Red Dot Sights Explained: How Reflex Optics Work

A red dot sight is one of the most common upgrades on modern handguns and carbines, and for good reason: it simplifies aiming down to a single act. Put the dot on the target, press the trigger. This guide explains what a red dot actually is, how the optic projects that floating dot, and how the main design choices — emitter style, dot size, reticle and brightness — affect the way a sight behaves in practice.

What a red dot sight is

Broadly, a reflex sight is any optic that superimposes an emitted reticle — usually from an LED — against a lens. The standard, non-magnified red dot you see on pistols and rifles is a reflex collimator sight. You look through a partially mirrored lens and see an illuminated aiming point, normally a dot, that appears to float out on the target.

Because the reticle is generated by the sight itself — not painted on the glass, and not part of the firearm — placing the dot on the target is the whole aiming act. That is the source of the red dot’s reputation for speed, and why it has largely taken over from iron sights on defensive and duty firearms. The terms “red dot,” “reflex sight” and “collimator sight” are often used interchangeably. The emitter is frequently a red LED, but many sights also offer green; the operating principle is identical.

How the optic works

A standard red dot uses a light-emitting diode (LED) sitting at the focus of collimating optics, which generates the dot-style reticle. The lens facing you is curved and carries a partially silvered, wavelength-selective coating designed to reflect just the red part of the spectrum while letting most other light pass straight through.

In plain terms, the coating bounces the narrow band of light the LED emits back towards your eye, while nearly everything else passes through so you still see a clear target. Deep-red LEDs around 670 nanometres are commonly chosen because they are bright, contrast well against natural backgrounds and pair well with these coatings.

Because the LED sits at the focal point of the curved lens, the light leaving the lens is collimated — the rays exit essentially parallel, as though the dot were located very far away. The sight throws that parallel beam of red light back at your eye, and your eye, focused on the target, reads the beam as a dot sitting out on the target rather than on the glass.

The “parallax-free” idea — and its limits

Red dots are routinely marketed as “parallax free.” In practice this means that small shifts in your eye position do not move the apparent point of impact by much — a genuine advantage during rapid shooting or from awkward positions.

The qualification matters, though. A collimated image is truly parallax free only at infinity, with a small error circle for any target at a finite distance. Some makers tune the optical focus to minimise parallax at a chosen distance, often roughly 25–50 m. Treat “parallax free” as “very low parallax for practical purposes,” not “zero error at every distance.” The residual error is bounded by the size of the optical window, so it is small — but keeping the dot reasonably centred in the window remains good practice.

Open-emitter versus enclosed-emitter designs

Reflex sights come in two broad construction styles, and the difference is mostly about where the LED emitter lives and how well it is protected.

On an open-emitter sight the emitter sits behind the glass and is open at the back. These tend to be smaller and lighter, generally cheaper, and offer a larger field of view, which makes target acquisition fast and helps you keep situational awareness. The trade-off is exposure: the frame forms a kind of cup that can fill with water, snow, mud or pocket lint, and debris settling there can block the projected dot. Clearing it may mean digging into the cup with a finger or a shirtsleeve.

On an enclosed-emitter sight the emitter is sealed between two pieces of glass. These are generally larger and box- or tube-shaped, and typically more expensive, but the sealed design protects the emitter from mud, rain and dust. If the front lens gets dirty, it usually only needs a flat wipe. This obstruction risk is the central reliability trade-off between the two styles. One practical note on adjustments: in a fully enclosed tube-style sight, adjusting elevation and windage moves the whole assembly, whereas an open emitter has a single lens, typically shrouded in a small hood.

Either way, a red dot needs a secure, repeatable mount. Our range of red dot sight mounts covers the common footprints for pistols and carbines.

Dot size in MOA: speed versus precision

Dot size is given in MOA (minute of angle), an angular unit. As a working figure, 1 MOA is generally rounded to about 1 inch at 100 yards. Because the dot is an angular size, the physical patch of target it covers grows with distance.

• 2 MOA covers roughly 2 inches at 100 yards. A smaller dot obscures less of the target and favours precision and distance.
• 3 MOA is a deliberate middle ground between speed and precision.
• 6 MOA covers roughly 6 inches at 100 yards. A larger dot is faster to find and suits close-range work.

Larger dots in the rough 4–8 MOA range are quicker to acquire and well suited to close-range shooting; smaller dots suit longer distances where covering less of the target matters. Worth remembering: turning brightness up can make a dot look bigger and “bloom.” For precision work, use the lowest brightness that still gives a crisp dot.

Reticle, brightness and battery

Most reflex sights use a simple dot, but other patterns exist. A dot-and-circle reticle places a small centre dot inside a larger ring: the ring draws the eye for fast acquisition up close, while the centre dot is used for precision. More elaborate patterns such as crosshairs are less common on simple reflex sights and require more complex optics. Whether a given reticle helps you is a matter of preference and training.

Because red dots are electronic, power and brightness are part of operating them. Multiple intensity levels keep the dot visible against bright backgrounds without letting it bloom in low light, and night-vision-capable sights reserve their lowest settings for use with a night vision device. Many duty sights run on a common coin cell, and long runtimes are a major selling point — so many users leave the optic on continuously and change the battery on a calendar schedule rather than relying on switching it off. Extreme cold can shorten battery life. Always confirm the battery type, runtime and replacement interval for your specific model, as these figures vary by sight and setting.

Brands such as Holosun offer sights across most of these emitter, dot-size and reticle combinations, which makes it easier to match a sight to how you actually shoot.

How red dots compare to other sights

The defining benefit of a red dot over iron sights is the single focal plane. With irons, the eye must reconcile a rear sight, a front sight and the target at different distances. A red dot lets you focus on the target instead of the front sight, which allows shooting with both eyes open for a wider peripheral view, faster target acquisition, an unobstructed sight picture and better low-light performance thanks to the illuminated reticle. A red dot is not superior in every case, though: it depends on a battery and electronics, the emitter or lens can be obscured, and it takes training to present consistently. Many shooters keep co-witnessing iron sights as a backup.

Several optic types look similar through the glass but work differently. A reflex red dot reflects an LED dot off coated glass at 1x. A holographic sight projects a laser-generated hologram onto the glass, is too large and heavy for handguns, and keeps its reticle the same size under a magnifier. A magnified or prism optic uses lenses to enlarge the image, which imposes eye-relief and parallax constraints that a red dot largely avoids — a red dot’s tolerance for eye position also suits firearms with heavy recoil. Magnifiers exist for both reflex and holographic sights, but neither offers magnification out of the box. If you are working with magnified optics instead, our scope mounts cover that side of the range.

If you would like help matching a red dot, emitter style or mount to your firearm, take a look through the shop or come and see us in Varna — we are happy to talk it through.