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I have a Kenko 3x teleconverter and am getting generally soft images (using a Canon 5d Mark II on a tripod).

I am trying to find out how much of this is due to the quality of the teleconverter and how much is due to the fact that it magnifies the flaws of the used lenses so much.

Is there any database of tested lenses (preferably that includes vintage glass so I can afford them) that I can use to figure out which lenses to use for such testing? I am thinking that lens sharpness should be the key here. (I am thinking telephoto primes 200mm and up, preferably on the longer end of that if I can find something affordable enough)

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  • \$\begingroup\$ With a 3X TC, it's always the TC. That's why there are no published databases. \$\endgroup\$
    – Michael C
    Commented Jan 21, 2020 at 4:47

4 Answers 4

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You can figure out whether the TC is the major cause of loss of sharpness by measuring lens resolution with and without TC.

  • Measure lpmm using a resolution test chart without TC. If you max out the resolution of the chart, increase the distance between the camera and the chart. Most good vintage primes will easily max out that chart.

    lpmm = (line pairs)/(height of image on sensor)

    If the chart takes up half the sensor height, use half the physical size of the sensor in the calculation.

  • Measure lpmm with TC and multiply the result by the conversion factor.

  • If the adjusted lpmm with TC is less than the lpmm with lens alone, the TC is the likely major cause of reduced sharpness.

  • If lpmm is greater or equal, then reduced sharpness is likely caused mainly by lens limitations. (TC effects can be considered negligible.)

    Is this even possible? – I can't find the link anymore, but I read a review of a lens that performed better with TC than without. It could be caused by aperture effects or the TC somehow correcting lens defects, like when humans wear glasses. (If a TC corrects a particular lens' defects, it could be considered a bad TC because it probably wouldn't work well with other lenses.)

    There could also be small measurement errors. If TC-lpmm > lens-lpmm as a result of small measurement errors, the loss from TC could be considered inconsequential. (We're not working with lab grade equipment to measure MTF.)

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  • \$\begingroup\$ I see where you're going, but is it ever possible for lp/mm (* conversion factor) to be greater than without the TC? Seems to me that because, by definition, MTF is always strictly <= 1.0 (100%), and nothing has perfect optical quality, adding anything in the optical path will not actually increase resolution. That is, 0.99 * anything less than 1.0 will produce something less than 0.99. Am I wrong? \$\endgroup\$
    – scottbb ♦
    Commented Jan 19, 2020 at 19:35
  • \$\begingroup\$ I can't find the link anymore, but I read a review of a lens that performed better with TC than without. It could be caused by aperture effects or the TC somehow correcting lens defects - like when humans wear glasses. There could also be small measurement errors. If TC-lpmm > lens lpmm as a result of small measurement errors, the loss from TC could be considered inconsequential. (We're not working with lab grade equipment to measure MTF.) \$\endgroup\$
    – xiota
    Commented Jan 19, 2020 at 19:41
  • \$\begingroup\$ Hmm. I wouldn't think to include measurement error, but it's a fair point. I like your thinking. \$\endgroup\$
    – scottbb ♦
    Commented Jan 19, 2020 at 19:44
  • \$\begingroup\$ Simple: If the lens is great in the center but has horrible corners, the teleconverter will discard the corners.... \$\endgroup\$
    – rackandboneman
    Commented Jan 19, 2020 at 21:57
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Second answer, since it is a completely different alternative approach:

Have or borrow a smaller sensor camera, and stick/adapt the lens to it. A 20MP APS-C camera (could be another EOS) will be equivalent to a perfect 1.5x/1.6x converter, an MFT camera 2x (though it will likely be 18MPix, probably negligible difference). If the lens is great on these, blame the converter.

Or borrow a really high resolution camera (A7rIV or something) and check the center...

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  • \$\begingroup\$ A problem with this solution is it introduces multiple cameras and sensors with different pixel pitches and performance. Some cameras intentionally blur the image (via the anti-aliasing filter). But it's a practical approach to get a rough idea and if using the lenses and TCs in a mixed camera environment. \$\endgroup\$
    – xiota
    Commented Jan 21, 2020 at 20:43
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Your best bet is testing a couple lenses (vintage tele primes are often not that expensive) yourself without the teleconverter, and checking the results under magnification. Sample variation can be a big problem here - I was tempted to recommend the Pentacon 200mm f4 or a Tair3s (russian 300mm) ... IF you can get a great copy.

Testing with 135mm lenses might be the best option, since a handful different ones can be had for a song - not so much with 200mm and up. And they tend to come in faster apertures.

You want a high resolution lens more than a high contrast lens... so MTF curves at 30 or 60 lpm will be the most relevant.

Do use a tripod, cable release, and scrupulous live view manual focus for such testing. Take all test shots double. Nothing ruins your day like having been outdoors heavily packed to do formal lens testing... and finding a few essential samples having camera shake. Also remember that adapted lenses have no EXIF, so make a laundry list (or snap your planned test lineup with a phone camera) and STICK to it. Trouble lining up test shots with equipment sucks.

Mind that a 3x teleconverter will magnify any flaws not three but nine times when you look at the AREA the magnified flaw will take in the resulting picture.

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  • \$\begingroup\$ Why will it be nine times? That is the light loss for sure but the linear resolution should only be three times. \$\endgroup\$
    – lijat
    Commented Jan 19, 2020 at 21:30
  • \$\begingroup\$ You recomending 135mm lenses is interesting, do you think the smc pentax-m 135 f3.5 I have laying around might be a good candidate? I would have thought I needed to exceed 200mm at least to have sufficently straight light rays for the teleconverter to work at its best. \$\endgroup\$
    – lijat
    Commented Jan 19, 2020 at 21:32
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    \$\begingroup\$ Linear resolution, yes. But visually, a chromatically and spherically aberrated mosquito can be turned into an elephant visually.... \$\endgroup\$
    – rackandboneman
    Commented Jan 19, 2020 at 21:34
  • \$\begingroup\$ True, teleconverters appear to be optimized for certain focal length ranges, check the documentation on the kenko... but it seems to be more a case for "below 300" and "above 300", as eg minolta did... \$\endgroup\$
    – rackandboneman
    Commented Jan 19, 2020 at 21:35
  • \$\begingroup\$ That pentax is a K mount lens, adapting it to an EF teleconverter should in theory be possible... \$\endgroup\$
    – rackandboneman
    Commented Jan 19, 2020 at 21:37
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I can't find a list of compatible lenses from Kenko.

Canon as a list of lenses that are officially supported with Canon extenders, see here. AFAIK they are designed to work well with an extender.

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