|ABOUT PARALLAX ...
Optical sighting devices often use the term "parallax free" as a unique feature.
Sadly, it is misrepresentation unless it provides adjustability for parallax error at various ranges (some rifle scopes have an adjustment for this purpose). Optical devices are designed to be free of parallax at a specified distance. With that said, it is obvious that all optical devices experience some degree of parallax error when used at distances other than the engineered distance. In reality this variation is negligible! Optical designs are engineered so that parallax error is minimal within the suggested operating range of the device. Basically, the parallax error is so minute that it has no effect on performance.
"Parallax Free" is the ability of an optical device to operate with minimal or no parallax within its anticipated operating range.
Hopefully this will bring some clarification to an often intentionally misused phrase.
Parallax: an apparent change in the direction of an object, caused by a change in observational position that provides a new line of sight.
Optical: 1. Of or pertaining to sight. 2. Designed to assist sight.
Sight: 8. A device used to assist aim by guiding the eye, as on a firearm or surveying instrument.