There never was a 1.4x teleconverter for M42. Plenty of 2x, but they were not the best quality. I do use a lot of M42 lenses with my Fuji X series mirrorless camera. One way to use M42 lens and have a 1.4x teleconverter is to use the Fujifilm XF 1.4x TC WR. Mount it on camera, then use a M42 to Fuji X adapter, then mount the lens on the adapter. It is designed for a handful of Fuji XF zooms. It'll all be manual focus, of course, but it works.
You'd think that adding an adapter and a teleconverter to a vintage M42 ("universal thread mount", as found on #Pentax, #Yashica, #Fujica etc) would make for a combo that is too heavy and too long for normal use. It really depends on the lens you combine it with.
Here it is combined with a #Spiratone #reflex #Mirror #Lens, 300mm f5.6. The whole thing is smaller than my #Fujifilm 50-230, and about the same weight but almost twice the reach.
@Randy_Au if I want extra crisp at 300mm + 1.4 Tc, I will use fujifilm's 70-300mm lens which has 5.5 EV optical stabilisation and autofocus and ridiculous optical resolution.
Still a manageable lens, especially compared to the 100-400 (but heavier than both lens previously mentioned)
@f_dion cool! Surprising amount of detail given the focal length.
It's great that modern sensors can push to 3200+ with relatively manageable noise and lets us do fun things w/ such twitchy lenses
Francois Dion
Randy Au 🙃