Re: Explain benefits of front focal plain
The benefit mainly corresponds to any reticle that has multiple hash marks or dots that subtend to varying angles of measurement. It also affects reticles that are designed for a specific ballistic path. This style of reticle includes mil dot or mil hash reticles that may subtend every 1 MIL or 1/2 mil, etc. There are MOA reticles setup the same way but may subtend every 1 MOA or 2 MOA. Also ballistic reticles like the Zeiss Rapid Z or NF's velocity reticle would apply.
On a SFP scope the correct subtention (correct angle of measurement) only occurs at one magnification setting, which is usually the highest. So this means at all other magnifications your ballistic charts regarding hold over/under, MIL/MOA drop etc cannot be directly transferred to the reticle any longer unless a multiplier is applied to the reticle subtentions listed for your scope. This would also affect ones ability to range objects at varying powers or to hold off for windage at various powers.
As an example, a 2.5-10x42 SFP scope with a mildot reticle will typically be setup to subtend correctly at 10x, meaning that when on 10x the distance from each dot is 1 MIL. So if your ballistic chart tells you to hold off 2 MILS for a given distance then you simply use the 2nd mildot to hit the target. But if you are on 5x the correct subtention between dots is 2MILs. The power dropped in half so the correct subtention doubled. So now to hold over 2 MILs, like in the example above, you only need to drop down and use the 1st mildot to hit dead on. At 2.5x 1 mildot would subtend 4 MILs so for me to hit dead on I would need to hold in between the cross hair and the 1st mildot at the 1/2 MIL point in order to have a hit on target.
With an FFP scope it wouldn't matter what magnification setting you had your scope on because the reticle changes size in relation to the target so the subtention is correct over the entire magnification range.
This may lead to asking about what the negatives to a FFP scope are. IMO not many.
* The reticle size, and your ability to see the details of the reticle, drops as the magnification setting goes down. Fortunately, the majority of the times you would need a lower power magnification, closer shots, running shots, etc., you aren't going to be in need of additional hold off points. The main reticle cross is all that you will need to make the shot. And in today's market I have seen very few FFP reticles that aren't acceptable for this use.
* On the flip side as power goes up the reticle lines could become too thick for what some people would want for exact aiming and bullet placement. IMO for big game hunting this is a complete non-issue. Having said that I am not a big fan of high power scopes with 5-6x erectors on them. I do still use SFP reticles on my varmint guns but there are a couple FFP reticles that I am going to try out that seem thin enough to allow success.
* They are more expensive than their SFP counterparts.