Originally posted by pres589 I know there's no real way to measure this, but, just how much better is this thing than the Samyang 135 f2 which seems available new in the USA for about $500 USD?
Like HarisF1, I have both.
Sharpness: Samyang is sharper wide open. Pentax has some CA from f1.8-2.2, and it catches up to the same league about F2.4. From 3.2 up, both are super sharp and well corrected.
Color Rendering: Pentax is better. I don't like the blues of the Samyang. It's a pretty big difference for portraits.
Bokeh: Under some circumstances Pentax bokeh balls make colored outlines. Despite this, the Pentax bokeh is more pleasing.
Transmittance (T-stop): The Pentax passes significantly more light at F2 than the Samyang does at F2. Wide open it handily beats the Samyang in t-stop, making it superior for (non-astro) low light shooting.
Astroprotography: Near wide open, the Samyang has superior color correction from about 420nm on up to the near UV, making it superior for star photos.
Build/feel/usage: Pentax is much heavier. Pentax quality/feel wins by a mile. Also, you may have to try more than one copy of the Samyang to get one that's uniformly sharp in each corner. (my first one wasn't)