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.)