The Firing Line Forums - View Single Post - AR15 1-4X scope

btmj · February 29, 2012, 03:54 PM

The beam of light that exits the occular lense has a certain diameter. That diameter can't be any bigger than the occular lense, but in general it is equal to the objective lense diameter divided by the magnification.

Your eyeball at full dilation (at night) can accept a beam up to 7.5 mm (I think... I am testing my memory here). Typical daytime pupil can accept a beam of about 3mm I think. A beam that is larger than your pupil means you can move the scope relative to your eye and still have full view... ie the sweet spot is big.

Example: A 1-4X 20mm scope: at the 1X setting, the beam hitting your eye is 20 mm, which gives you a lot of leeway in positioning that scope. At 2X, the beam is 20 / 2 = 10mm, which still gives you a lot of leeway. At 3X magnification, the beam is 20 / 3 = 6.6mm. At this point, the beam will be just a little smaller than your maximum dilated pupil, but still bigger than your normal daytime pupil. Now at 4X magnification, you certainly start to notice a tunnel vision effect. I will say that it is nowhere as bad as a 4-16X40 at maximum 16X magnification (for instance).

February 29, 2012, 03:54 PM	#17
btmj Senior Member Join Date: November 1, 2011 Location: Near St. Louis, Missouri Posts: 864	The beam of light that exits the occular lense has a certain diameter. That diameter can't be any bigger than the occular lense, but in general it is equal to the objective lense diameter divided by the magnification. Your eyeball at full dilation (at night) can accept a beam up to 7.5 mm (I think... I am testing my memory here). Typical daytime pupil can accept a beam of about 3mm I think. A beam that is larger than your pupil means you can move the scope relative to your eye and still have full view... ie the sweet spot is big. Example: A 1-4X 20mm scope: at the 1X setting, the beam hitting your eye is 20 mm, which gives you a lot of leeway in positioning that scope. At 2X, the beam is 20 / 2 = 10mm, which still gives you a lot of leeway. At 3X magnification, the beam is 20 / 3 = 6.6mm. At this point, the beam will be just a little smaller than your maximum dilated pupil, but still bigger than your normal daytime pupil. Now at 4X magnification, you certainly start to notice a tunnel vision effect. I will say that it is nowhere as bad as a 4-16X40 at maximum 16X magnification (for instance). Last edited by btmj; February 29, 2012 at 04:06 PM.