First or Second Focal Plane?
It refers to where in the scope the reticle is located, be it before the magnification assembly or after the magnification assembly. Its location makes a real difference to how are you can utilise that scope, especially around the reticle
Chances are, most scopes you have used, have been second focal plane. This means that as you zoom through your magnification range, the reticle doesn't visually change in size. On something like a duplex reticle when you have a crosshair aiming mark and that is about there is, it makes little difference.
A first focal plane scope, with a reticle with additional information, will ensure that information is correct no matter the magnification you're on. If you have a reticle, like a BDC (bullet drop compensated) and you've got hashmarks that are 1 MOA away apart from each other, on a second focal plane they will actually only be one 1 MOA apart at a given magnification. Commonly this is at the top magnification of scopes that go up to about 20 power. Higher magnification scopes may nominated a specific power that the reticle is correct at.
Those hashmarks will actually change the amount that their measurement as you adjust the magnification. So, on a 12 power scope, if you drop down to 6 power, you will actually see twice as much information between those two lines that were one MOA apart. Now they are 2 MOA apart, as the you have halved the magnification. That's fine on six power, its a pretty easy calculation to make, but when you're on 7.8 power, its not so easy to do in your head.
Enter the first focal plane scopes.
First focal plane puts the reticle physical and a different location so that you actually are zooming up on the reticle as well as the picture down range. As the reticle adjusts inline with the down range image, those hashmarks that were 1 MOA apart on 3 power, are still 1 MOA apart on 12 power.
However, as you zoom out those two marks get a lot closer and closer together. They retain the same amount of information between them but because they are linked to magnification will now gets smaller and smaller. Herein lies one of the downsides of first focal planes scopes.
Down sides of FFP
Reticles that were used on second focal plane scopes, and have been moved over to FFP scopes often become difficult to use. At low magnification, they don’t give you easy to see aiming marks, as the lines are too close together, and at high magnification, the line thickness can actually block out what you are attempting to aim out.
This is an issue that has been the focus of competitive shooters, professional operators and optics companies in recent years, especially with the rapid growth of things like the Precision Rifle Series. There have been significant developments in reticle design to make the most of the FFP benefits. Floating dots, contrasting hashmarks and open grids are no common to see on even some of the cheaper FFP optics on the market.
When SFP is better
This does not mean that a SFP scope is no longer useful. As is usually the case with these things, it is all about the application you are using it for. So if you are wanting to shoot dear at pretty standard distances, SFP is the ideal option. If you are after pigs with your 30-30 and want to put a lower power scope on there, definitely choose the SFP option, they are generally cheaper as well.
Even if you are shooting benchrest competitions SFP would be a great choice. You are generally going to be set on one power and you are making most of your adjustments by using the elevation and windy turrets. You will have little need for any holding marks in the reticle.
The other factor maybe that you're on the same power for the entire match, perhaps in F-class. You might stay on 45 power therefore, a reticle it changes with magnification makes no sense because you always shoot on the same zoom power.
FFP scopes have found their home in dynamic styles of shooting where adjustment to magnification and utilising more complex reticles have significant benefits. Precision Rifle shooting, long range hunting and even spotlighting can see significant benefits to using FFP scopes.