First Focal Plane Vs. Second Focal Plane Scopes
This is a good place to explain subtensions, which are the distances between the reticle marks or stadia lines within your scope. If you are using a duplex reticle, an example of a subtension is the distance between the crosshair and the bottom post. If you are shooting a scope with a mil reticle, the distance between each mil is a subtension.
Subtensions are what really differentiates between the two types of scopes. In a FFP optic, the subtensions will be the same through the entire power range of the scope. If using a mil reticle, and the target is two mils tall at 7-power, it will be two mils tall at 14-power as well. If you were using a RFP scope, and the target was two mils tall at 14-power, it would theoretically be 4 mils tall at 7-power, and if the scope went to 3.5-power, would be 8 mils tall. Essentially, in a RFP scope, the relationship of the reticle to the target doubles as the power is decreased by half.
Why is this important? Because if you are thinking of using aiming features within a scope such as a Mil Reticle, or Ballistic Reticle, these features would only be useable on max power in a traditional RFP Scope (or by doing the math of halving/doubling aspect when the power is changed), but in a FFP scope reticle subtensions are useable throughout the entire power range.
Why would you choose a RFP scope over a FFP version? First off, cost is a major consideration. FFP's as a rule, run anywhere from $500-1000 more in high quality models. This alone can be the deciding factor for many people.
Another factor would be close shots in heavy cover. With a RFP scope set at low power, especially with a duplex reticle, your eye is automatically drawn to the center by the heavy lines, making the reticle easy to find. When using a FFP scope at low power, the reticle is often reduced to an extent where it can be hard to find. Both of these instances are generalizations, because there are reticles in both FFP & RFP scopes that would work very handily in this same scenario.
A third factor, especially concerning long range shooting, is RFP scopes will generally have a much finer crosshair at distance, allowing for much more precise shot placement. Because the reticle does not magnify with the image, the reticle appears smaller against the target. When looking through a FFP scope, the reticle is magnified at the same rate as the target, so at maximum power the reticle is much larger than at lower powers, therefore covering a larger portion of the target.
Again, these are generalizations, because a FFP scope can be made to have just as small a reticle at max power as a RFP scope, but in this scenario, can become almost unusable in the lower power settings, practically disappearing, and often becoming just an aiming point.
Subtensions are what really differentiates between the two types of scopes. In a FFP optic, the subtensions will be the same through the entire power range of the scope. If using a mil reticle, and the target is two mils tall at 7-power, it will be two mils tall at 14-power as well. If you were using a RFP scope, and the target was two mils tall at 14-power, it would theoretically be 4 mils tall at 7-power, and if the scope went to 3.5-power, would be 8 mils tall. Essentially, in a RFP scope, the relationship of the reticle to the target doubles as the power is decreased by half.
Why is this important? Because if you are thinking of using aiming features within a scope such as a Mil Reticle, or Ballistic Reticle, these features would only be useable on max power in a traditional RFP Scope (or by doing the math of halving/doubling aspect when the power is changed), but in a FFP scope reticle subtensions are useable throughout the entire power range.
Why would you choose a RFP scope over a FFP version? First off, cost is a major consideration. FFP's as a rule, run anywhere from $500-1000 more in high quality models. This alone can be the deciding factor for many people.
Another factor would be close shots in heavy cover. With a RFP scope set at low power, especially with a duplex reticle, your eye is automatically drawn to the center by the heavy lines, making the reticle easy to find. When using a FFP scope at low power, the reticle is often reduced to an extent where it can be hard to find. Both of these instances are generalizations, because there are reticles in both FFP & RFP scopes that would work very handily in this same scenario.
A third factor, especially concerning long range shooting, is RFP scopes will generally have a much finer crosshair at distance, allowing for much more precise shot placement. Because the reticle does not magnify with the image, the reticle appears smaller against the target. When looking through a FFP scope, the reticle is magnified at the same rate as the target, so at maximum power the reticle is much larger than at lower powers, therefore covering a larger portion of the target.
Again, these are generalizations, because a FFP scope can be made to have just as small a reticle at max power as a RFP scope, but in this scenario, can become almost unusable in the lower power settings, practically disappearing, and often becoming just an aiming point.
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