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Old 21-05-2019, 11:41 AM
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Atmos (Colin)
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Join Date: Aug 2011
Location: Melbourne
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Quote:
Originally Posted by billdan View Post
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.
Apologies, wrote it out wrong! I’m used to doing it on my phone calculator and don’t use an equation as such.
SD*((FL-STF)/FL)+(STF/FR)
So
43.3*((1000-55)/1000)+(55/4)
=54.67mm
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