FFP is First Focal Plane.... The image of the target in a scope is formed in two places, once by the Objective lense (converging at the first focal plane), and again by the Erector lenses (converging at the second focal plane).... The eyepiece then looks at that second image, making the light rays parallel again for your eye to use properly.... Without the Erector lenses, the image formed by the eyepiece would be inverted, like an Astronomical Telescope is.... In a fixed power scope, it doesn't matter where the reticle is, at the first or second FP.... However, in a variable power (zoom) scope, the image at the second focal point changes in size as you zoom the magnification.... Since the reticle is fixed in physical size, in a second focal plane scope, the image changes size, relative to the reticle, which causes the Mil-Dots to subtend a different dimension on the image, depending on the magnification.... For example, in a 5-15 X scope, if the Mil-Dots are calibrated to 10X, and they are 3.6" apart at 100 yards at that power.... at 5X they will be 7.2" apart.... and at 15X just 2.4" apart....
In a FFP scope, the reticle is placed where the first image is formed by the objective lense, and so becomes "part" of that image.... I don't think this can be done with a scope with an Adjustable Objective (AO), I believe all FFP scopes either have no parallax adjustment (fixed focus) or are side focus designs.... When the Erector lenses are zoomed to change the magnification, the reticle zooms right along with the image.... When you look through an FFP scope, and zoom the image, the reticle appears to get larger and smaller, but the MilDots stay the same distance apart, RELATIVE TO THE IMAGE.... so they always subtend 3.6" of the target, regardless of the magnification.... So much for FFP, now what is "Mil-Mil" ?....
First of all, you need to know that a "Mil-Dot" means that the dots on the reticle are 1 "milliradian" apart.... That is 1 yard at 1000 yards (the 1/1000 is where the "milli" prefix comes from).... That means 1 "Mil" is an angle subtending 3.6" at 100 yards.... Most scopes use 1/4 MOA (minute of angle) clicks on the turrets to adjust the POI (some are 1/8 MOA).... Some scopes use 1/4" (or 1/8") clicks.... Since 1 MOA at 100 yds. is just over 1", each click is about 1/4" (or 1/8", respectively).... A scope that is "Mil-Mil" means that each click of the turret is (typically) 1/10 of a Mil-Dot.... in other words the clicks on the turret are in the same system of units as the dots in the reticle (milliradians).... The adjustment in (most) Mil-Mil scopes are therefore slightly coarser than in a conventional "Mil-MOA" (or "Mil-Inch")scope, because 1/10 Mil-Dot is actually 0.36" at 100 yards.... however, there are (in theory) exactly 10 clicks per Mil-Dot, which makes it easy for sighting in, or adjusting your POI using clicks.... With a scope where the turrets are calibrated in MOA (inches), there are about 13.6 (or 14.4) clicks per Mil-Dot.... pretty inconvenient if you are moving the turrets between targets.... However, if you only use the turrets to sight in, and then use the Mil-Dots for holdover.... there is no PRACTICAL difference, and in fact 1/4 MOA clicks are a finer adjustment than 1/10 Mil clicks....
Bob
