Sights with SFP reticle
2nd focal plane (SFP)
— reticle of an optical sight (see "Type"), located in the so-called second focal plane — roughly speaking, in the area of the eyepiece.In contrast to the FFP (see the relevant paragraph), such reticles are found not only in scopes with magnification adjustment, but also in models with fixed magnification. Actually, with a constant magnification, it simply makes no sense to install a grid in the first focal plane: this design has no advantages over SFP, and it costs much more. But in models with adjustable magnification there is quite a practical difference. It consists in the fact that the grid in SFP does not change in size when the multiplicity changes — in other words, the image in the field of view “stretches” and “shrinks”, while the grid remains unchanged. This significantly complicates the use of goniometric elements, since the angular size of these elements varies in inverse proportion to the multiplicity. For example, if at 5x the distance between two adjacent points is 6 MOA, then at 15x it will decrease to 2 MOA.
Thus, the “true” angular size indicated in the characteristics, the marking elements have only at a strictly defined degree of increase, in other cases, this size must be recalculated using special formulas. However, the simplicity and low cost of sights for many users outweigh this drawback. And in some cases, it has no effect on convenience at all; a typical example is hunting sights, whose reticles often do not have goniometric elements at all.
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