Red Dot and no-magnification optics are probably the greatest increase in efficiency for firearms outside of the invention of the brass case. Using them greatly increases the chance of getting solid hits on target, as it removes the mental and eyesight gymnastics of aligning four objects into a single plane, reducing it to three.

However, like their magnified optic cousins, red-dots also suffer from parallax, which typically appears at short-ranges when the eye is not centered through the optic. For those not familiar with the term, parallax refers to “the effect whereby the position or direction of an object appears to differ when viewed from different positions, e.g., through the viewfinder and the lens of a camera. (courtesy of Google Dictionary)”

Breach-Bang-Clear has put together an excellent primer on parallax staring with the basic explanation of just what is parallax?

Then, they proceed to test some of the most well-known optics including:

• Aimpoint Micro H1 (4 MOA)
• EOtech EXPS
• Trijicon MRO
• Leupold LCO
• Trijicon RMR 01 (3.25 MOA)
• $40 Tru-Glo Walmart Special

Parallax is easily on nearly all of the optics:

Breach Bang Clear also goes into detail on a few other topics that effect red-dots such as refraction (the apparent displacement of an object as light travels through medium) and aberration.

Check out their great primer over at Breach Bang Clear.