For scopes with reticle on first focal plane, the reticle will be proportionally changed when you change the power setting (the higher the power, the thicker the reticle). For scopes with reticle on second focal plane, the reticle will stay the same size/proportion no matter which power setting. You don't normally see an American brand name scope with the reticle in the first plane. Europeans normally can shoot in dimmer light than we can so some of them tend to like the reticle in the first plane so it is more bold.
I have an old Weaver K8 with external adjustments on the rear where the ring would normally be. The reticle is on the 1st focal plane. When set at the lowest power, the reticle looks almost too small to use in the woods. When set at 8x, it looks like a couple of crossed telephone poles!
The biggest advantage to a first focal plane reticle is that since the reticle changes size with the magnification of the scope, any range estimation features will work no matter what power you have the scope set for. So, if you use a mil-dot reticle, the space between two dots will always be 1 mil (about 3.6") whether you are at 3x, 10x, or 50x. Pretty handy if you know how big something is, but don't know how far away it is...
The biggest disadvantage is that for a first focal plane reticle to be thick enough to use at lowest power, they tend to get pretty fat at the highest power and obscure more of the target than most people like.
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.
Understanding the difference between 1st and 2nd focal plane reticles that have more than 1 stadia point in them can go a long ways towards establishing long-range shooting systems, reticle-rangefinding, and point blank range rangefinding systems. The main point is that the subtension between stadia points doesn't change in 1st focal plane as has been mentioned, but the 2nd focal plane reticle allows for a lot of flexibility for downrange zeroing and rangefinding in that it is an inversely proportional system--as power INCREASES reticle subtension DECREASES.
The l is not an I (God forbid ), but it is an L. No wonder my friends requests have all been denied!!! ;D
I remember Dad talking about that, and I believe he experimented a little with what you are talking about. I think he preferred the 1st focal plane riflescopes for hunting applications because, for him, it was so simple to understand and to use for hunting Tx whitetails under field conditions. The old KISS principle.
The trick to this is knowing the size of your target. Dad measured whitetails for some time. He went to processing places during deer season, for several seasons. They would let him go into the storage lockers, tape measure in hand. He told me that deer in his area consistently measured 13 inches from sternum to the top of the back. The larger heavier deer were still 13 inches, but they were broader, and had more mass. I myself have measured a few deer, and they seemed to be what he said. When Dad was doing this, he lived in Abilene, and I think his measurements were pretty much in that area.
Most people are not adept in estimating range, myself included. But if you can throw up your scope, and almost instantly see that the top portion of a post covers 1/2 of a deers body, then that deer is roughly 200 yards.
I like the Simmons scope I mentioned. It is a 3.8 x 12, with the diamond reticle. It is a good hunting scope. I don't hunt in open spaces much, so I have not used it much for the range finding and ballistic compensation, but I can when I need to. For my hunting, what I like is that in dim light, you can turn the power up to about 9, and see the reticle good enough to make a shot.
This is not hype or theory. I can state this because I have it this years.
Sorry about the I + L mixup--i understand your concern there.
I have also put a lot of time into researching rangefinding and downrange zeroing with reticles, and it has been extremely educational for me. I have found that with a little practice most folks can get within 3% of lasered ranges to very long-range on tgts. of known dimension even by using the simple plex if the subtension measurement is wide enuf, and most importantly known.
1 of the biggest advancements I ever made was when i realized that the mil-dot mil-ranging formula is not specific to the mil-dot reticle at all and can be used with precise accuracy with any reticle out there. U would not believe the misinformation there is floating around out there regarding reticle-rangefinding--even from the "experts." Once every variable in the mil-ranging formula is understood it can answer any question that anyone would ever need to know about the subject.
Here are some of my findings regarding point blank range, and ballistic reticle rangefinding under Item C) Reticle-Rangefinding--
A forum community dedicated to the great outdoors and hunting enthusiasts. Come join the discussion about hunting, fishing, survival, archery gunsmithing, optics, reviews, accessories, classifieds, and more!