please explain what a second focal plane is. — 2 Answers

Second focal-plane, which is typically found in moderately powered hunting optics like the Diamondback 3.5-10x50, will allow for a very visible reticle at any magnification, but one that would require the user to be on what is known as the "subtension magnification", in this case 10x, for using any subtensions below, above, or to the sides of the center intersection as designed. In short, their values are only accurate at this subtension value, and in the case of this optic at 10x, they would be 1.5, 4.5, 7.5, and 11MOA as measured from the center on the vertical, and 2MOA each as measured from the center on the horizontal. Because these optics are typically utilized as hunting optics, and for ranges from 100 - 500 yards, we can typically assume that for shots at the longer ranges, higher magnification would be utilized, and there should be little worry about being off of that subtension magnification for accurate holdovers, and conversely, shorter distance shots would allow for demagnifying - but still requiring a highly visible reticle - which the second focal-plane offers.

For contrast, a first focal-plane reticle scales in-time with magnification change, keeping those subtension values accurate at any magnification - however - it will appear as if the reticle is growing and shrinking with magnification change. On a high-magnification optic - say 6-24x50 - this is often seen as advantageous, as it allows the user to utilize holdovers at any magnification at which he or she could see the reticle, and as is often the case, may find it beneficial to be at lower-than-maximum mag. A possible detractor to first focal-plane would be the limited visibility of the reticle at the bottom end of the magnification range, as it "shrinks" considerably.