Best way to compare two 50mm lenses?

Started 10 months ago | Questions thread
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CircaM
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Best way to compare two 50mm lenses?
10 months ago

I have a Pentax K-r and two 50mm lenses.

I don't need 2 50mm's so I want to test which one will take the best images and sell the other one.

One is a SMC Pentax-A 50mm f/2.0 manual.
The aperture ring works fine but sometimes the button doesn't press down, then it won't turn, and sometimes the button just doesn't pop back up to lock the ring at the aperture.

The other is a MultiCoated Sears 50mm f/1.7.
Which is in slightly better condition, smooth, but is a "Sears" lens.
It does say "auto" on the lens, but I don't know what is "auto" about it.

What is the best way to compare the image quality of both of those lenses so I can determine which to sell and which to keep?

 CircaM's gear list:CircaM's gear list
Pentax K-r Pentax smc DA 18-55mm F3.5-5.6 AL Pentax smc DA 50-200mm F4-5.6 ED Adobe Photoshop CS6 extended +1 more
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Pentax K-r
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