sscoyote
Well-Known Member
Dave I know this may sound wierd, but this will teach u more about rangefinding with a reticle than anything else u could do. Look out any windows u may have in your house, and see if there are any objects u could use as an "optical std." of sorts, like a street sign, telephone pole, whatever . No go out and as discreetly as possible, measure it, write it down somehwere, and laser back to your window. Make sure u get a couple of these stds. at 300, 400 , 500 whatever ranges. Now grab an old scope with any reticle that has mre than one stadia point in it--simple plex, ballistic or rangefinding reticle will work. Now here's the mil-dot, mil-ranging formula that can be used with any reticle measurement (subtension) there is since the mil-ranging formula is not specific to the mil-dot 3.6 IPHY subtension. Some think that the mil-ranging formula was created for the milliradian, but it's actually just the opposite. All angular systems of rangefinding with a reticle are based on this formula (inches to yds.)--
tgt. size (") x range of reticle subtension measurement (usually 100 yds.) / reticle subtension itself (") / quantity of subtension tgt. occupies (decimal equivalent) = range (yds.)
Looks complicated, super simple to apply-- suppose u're ranging a 10" tgt. with the simple plex reticle that subtends 4 inch per hundred yds. x-hair to post tip at the optics highest power. When u look at it thru the reticle u see that it occupies 1/2 the "gap" (0.5 decimal equivalent). Now just substitute the variables in the simple equation--
10 x 100 / 4.0 / 0.5 = 500 yds.
Now suppose your plex-reticled scope is a Simmons or Tasco, maybe Bushnell. The catalogs don't have a subtension pg. so u may not know what the reticle subtension is. If u think about it u have several stds. that u can use out there. Just work the formula backwards, sort of (reverse milling), and u can calculate the 100 yd. subtension. Using the example above here's how we could calculate the subtension--
10 x 100 / X / 0.5 = 500
X=4.0
See how this stuff works? U can also calculate a magnification that will allow u to bracket a particular target if u're using the common second focal plane scopes. I hit on this myself when i was looking out the window of my house and doing what i just described above. I noticed that as i changed the power of the optic the size of the reticle at any distance away got bigger as i lowered the power (actually the image gets bigger and smaller and the reticle stays the same). It was easy to see that it was proportional, and i could calculate a power that would bracket a certain tgt. size. But the system is actually inversely proportional since as magnification INCREASES, reticle subtension DECREASES...and it worked most of the time assuming the power ring is calibrated properly.
It is very important to understand these concepts if you're trying to increase your knowledge of optics and their applications. It has been quite an education for me playing with this stuff.
tgt. size (") x range of reticle subtension measurement (usually 100 yds.) / reticle subtension itself (") / quantity of subtension tgt. occupies (decimal equivalent) = range (yds.)
Looks complicated, super simple to apply-- suppose u're ranging a 10" tgt. with the simple plex reticle that subtends 4 inch per hundred yds. x-hair to post tip at the optics highest power. When u look at it thru the reticle u see that it occupies 1/2 the "gap" (0.5 decimal equivalent). Now just substitute the variables in the simple equation--
10 x 100 / 4.0 / 0.5 = 500 yds.
Now suppose your plex-reticled scope is a Simmons or Tasco, maybe Bushnell. The catalogs don't have a subtension pg. so u may not know what the reticle subtension is. If u think about it u have several stds. that u can use out there. Just work the formula backwards, sort of (reverse milling), and u can calculate the 100 yd. subtension. Using the example above here's how we could calculate the subtension--
10 x 100 / X / 0.5 = 500
X=4.0
See how this stuff works? U can also calculate a magnification that will allow u to bracket a particular target if u're using the common second focal plane scopes. I hit on this myself when i was looking out the window of my house and doing what i just described above. I noticed that as i changed the power of the optic the size of the reticle at any distance away got bigger as i lowered the power (actually the image gets bigger and smaller and the reticle stays the same). It was easy to see that it was proportional, and i could calculate a power that would bracket a certain tgt. size. But the system is actually inversely proportional since as magnification INCREASES, reticle subtension DECREASES...and it worked most of the time assuming the power ring is calibrated properly.
It is very important to understand these concepts if you're trying to increase your knowledge of optics and their applications. It has been quite an education for me playing with this stuff.
