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.

A prism scope is a fixed-power riflescope that uses a glass roof prism, not a train of erector lenses, to turn the image upright. The objective alone would focus the target upside down and reversed. The prism bounces the light through six reflections, the two faces of its roof counted separately, and that flips the image both ways while the light leaves on the same axis it came in on.

The same reflections fold a long light path into a short block of glass. In the teaching model on the bench, 98 mm of path fits into 48 mm of prism, so the image reaches the reticle about 50 mm sooner than it would with the path laid out straight; press Unfold to see the difference. That is why a 4x32 like the Trijicon ACOG TA31 is only 5.93 in long. Trijicon lights the TA31's reticle with a fiber optic collector by day, which sets its brightness to the light, and with tritium at night, so it needs no battery.

A prism scope trades zoom for size: it stays at one power, so it is shorter and simpler, and its reticle marks read the same all the time because there is no other power for them to be wrong at. The ACOG TA31 is always 4x with a 7.0 degree field and 1.5 in of eye relief, and its reticle is a bullet drop compensator for 5.56 from 100 to 800 meters. Up close, Trijicon has you shoot it with both eyes open out to 300 meters.

An LPVO covers true 1x for close work and 6x to 10x for distance in one optic, with more eye relief (about 95 mm on the NX8 1-8x24) but a longer tube, and the FFP or SFP choice above. Compare fixed and variable optics in the optic selection matrix.

A red dot magnifier is a small afocal telescope: parallel light goes in, and parallel light comes out at a wider angle. The red dot's collimator already sends the dot out as parallel light, just like the light from a distant target, so the magnifier enlarges the two together and your eye still focuses at infinity. Inside, the objective focuses an upside-down image, an erector lens turns it upright and the eyepiece spreads it three times wider.

That is why a 3x magnifier makes a 2 MOA dot look 6 MOA across while it still covers 2 MOA of the target, about 1 in at 50 yards. The magnifier does not set your zero: EOTech says the G33's adjustments only center the reticle in its window, and Aimpoint says its magnifiers use the sight's own dot, so there is nothing to re-zero. Flip it aside on its switch-to-side mount and you are back to the red dot's 1x. See how the dot itself is made in the 3D red dot cutaway and compare combos in the red dot magnifier combo guide.

A red dot with a magnifier gives you a real red dot at 1x, with the unlimited eye relief Aimpoint lists for the Micro T-2, and a fixed 3x when you flip the magnifier in. The G33 brings 2.2 in of eye relief and a 7.3 degree field at 3x, and adds 10.6 oz with its mount.

An LPVO gives every power from 1x to its top power in one optic, with a reticle built for holdovers, but your eye has to sit at its eye relief even at 1x. Pick the combo for fast close work with an occasional 3x look; pick an LPVO when you want holdover marks and more than 3x. The AR-15 magnifier guide ranks the magnifiers themselves.