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Old 21-05-2019, 11:29 AM
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billdan (Bill)
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Join Date: Mar 2012
Location: Narangba, SE QLD
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I think Colin's math is a little incorrect.

Quote
For a FF DSLR and a 10” F/4 Newtonian.

SD * (FL/(FL-STF))+(STF/FR)
43.3*(1000/(1000-55))+(55/4)
= 54.67mm


But 43.3*(1000/(1000-55))+(55/4)
= 43.3*(1000/945) + 13.75
= 43.3 * 1.058 + 13.75
= 59.56mm

If we used this formula instead

SD / (FL/(FL-STF))+(STF/FR)
= 43.3/(1000/945) + 13.75
= 43.3 / 1.058 + 13.75
= 54.67mm

I'm not sure which one is correct, 59.56mm using Colins formula as written, or 54.67mm uing the modified formula.

Either way, 50mm filters will give you vignetting at that 55mm distance from filter to sensor at F4.
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