Hey everyone, I've spent a lot of time with these two lenses so I can give a better comparison (for those who care):
Colour: The K 55's colour is more vintage and significantly less accurate / saturated. This is not always a bad thing, but it does not pop like the 50.
Sharpness: The K55 is sharper than the 50 until f4 ish if you are pixel peeping. The k55 also renders higher contrast, while the 50 is softer and brighter (especially wide open). The difference is similar to the difference between the FA 35 and the DA 35 2.4.
Bokeh: Neither lens has killer bokeh when you stop down, but I tend to prefer the 55 because it is more creamy / better subject isolation. The 50 resembles a impressionistic painting. If the background is complicated, the 50 tends to render a harsh bokeh which can be distracting (but other times can look pretty cool).
Focal length: Seemingly unimportant, but possibly the largest difference between these lenses! Colour and contrast are things one can process with raw. Bokeh is more or less similar, with a slight edge given to the K55. However neither can match a 1.4, from what I have seen. However, the 55 is simply a better portrait lens. It is a little tighter and seems to render differently (smoother) since it is more zoomed in. The transition from in-focus to out of focus is very smooth and, I believe, 3-D. The extra 5mm leads to out of focus circles rendering much larger, which can be smoother.
Now all in all, I still want to keep both. However, this focal length in ASP-C land is very much a portrait length, and for this reason the K55 wins. I believe it wins primarily because it is 55mm over 50mm, and increased center sharpness WITH soft edges and backgrounds. The 50mm seems sharp to a fault when stopped down. I suppose you should consider that since I am looking at this as a portrait lens, my interpretations of the lenses' performance is a little skewed. However, the focal length tends to lend itself to portraiture.
Either way, if you own either lens and are wondering about the other one, be confident that they are more similar then they are different. If you are looking for an inexpensive portrait lens, I don't think you can do much better than the K55 1.8 for sub-50 dollars
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