Second Focal Plane vs. First Focal Plane.
There are two types of variable power scopes out there.
Most scopes that you will encounter are 2nd focal plane.
This means that the reticle is placed in the 2nd focal plane within the scope's optical design.
2nd focal plane reticles will not change in size when you adjust the magnification of the scope.
2nd focal place reticles are only accurate at ONE POWER SETTING.
In most cases, the reticle is accurate at the highest power setting that the scope offers.
You must know what power setting your scope is accurate at and USE THAT SETTING for all measurements.
Alternatively, we have First Focal Plane scopes.
First Focal Plane scopes have the reticle placed in the first focal plane of the scope's optical design.
This means that the reticle appears to change size within the scope as you adjust the magnification.
The reticle is actually staying the SAME SIZE in relation to the target you are observing.
This is a huge benefit because the reticle is now accurate at ALL POWERS.
This allows you to hold-off or MIL targets at any power without having to be concerned that you are on the proper scope setting.
The major advantage of first focal plane scopes is being able to use the reticle at lower powers where you have more field-of-view.
This increased field-of-view enables you to move quickly from one target to the next.
The major disadvantage of first focal plane scopes is that the crosshairs often become very thick at higher powers.
The thick crosshair can obscure small targets, which makes it harder to be precise on very small targets.
The advantage of second focal plane scopes is that the reticle size does not change so you can be more precise on small targets.
The major disadvantage of second focal plane scopes is that they must be used on a specific magnification setting in order for the reticle to be accurate.
This usually means that you will not have as much field-of-view as you will be on a higher power setting.
Fixed power scopes all behave the same as a first focal plane because they do not have any magnification adjustment.
Last edited by ar15barrels; 04-06-2010 at 10:03 PM..