Hi all!
I recently bought both a brand new Pentax DA F1.8 50mm, and since it doesn't work with my (manual) macro tubes, I bought an AUTO Sears MC F1.4 50mm off of ebay ($62 + shipping). I quite like both lenses; the sears is in amazing shape, it really feels like a new lens. Not sure if it's been serviced, etc., but smooth controls, great aperture function, no fungus, etc.
Just for fun, I did some systematic testing. All of the below photos are taken with a pentax K-50. I used a tripod and kept the lighting consistent.
NOTE: the Sears images look a little yellow, although the lens does not seem to show yellowing (to the eye). My light in this room is quite yellow, and I noticed that the Sears is not yellow in sunlight. I am not sure why the auto WB isn't taking care of it? I decided /not/ to touch these up to fix it. These images are JPG out of the camera, so no RAW processing on my end either.
All images at ISO 100, and taken with a small lens hood on. I used a remote IR trigger to avoid shake. I'll also note that the Pentax can go well beyond the F16 that the sears can, but I didn't bother to test that.
Speed
These images were taken in very dim light, to try and pull out any speed differences.
As you can see they are very close. The yellow tint (see above) of the sears makes it look a little brighter, but the histogram tells me that the Pentax is slightly faster, even comparing the 1.4 - 1.8
Sharpness
You can see that the lenses again, are quite similar. the F1.4 really starts to add a softness, a haze, though. Both lenses seem to sharpen up as we close the aperture down
I apologize at the poor alignment. Must have moved during lens change and I am too lazy to do this manually. This image is at 100%
Bokeh
You don't do F1.4 for sharpness, you do it for bokeh and speed.
I did a comparison of both lenses at wide open and stopped down to the next step on the manual lens
As you can see, they are very similar. The 1.4 does give an extra kick, e.g., with the flare just to the left of the cube. At 1.4 the lens gives a sharp double which can look interesting (e.g., foliage behind a portrait). Even at 2.0 the Sears lens seems to give a but more blur. I didn't do DOF comparisons and its hard to see in these images.
Well that's it! To restrict me to full manual - no HSS, no P-TTL, I will probably use the Pentax (it's also a lot lighter). The Sears also flares more but it's nighttime and there is no sun to help show that
Overall, it's a good lens. And I reiterate what I said earlier - the yellowness seems to be strange as I did not get it with outdoors light and I can fix it when I develop from raw...