The difference between SFP and FFP
The main differences between SFP (Second Focal Plane) and FFP (First Focal Plane) reticles in scopes are:
Focal plane location:
SFP reticles are located in the second focal plane, behind the objective lens. FFP reticles are located in the first focal plane, in front of the objective lens.
Reticle size relative to target:
With SFP, the reticle size stays the same but the target image gets smaller at higher magnification. With FFP, the reticle and target image both get larger/smaller together as magnification changes, keeping the same aiming point.
SFP suffers from parallax error at lower magnifications since the reticle is not at the focal plane. FFP avoids this since reticle and image plane are always at the same focal distance.
SFP and FFP prices
Generally, FFP scopes are more expensive than SFP due to the more complex optical design needed to place the reticle in the first focal plane.
FFP is better for long-range precision shooting where holdover points need to remain consistent at all magnifications. SFP works well for most hunting/tactical uses where quick target acquisition is important.
So in summary, FFP provides a more natural sight picture and is parallax-free aiming at all magnifications, while SFP is simpler but has parallax and changing reticle issues at higher mags.