I thought the K 55 1.8 was based on the old Biotar lens design while the A 50 1.7 is a Planar derivative. Both seem to be good lenses; I like how compact the M 50 1.7 is and packs better build quality than the A. I actually purchased an A version with the common broken aperture ring issue mentioned already in this thread. At some point I'd like to fix it.
Which is better for sharpness and contrast wide open, the 55 or the 50 1.7? I think the M 50 1.7 was supposed to be a Leica-beater back when it was new and I'm not sure if the K 55 1.8 is quite up to that standard. For portraits the K 55 1.8 seems plenty fine though from sample images.
I'd kind of like a K 55 1.8 for use on a mirrorless kit along with my M 28 f2.8 mk2.
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