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Old November 19, 2012, 08:36 AM   #1
tpcollins
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Calculating scope subtentions at different powers

Simple question - I have a Swarovski 3-9x36m scope with the 4A reticle. Per their specs, the subtention from the crosshair down to the edge of the vertical post at 100 yards is 25.2" at 3x and 8.3" at 9x. Is there a special formula to calculate these distances for 4x thru 8x as well? I would assume a linear taper would be too simple . . . thanks.
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Old November 19, 2012, 10:47 AM   #2
Creeper
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No special formula... just math.

If 9X is 100% and you have 8.3" of subtention, then at 3X, which would be 33.3% of the 9X total magnification, you'd have a subtention of 24.9"... so, not perfect, but still, that's pretty close to the listed 25.2".

Using the 8.3" as an accurate reference of subtension, then it progresses as follows (rounded up) for 100 yds.

3X = 24.9"
4X = 18.7"
5X = 15.0"
6X = 12.5"
7X = 10.7"
8X = 09.3
9X = 08.3

You really need to get a large 1 inch grid target and test the subtention values... should be pretty close, but ya' never know, one or several of the magnifications may not be true X.

It's been awhile since I've done this math. It looks right, but if someone sees an error, please provide the correct calculation.

Cheers,
C
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Old November 19, 2012, 02:11 PM   #3
tpcollins
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Good job Creeper, I called Vortex Optics and they gave me the exact same formula. Thanks.
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Old November 19, 2012, 04:02 PM   #4
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No problem... it's good to exercise the old grey blob now and again. You get a chance, I'd love to see how close the math is to your confirmed target subtension. I'm betting Swarovski is pretty close.
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Old November 19, 2012, 06:15 PM   #5
tpcollins
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Creeper - I was at the range recently to tune up for our firearms season and took a sheet of paper with two black lines 8.3" apart and posted it at the 100 yard target line - at 9x my subtentions were dead nut on. I'm good to go, thanks for reaffirming.
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