I liked the Pentax FA 400mm f/5.6 ED IF right away; it’s not particularly heavy, and can be used with the camera mounted on the tripod head, no need to mount the lens; this is more stable. I heartily recommend it to any 645D user.
At long focal lengths, blur from the shutter itself can be a problem, even with mirror lockup. In this case, I used ISO 400, and I also mounted the camera to the tripod (not the lens), which is a more stable setup (but not suitable for heavier lenses). Even so, 1/60 is a problem.
This is the entire frame as shot (shade).
Shutter speeds at f/5.6 - f/16 were 1/500, 1/250, 1/125, 1/60. The 1/60 shot shows some degradation. Some of this is from diffraction, but it also looks like some motion blur is present, suggesting that the shutter itself is an issue (even with mirror lockup), as with the 300mm f/4.
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