When you hear terminology like “focal plane” it sounds like serious physics is about come at you like an amped up spider monkey, but it’s actually a pretty simple concept. Here’s why. The reticle that you see through a scope is a physical thing. Back in the day, reticles were made from actual Wooly Mammoth hairs Crazy Glued onto the glass lenses. In more modern scopes, the reticle is usually a pattern etched onto glass. The important thing is that’s the reticle is a “physical” object, so it has to be placed somewhere in the scope tube.
That’s where first and second focal planes come into play.
In a first focal plane scope, the reticle is placed in front of the magnification lenses. Think about this for a second. Just like the target and anything else you look at, whatever is in front of the magnification lenses is going to get… magnified. That means that the reticle itself will grow and shrink in size if you change the magnification on a variable power scope. Of course, on a fixed [read non-adjustable] magnification scope, the reticle will always look the same, even if it’s in the first focal plane, because there’s no magnification setting to change. Generally speaking, first focal plane scopes are more of a pain to manufacture, so you’ll often seem them priced higher than a second focal plane scope with comparable features. However, technology benefits march on and over the past couple of years, first focal plane scopes have become much more affordable.
With a second focal plane scope, the reticle is located behind the magnification lens, or closer to your eyeball. Since whatever the scope is viewing has already been magnified, the reticle appearance won’t change as you adjust magnification power up and down. The reticle appears the same at any power setting.
So, that’s all there is to first and second focal plane scopes. The important things to know are the ramifications of reticle placement, so let’s get into that next.
Read the rest: First and Second Focal Plane in Rifle Scopes for Dummies