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You also need to consider the thickness of the filters and it's effect on the effective flange to sensor distance.
A 3mm astrodon filter will effectively shorten the light path by 1mm.
I tried my 14-24 on my QSI and when focused on the stars the focus marker would read ~1m. I got a 1mm shim to increase the distance and it improved things.
My suspicion is that a complex zoom lens will require a very specific flange to sensor distance to work correctly and produce a flat field.
The G series Nikon lenses without aperture rings are also a pain.
I had much better results with a old prime lens. It's field flatness wasn't affected by the altered light path distance.
DT
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