How a Scope Works FFP vs SFP in 3D

A riflescope is a chain of lenses that builds two images inside the tube. The objective lens at the front focuses the target into a real image at the first focal plane, and that image is upside down and reversed. The erector, a set of lenses riding in a tube in the middle of the scope, flips it back upright and relays it to the second focal plane. The ocular, or eyepiece, then turns that image into the parallel light your eye focuses on.

The erector is also where the zoom happens. Its two moving lens groups change how much they enlarge the image while keeping it on the second focal plane, so the picture stays sharp at every power. In the cutaway the arrow at each focal plane shows the image flipping, and the traced rays show the objective bundle narrowing down to the exit pupil behind the eyepiece.

The difference between first and second focal plane scopes is which image the reticle is laid over. A first focal plane reticle sits ahead of the erector, so the erector magnifies it along with the target: it grows as you zoom in, and a 2 MOA mark covers 2 MOA of target at 1x, 4x or 8x. A second focal plane reticle sits behind the erector, so it never changes size, and its marks only match their printed values at the one power the maker calibrated, usually the highest.

Set the power to 4x on the bench above and press Compare under the eyepiece. The FFP view shows the crosshair shrinking with the target; the SFP view shows the same crosshair at full size, with each 2 MOA mark now spanning 4 MOA of the target. At 8x the two views match.

Choose FFP if you hold over, range or measure wind at in-between powers, because the marks stay true from 1x to 8x; the Army's Direct View Optic (SIG TANGO6T 1-6x24) and the XM157 fire control on a 1-8x30 LPVO are both first focal plane. Choose SFP if you shoot mostly at 1x and use holds only at maximum power: an SFP reticle keeps a big, readable center at 1x, and SFP is the more common layout on LPVOs.

The FFP catch on an LPVO is 1x. The whole reticle shrinks to a small cluster in the middle of a wide field, as the 1x eyepiece view above shows once you switch its lit center off, so a first focal plane LPVO works like a red dot only when its center is illuminated brightly enough to find fast. A true red dot has no erector or focal plane at all; see why its dot sits at infinity in the 3D red dot cutaway. Compare picks in the best LPVO guide and the best budget LPVO guide, or see where an LPVO fits against dots and prisms in the optic selection matrix.

The exit pupil is the diameter of the beam leaving the eyepiece, and at high power it equals the objective diameter divided by the magnification: a 24 mm objective at 8x gives 3 mm. At low power an aperture inside the erector usually sets the pupil instead, which is why a 1-8x24 does not deliver 24 mm at 1x; Nightforce lists 7.9 mm at 1x and 3.0 mm at 8x for its NX8 1-8x24, and the traced model above lands close to both.

Eye relief is how far behind the eyepiece that pupil sits, around 95 mm (3.7 in) on the NX8. A wide pupil at 1x forgives a sloppy head position; a 3 mm pupil at 8x has to line up with your own pupil, so the eye box shrinks and your head position has to repeat more precisely. Get the scope where your eye lands with the scope ring height calculator and the LPVO mount guide.

Elevation and windage turrets work by tilting the erector tube. They press on its front end, which pivots at its rear inside the scope, against a spring on the opposite side. Tilting the erector changes which part of the first-focal-plane image it relays to the center of the view, so the reticle lands on a different spot on the target while staying centered in the eyepiece.

Dial the elevation turret up and the reticle moves down on the target: turrets move the point of impact, so once you put the reticle back on the target the barrel points higher. The cutaway exaggerates the tilt six times so it is visible; in this model a 20 MOA dial moves the front of the tube less than half a millimeter. Practice the result in the zeroing simulator and hold or dial at distance in the range simulator.

The magnification ring turns a cam sleeve around the erector tube. Each erector lens group rides a pin that runs in a straight guide slot in the erector tube and a curved cam slot in the sleeve, so turning the sleeve slides both groups forward or back by different amounts. The cam curves are cut so the image stays on the second focal plane at every power; optical designers call this a mechanically compensated zoom.

Drag the magnification ring or pick a power and watch the two groups in the cutaway: both travel toward the objective as the power rises, one of them reversing briefly near the low end, while the pins stay lined up with the cam slots. That same ring setting decides whether SFP holdover marks read true; drill holds at the right power in the holdover trainer.