First, or Front, Focal Plane scopes have the reticle installed toward the front of the erector tube, the part of the scope that houses the erector lens and responds to adjustments in windage and elevation via the knobs on the top and side of the scope. This is significant because as the shooter adjusts the magnification of the scope the zoom lenses inside the erector tube also move toward or away from the reticle resulting in the reticle “growing and shrinking” as the adjustment is made. The crosshairs remaining a constant size in relation to the target.
FFP Scopes have the disadvantage of being expensive and in certain situations the reticle can actually obscure a target that may be particularly far down range. The constant reticle calibration is however a big advantage that many shooters find to be worth the extra cash. Especially for competitive shooters who may face a number of different ranges in a given match.
Second, or Rear Focal Plane scopes have the reticle installed toward the back of the erector tube. As such it is not subject to the perceived resizing when adjusting the magnification from the movement of the erector lenses. The reticle stays constant in relation to the shooters eye instead of the target.
Although cheaper to produce and purchase, a disadvantage of a SFP is that the reticle is only in correct calibration at one specific power. These scopes are best used when you're shooting at known distances.
If you're looking for a scope or other optic for your rifle visit the Black Rain Ordnance Optics section HERE.
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