So, a little testing of the old Pentax 1.4x-S and 1.4x-L converters.
My test subject was the compact edition of the Oxford English Dictionary (a reduced-size facsimile of the full edition; most people need a magnifying lens to read it), illuminated by an off-camera flash. I used the DA*200, as the best lens I have that is compatible with the L converter. Subject distance was about 9 meters; I wanted a subject and distance where it would be clear whether or not the TCs gave more detail. I took one series of shots with the lens alone, then a series with each of the TCs. I used f/8, ISO 100 (K-5) for all shots and increased the flash power by one stop for the TC shots. Focus using Live View, refocusing each time, and then comparing to find the best focus from each series. Same settings applied to all images, including what I consider standard capture sharpening. Crops are from near the center of the image circle, so I didn't examine edge performance.
100% crop from the lens alone and downscaled crops from the TCs:
Tighter crops, upscaled to double the above size (arranged in a different order compared to the above):
Both TCs magnified a bit more than 1.4x. I kept the camera in the same position, so with TC the lens was a little closer to the subject, but only 25 to 30mm, 0.3% of the subject distance. One TC gave about 1.43x and the other 1.46x.
Any suggestions for better testing would be appreciated!