I'm considering buying the Vortex Viper in either the 4x16 or 6x24x50 pst models. Each has an option for first and second focal plane. As I understand it, ffp allows at any magnification setting the reticles moa marks remain accurate. With sfp scopes, only the highest mag level makes these "marks" on the reticle are accurate. My thinking is this...if I decide to get the 4x16 I would get the sfp because I could live at the 16 power setting for most range shooting. However, if I wanted the 6x24 power I would get the ffp so that I coudl dial down the power setting without making the reticle off.