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Pentax DA 12-24 vs. Sigma 18-125 DC

At 18mm

I own the Sigma 18-125 and it has become one of my most used lenses. It is small, versatile and its sharpness it good when stopped down a notch. Only drawback: Its distortion. Now I also own the Pentax DA 12-24, an ultrawide zoom. But this Ultrawide does reach far into the region my Sigma 18-125 covers, too. I set out to compare the two lenses at 18mm and 24mm. Both lenses are designed for APS-C sensors and both are rather complicated designs - the one because of its ultrawide range, the other because of its massive 7x zoom range.
I chose this rather bad page design (horizontal&vertical scrolling) because I believe it makes comparing easier.

full frame	Centre	upper left corner	upper right corner	lower right corner	lower left corner
Pentax DA 12-24 f4 18mm	Pentax DA 12-24 f4	Pentax DA 12-24 f4	Pentax DA 12-24 f4	Pentax DA 12-24 f4	Pentax DA 12-24 f4
Sigma 18-125 DC f3.5 18mm	Sigma 18-125 DC	Sigma 18-125 DC	Sigma 18-125 DC	Sigma 18-125 DC	Sigma 18-125 DC
Pentax DA 12-24 f8 18mm	Pentax DA 12-24 f8	Pentax DA 12-24 f8	Pentax DA 12-24 f8	Pentax DA 12-24 f8	Pentax DA 12-24 f8
Sigma 18-125 DC f8 18mm	Sigma 18-125 DC	Sigma 18-125 DC	Sigma 18-125 DC	Sigma 18-125 DC	Sigma 18-125 DC

Conclusion:
Tthe Sigma appears to be slightly sharper, but shows more corner softness, especially when wide open. Not really visible here, but corner shadowing is less on teh Pentax (when at 18mm+). The Pentax has much less distortion. I have reshot the series twice on different days.

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