Quote:
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Originally Posted by Not quite Correct, see EDIT
The calculation that I have used is:
SD*(FL/(FL-STF))+(STF/FR)
SD - Sensor Diagonal
FL - Focal Length
STF - Sensor to Filter
FR - Focal Ratio
For a FF DSLR and a 10” F/4 Newtonian.
43.3*(1000/(1000-55))+(55/4)
= 54.67mm
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Now this is calculating vignetting. As Bojan has said, it’s about vignetting. What the example above shows is that with a 10” F/4 Newtonian on a FF DSLR, M48 threads are too small and should be at least 55mm to avoid vignetting.
EDIT: Correct equation is:
SD*((FL-STF)/FL)+(STF/FR)