DSC06945 Schneider Xenar 45mm f/2.8 is a pancake that can be converted to M42 and stays as pancake in terms of size.
Among the three Schneider Xena 45mm f/2.8, Xenon 50mm f/1.9 and the Tele-Xena 135mm f/4.0 that I have tried, the best image quality comes from the Schneider Xenon 50mm f/1.9. The performance on the 135mm f/4.0 is excellent but the long minimal focusing distance at 14 feet make it harder to use unless your subject focus is far. The 45mm f/2.8 is the cutest one as an altearnative pancake with reasonable performance. A word of caution on the DKL mount, many of the DKL to M42 or DKL to K mount adapters are priced in the $35 to $50 range many of them are poorly made with all sorts of failing issues after few usage. I ran through three DKL to M42 adapters to get one good one out of three. They all work initially and after a few mount, they lose the tight mounting or fail to lock the DKL lens into place.
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