Rifle scopes are based on refracting telescopes. Using image erector lenses to present the shooter with an upright image, they have two planes of focus where the reticle can be placed, at the focal plane between the objective lens and the image erector lens (first focal plane [FFP]), or the focal plane between the image erector lens system and the eyepiece (second focal plane [SFP]).
On scopes that have a fixed power, there is no significant difference. But scopes that are capable of varying in power present the shooter with things to consider. On a first focal plane scope, the reticle expands and shrinks along the image as the shooter adjust the magnification. On the second focal plane scope, the reticle would appear the same shape and size while the image grows and shrinks.
Focal plane advantages and disadvantages
The main disadvantage of the second focal plane designs comes with the use of range estimation, wind holds, hold-overs, and hold-unders when utilizing the Mil-Dot reticle. The problem comes with the proportion between the reticle and the target being dependent on selected magnification. These reticles will only work properly at one magnification, usually being the 14 or highest power.
While the first focal plane designs are not susceptible to magnification-induced errors, they have their own disadvantages. It’s challenging to design a reticle that is visible through the entire range of magnification e.g. a reticle that looks fine and crisp at 24X may be very difficult to see at 6X magnification. Conversely, a scope with a reticle that is fairly easy to see at a 6 magnification may become thick enough at a 22-24 magnification to make a precision shot difficult.
Variable power telescopic sights with first-plane reticles have no problems with point-of-impact shifts. Variable power telescopic sights with second focal plane reticles can have slight point of impact shifts through their magnification range caused by the positioning of the reticle in the mechanical zoom mechanism in the rear part of the telescopic sight.
Normally, these impact shifts are insignificant. But accuracy-oriented users that wish to use their telescopic sight trouble-free at several magnification levels often opt for first focal plane reticles. Around the year 2005, Zeiss was the first high-end European telescopic sight manufacturer who brought out variable-magnification military-grade telescopic sight models with rear-second-plane-mounted reticles. They get around impermissible impact shifts for these sights by laboriously hand adjusting every military-grade telescopic sight.