HOW DOES IT WORK?

An aperture does three things:

• It has a small vertical opening that blocks light rays from above/below the subject from coming into your eye and creating blur in the vertical direction.  Vertical blur will make horizontal lines (like the top of your sight post, and the bottom of the target) appear fuzzy.

• It has a small horizontal opening which blocks light rays coming into your eye from the left and right of your image and creating blur in the sideways direction.

• Its overall opening size determines how much light gets in, and how bright your image is.

Traditional apertures originally adapted from photographic pinholes have circular openings, designed to give the photograph the same focus in both horizontal and vertical directions.  All you can change is diameter.  Choosing the diameter of a circle defines the vertical opening, and the horizontal opening, and it defines the brightness of the image, since total open area of a circle is calculated from the diameter using the formula A = ¼*π*D².  By choosing a circular aperture with a small vertical diameter to help your eye focus on the critical horizontal top edge of the sight post, you are forcing yourself to have the same focus on vertical edges, and you are forcing yourself to accept the brightness defined by the circle’s open area. 

Unfortunately, a service rifle shooter’s true focus needs are not the same as a photographer’s, and a round opening is not the best choice for shooting.  When you shoot with a front sight post, focus on the top horizontal edge of the post, and the bottom horizontal edge of the target is much more important than focus on the sides of the post or target.  You need good horizontal focus so you can tell just when the post is touching the bottom of the target.  If the sides of your post are blurry, this doesn’t hurt you, because you have two sides of the sight post visible, and the human eye can center the target between two blurry edges as easily as it can center the target between two sharp edges.

Bright Focus sights use a rectangular aperture opening, so you can choose the vertical dimension of the opening, and the horizontal dimension, and they don’t have to be the same.  Here’s the benefit:  you can choose a small vertical opening dimension to give you fantastic focus on the horizontal edge between your front sight post and your target.  Then, you chose a large horizontal opening dimension to keep the total area of the aperture large, and to keep the image bright.  This gives up a little focus on your vertical edges, but this is not as important as getting a bright image.

SHOW ME !

A special pinhole camera was built to use actual rifle National Match sight apertures as the pinhole.  The film records what the shooter’s eye sees.  A post representing a front sight was placed 24” from the pinhole lens, and a square target was placed far from the camera, so both near and far focus effects are seen. 

Digital image analysis shows that horizontal edges in the BRIGHT FOCUS image have 30% less blur, with no loss in image brightness.

Question: What about the loss of focus on the vertical edges? 

Answer: To a service rifle shooter, vertical edge focus is not as important as horizontal edge focus.  There are two sides to a sight post, and two sides to a target.  The human eye has a great ability to center objects, even if they are out of focus.  For a shooter, gaining horizontal focus by giving up some vertical focus is a good deal.  It helps you see better, aim better, and shoot better.

Question:  What about internal reflections, since the aperture is so close to the front face of the sight?

Answer:  Bright Focus Rectangular Apertures are designed to avoid internal reflections in two ways: first the aperture itself has the rectangular opening laser cut in it.  However instead of the laser shining straight down and cutting a flat edge (which would give a reflecting surface), the laser is programmed to wobble a few thousandths of an inch at 1kHz, so as it cuts through the metal, it actually creates a V shaped cut.  Thus, after it breaks through, the actual edge your eye sees is a sharp edge on the inside face of the aperture, and the edge of the aperture is a surface that slopes away from your eye.  See Fig. 3 for a cross section of one of the apertures, showing this laser cut edge.  In essence, this is a micro sun shield.  The second element of the design to reduce reflections is that the internal tube of all Bright Focus hoods are made with a very fine thread cut on the inside of the opening.  Since this internal opening is now a series of peaks, rather than a surface parallel to the direction you are looking, the amount of surface which might reflect light back to your eye is dramatically reduced.

Link to White Paper Technology report on use of rectangular apertures on the M-16 rifle.

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