. It can also introduce aberrations of it's own, which may be additive or not, depending on the lens it's mounted on.

    A few years ago I did some comparisons with a Canon body (50D, I think), using my 70-200 f/2.8 standalone, and with the 1.4x and the 2x matched TCs from Canon. I took the image without TC, and cropped it to match the FOV of the lens with each TC, and enlarged it to the same size as the images taken with TC in Photoshop. I no longer remember what method I used to enlarge; probably bicubic, but I'm not sure.

    I came to the conclusion that the lens + 1.4x TC was slightly sharper than just cropping the base image, but the lens with 2X TC was inferior to cropping and enlarging the base image. Not by much, but even the TC & lens combination designed to be used together caused more loss of sharpness than simply cropping and enlarging. Using the 1.4x TC and cropping to match the FOV of the 2X was definitely sharper. I haven't used my 2X TC much since then.

    That said, I don't think the bokeh in the images above looks bad at all. The image of the butterfly suffers from not enough contrast between the bug and the background, but I don't think that's down to bokeh.
     
  3. Jcarrith

    Jcarrith Mu-43 Regular

    131
    Apr 26, 2012
    Colorado
    Given that, is it your opinion that extension tubes are preferable to teleconverters if your goal is a smaller area filling the sensor? I understand that focus distance is better with a tele, the question is image quality.
     
  4. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    I think the answer is "yes, but." It's hard to predict how any given TC will interact with any lens. But in general, a TC will reduce IQ, it's only a question of by how much.

    Paired with a macro lens, that's already optimized for close focus, extension tubes will give you better sharpness. Using extension tubes with a lens that's optimized for distance use (e.g., any non-macro) may not work very well, either.

    Unfortunately, as you mention, a macro lens + extension tubes can put you very, very close to your subject.