[OM] Keeper--yes, no, or can't tell?

[Mike Gordon via olympus]

Will be using this lens wide open or nearly so for astro in a couple weeks so 
made another exchange with B&H to try and get a sharp/centered copy. A target 
is fine but my chimney is easy and readily available and good enough to check 
for even sharpness in corners at infinity.  

Third copy seems pretty well centered based on corner sharpness:


https://www.olyendomike.com/Third-copy-Sigma-2014/


I am not sure it is as sharp in the center and left side as the second copy but 
the testing conditions were different. The chimney was heavily backlit both 
times but more severe with the third copy and that might washout some contrast 
which can give the impression of poorer resolution. I put the center images in 
separate tabs in my browser and went back and forth. All the images are cropped 
SOOC jpegs so processing is the same.

Center comparison at F2.8:

https://www.olyendomike.com/Third-copy-Sigma-2014/i-jhLFXnx/A
https://www.olyendomike.com/Second-copy-Sigma-20mm-f14-DGDN/i-BsZDLg4/A




Right Lower Corner at 200% second copy vs third copy:

https://www.olyendomike.com/Second-copy-Sigma-20mm-f14-DGDN/i-3Mhh8FV/A

https://www.olyendomike.com/Third-copy-Sigma-2014/i-MttBpJ7/A


Perhaps I'll repeat a couple shots w/o the backlight and be sure the center is 
as good on the current third copy as the second. B&H closes for Passover soon 
so have to decide quickly. The Center F2.8 comparison bothers me, though the 
lighting differences may negate a valid comparison.  Dustin Abbott's 
comparisons at different apertures showed the contrast is nearly as good at 
F1.4 and maxes out near F2.8.  The sharpness improvement at F2.8 vs 1.4 was 
dramatic in my testing of all 3 copies.  I'd rather have a bit sharper copy in 
the center that was mildly decentered.  

My Sigma 14-24/2.8 is very sharp at F2.8 especially at 14mm.

Pixel peepers and any other comments welcome, Mike


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