sscoyote
Well-Known Member
Johnny, what they're doing is using a reticle system that has stadia lines that measure (subtend) 1.05 inch per hundred yds. Now when they look at a target of known dimension at distance they bracket it between the MOA lines, and take a bracket reading, by interpolating (guessing) how many MOA lines the tgt. occupies using the decimal equivalent (3 MOA and 7 tenths to # 4). If the scope they're using is a 2nd focal plane scope (the reticle is located in the 2nd focal plane) then the optic has to be set on a certain magnification for the subtension to be correct. If it's a 1st FP scope the subtensions will remain the same as the power is changed. A 1st focal plane reticle is also refferred to as a magnifying reticle, and if u have 1 u will know it as the reticle changes size as u change the power. Most American scopes r 2nd focal plane
Subtension is a difficult concept to understand, and takes some thinking to get it right in your head (did for me at least). The best way to understand it is to get a scope that has a plex reticle in it (any multi-stadia reticle will work). Look out the window of your house at something and then change the power. U should easily see that as u lower the power the object takes up more space between any 2 stadia in the reticle, and as u increase it it takes up less space (subtension is inversely proportional to magnification--a very important concept to understand)--the reticle stays the same but the object gets smaller and larger.
Subtension is a difficult concept to understand, and takes some thinking to get it right in your head (did for me at least). The best way to understand it is to get a scope that has a plex reticle in it (any multi-stadia reticle will work). Look out the window of your house at something and then change the power. U should easily see that as u lower the power the object takes up more space between any 2 stadia in the reticle, and as u increase it it takes up less space (subtension is inversely proportional to magnification--a very important concept to understand)--the reticle stays the same but the object gets smaller and larger.
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