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Old 01-24-2021, 12:02 AM
ShootingSight ShootingSight is offline
Join Date: Jan 2010
Location: Cincinnati, OH
Posts: 292
Default What the human eye really sees

Last week, I crossed something off my 'to do' list that has been there 10 years. I replicated with a camera what the human eye sees.

The human eye is between 20-25mm in diameter, so a 20-25mm lens. Pupil/aperture is between about 3/8" and 1/8", so between about f: 2.8 and f: 8, though with an aperture sight can be about f: 22-25.

I set up my camera with a 24mm lens, and set the slide (long slide - 9.5" sight radius) of a 1911 so the rear sight was 24" from the lens (about my arm length), and I hung a 20 MOA target 50 feet away.

I then photographed across a matrix of focal points and aperture sizes to replicate what the human eye sees at different brightness/aperture settings, and with different focal points.

A point of math. Focal distances can be expressed as diopters, which are inverse focal lengths. This is an abstract mathematical concept, but it simplifies focus, as it describes both distance, and blur. You need zero diopters to focus perfectly on the target, you need 1.5 diopters to focus on the rear sight. Front sight is between 1.0 and 1.25 diopters away. Depth of field is expressed as a tolerable level of +/- diopters. If you focus at 1.00 diopter and an object is at 1.25 diopters, you have a 0.25 diopter degree of blur. Importantly, if you focus at 0.75 diopters, you have a +0.75 diopter blur on the rear sight, a -0.75 diopter error on the target, and a +0.25 to +0.50 diopter blur to the front sight.

Aperture size is driven by brightness. In an indoor range, you can likely achieve an aperture that tolerates a +/- 0.75 diopter blur. So your strategy is to focus at +0.75 diopters, so the target and rear sight each have a 0.75 diopter blur. In brighter light, you can reduce your aperture and achieve a +/- 1.00 diopter depth of field. Now you can focus at +1.00, so your target has tolerable focus, but your diopter error to the sights is reduced, so you have a tolerable target blur and sharp sights.

Art Neergaard
ShootingSight LLC

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Old 01-26-2021, 10:22 PM
ptf18 ptf18 is offline
Join Date: Dec 2010
Posts: 207

Art. Thank you. I've recently dragged my "vintage" rifles out of the safe. Peering down the barrels with 67 y.o. eyes (although they have been "fixed" via cataract lens replacement) I have an better "appreciation" of what you have (and in the past) offered with your "explanations" of vision and firearms usage.
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