My dad was a gun writer, did not have much published, he had pretty technical tendencies. He did have an article published in rifle magazine, it was on this subject. I believe it was titled "Whitetails at Longish Range".
Dad was an advocate of having the reticle in the 1st focal plane. The reticle subtends the same area at a given range no matter what power the scope is on. He had an old Kahle's variable scope with a post, the area above the horizontal line covered 3 inches at 100 yds. It does this at 3 power on up to 9 power.
This has a couple of practical applications.
If you know the size of your target you can roughly determine what the distance is. Since we are in Tx, Dad measured countless whitetail deer, and found they were almost 13 inches brisket to top of back. For practical purposes, you could just call it an even 12 inches. If this 3 inch portion of the reticle happened to cover 1/2 the deer's body standing broadside, that deer is about 200 yds. If it covers 3/4 of the body, that deer is about 300yds, if it covers the entire body, then it's about 400yds.
The 2nd application is with ballistic compensation. Many non-magnum rounds, if zeroed dead on at 200yds, are going to drop about 9 inches at 300 yds. Remember our 3 inch section of the post? Well, at 300 yds, that 3 inch section subtends 9 inches!!!
Dad zeroed in his 30-06, so that the top of that reticle was dead on at 200yds. At 300 yds, the horizontal bar compensated for the 9 inches of drop. I believe Dad was partial to 165 gr bullets, handloaded a lot, and his load chronographed somewhere around 2700 fps.
With the reticle in the first focal plane, you can do this no matter which power the scope is dialed in.
Dad practiced this quite a bit and was said to be pretty deadly. I never actually witnessed him sighting in a deer at 300 yds, determining the distance, then rolling him over with the horizontal bar. Nonethess numbers don't lie.
I found a scope with similar characteristics. Made right here in America, by Simmons. I think they call it a smart reticle or something. It has a diamond shaped reticle, and it is in the first focal plane. I bought mine about 10 years ago, I am guessing they still make it. The entire diamond covers 6 inches at 100 yds, but it is bisected with the horizontal line. So, each section, upper and lower, covers 3 inches at 100 yards. So this will do the same thing, if you set it up right.
Dad passed away 3 years ago, he was a very dynamic, likeable person. He was a veterinarian by trade, and was what you might call "proficient" as a hunter, reloader and shooter.
