FFP vs. SFP
When choosing a riflescope, you need to consider which focal plane of the sighting line is installed on the scope — the first focal plane (FFP, also called front focal plane) between the objective lens and the erect image lens, or The second focal plane between the erect lens and the eyepiece (second focal plane, referred to as SFP, also called rear focal plane). On these two focal planes, the target image can perfectly coincide with the reticle image without being affected by parallax.
The biggest difference between the two focal planes is that the FFP is in front of the magnification adjustment part of the scope, and the SFP is behind the magnification adjustment part. In other words, the FFP reticle image will receive optical magnification together with the target image, while the SFP reticle image will not.
When changing the magnification, the FFP marking line is synchronized with the target. The scale on the marking line can always maintain the same angle measurement regardless of whether it is 5 times, 15 times or 25 times magnification; while the SFP marking line is only at a specified magnification. (Many manufacturers often set it at 10 times for convenience) to measure the accurate angle. Once the magnification is changed, it needs to be corrected by the conversion formula.
Although the FFP marking design is far superior to the SFP design, the cost is much higher. Therefore, only high-end scopes on the market will usually use the FFP design. Moreover, the FFP marking line will become too thick or too thin after being synchronized and enlarged, which may affect the aiming. If you do not use graduated markings, or use a fixed power of the scope, then the FFP design has no advantage at all.
If you shoot in low light (such as hunting animals that only come out during the short period of dawn and evening) or the background light is too strong, it often happens that the contrast is too dark to see the markings. In order to cope with this situation, some scopes have a reticle illumination function, that is, an additional light source is set in the mirror to illuminate the reticle to increase the amount of light reflected. The marking illumination design can be light from the outside with optical fiber or radioisotope (usually tritium), but most of the products on the market still use battery-powered light-emitting diodes (LED) to provide the light source.
Because the marking line illumination may produce vignetting on the retina under low light conditions, or because it is too bright than the background, the pupils shrink and the surrounding environment cannot be seen clearly, so generally only weak red illumination is used (red has the least impact on the pupil) .
Contact Person: Mrs. Christina Yi