Used with a 645z, the weather sealing and light weight make this an awesome performer in inclement weather. Fantastic contrast, sharpness and bokeh.

Exceptionally good lens. I test images based on the default raw processing settings of DXO Photolab, and this lens is in their database, so distortion and lateral color are automatically corrected. I tested this on a 645z.

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  Surgically sharp without being harsh

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  None

Spectacular lens. Extremely sharp on the Pentax 645Z, great contrast, and small footprint. Great grip. Little to no flare, even shooting directly into the sun. Only minor quibble is that there is some purple fringing against bright light. Fantastic value.

I am using on K-1 with cheap Chinese adapter, which is all you need because the expensive adapters provide no more functionality, although I wish my adapter had a tripod socket to allow tripod use with the heavier 645 lenses.

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  Sharpness, smooth rendering, ease of focussing.

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  Flare

I just carefully examined the optical performance of three copies of the 55 D-FA lens on a 645Z body. By any traditional, fussy standard, all three copies were exceptionally fine lenses, in my tests being essentially perfect from corner to corner from f/5.6 on up.

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  Superb overall optical performance from 5.6

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  Not quite up to wide-open star capture work, hard to focus for wide open at infinity

This is a really good lens. I was nervous about buying one because reviews are not unambiguously positive, and it's not spoken of much outside this forum. I'm glad I took the chance. I wanted something faster and lighter than my fantastic 45-85/4.5 SMC Pentax-A 645 zoom. This fits the bill.

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  Well built, reliable, sharp

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  A bit heavier than I'd like...

I really don't understand all the great reviews . This lens is sharp in the center and very soft in the corners and edges, even stopped down to f8. Also a fair bit of distortion . I was really shocked and dissapointed by how poorly it performs .

its perfect for the new 645z. the weight is awesome for the quality.

Great lens and the second most used lens in my kit. Great image quality, and well balanced on the 645z.

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  Great Performer & Lightweight

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  None that I can find

While I had big expectations, I was still surprised by the quality of this lens. It is incredible. Without a doubt, the best lens I own.Every photographer should have one.