Two fixtures can carry identical lumen figures, identical wattage and identical colour engines, and need completely different quantities to light the same stage. The variable doing all that work is beam angle — and it is usually the least-read number on the datasheet.
Beam angle decides how large a circle one fixture paints at a given throw distance. Coverage is that circle. Fixture count follows coverage. Nothing else in the specification changes the answer as much, which is why purchasing on output alone produces rigs with hot centres and dark edges.
In this guide
A beam angle is the angle within which output stays above 50% of the peak intensity. Draw a cone from the lens, measure where the intensity falls to half, and that is the beam angle. It describes the bright, usable core of the pool.
A field angle is the angle within which output stays above 10% of peak. It is a wider cone, and it describes where the light effectively stops. Between the two angles lies a soft rim of diminishing light that is visible on a wall but too weak to illuminate anything.
Field angle typically runs 1.4 to 1.6 times the beam angle — a 15° beam usually has a field angle somewhere around 22°. That ratio is the reason a quoted coverage figure is never quite what you measure on site, and it is the reason a datasheet that lists one number without saying which one it is cannot be planned with.
When a listing says “beam angle”, ask whether it is the beam angle or the field angle, and at what measurement distance. A manufacturer quoting a field angle as “beam” is quoting a number about 50% larger, which sounds better on a comparison table and makes the coverage chart optimistic at the edges. It is rarely deliberate misdirection — the two figures get used interchangeably in the trade — but for planning purposes the difference is real.
| Term | Defined at | What it tells you |
|---|---|---|
| Beam angle | 50% of peak intensity | The size of the bright core — the pool that reads on stage |
| Field angle | 10% of peak intensity | Where the light effectively ends, including the soft rim |
| Ratio in practice | Field ÷ beam | Usually 1.4–1.6×, depending on the optic and how hard the edge is |
| Edge hardness | Optical design | A hard-edged profile and a soft wash can share a beam angle and look nothing alike |
One geometry formula does most of the work in this subject, and it is simple enough to do in your head at a reasonable approximation:
pool diameter ≈ 2 × throw distance × tan(beam angle ÷ 2)
For small angles there is a shortcut that is accurate enough for rig planning: a 1° beam spreads about 1.75 cm per metre of throw, so you can multiply the angle by the distance and read the result in centimetres. A 10° beam at 6 m covers roughly 10 × 6 = 60 cm by the shortcut, against 105 cm by the exact formula — so the shortcut is only safe below about 5°, and the full formula is worth the keystrokes.
Read that list again as a fixture-count decision rather than a beam-size list, and the commercial consequence appears immediately. The same quantity of light spread over a 16 cm circle and a 7.9 m circle is the same lumens concentrated into areas that differ by a factor of roughly 2,500 — so the fixtures are not substitutes for each other at any price.
A single pool is a circle. A stage is a rectangle. Covering a rectangle with circles requires either very large circles or several overlapping ones, and overlap has a cost: where two pools meet, the illuminance is roughly double that of either pool alone.
The working convention in stage lighting is to aim adjacent fixtures so their beam edges overlap by about 50% of the pool diameter — that is, to space fixtures half a pool apart. This raises the level in the seam between pools to roughly the level at the centre of each pool, which is what produces a smooth wash rather than a row of visible dark bands. Space them a full pool apart and you get scalloping; space them a quarter pool apart and you are paying for fixtures you do not need.
When a wash looks striped, the instinct is to raise the level or add fixtures. Neither fixes it. The level is already correct at the centre of every pool — what is missing is level in the seams. Re-aiming the existing fixtures to increase overlap fixes it for free, and buying fixtures to fill the gaps costs money while leaving the original spacing error in place.
Here is the calculation that makes beam angle the most commercially important number in a specification. Take a 6 m wide stage front wash at a 6 m throw, and work out how many fixtures are needed to cover the width with the 50% overlap rule above. Keep the light output constant and change only the beam angle.
The 76:1 spread is not a stunt; it is the arithmetic behind a decision lighting designers make every week. If the job is coverage, the wide optic needs one fixture where the narrow optic needs dozens. If the job is a shaft of visible light from a front-of-house position twenty metres away, the wide optic delivers nothing usable no matter how many you hang.
This is also why the total lumen requirement of a stage does not change with beam angle. The flux needed to hit a target illuminance is set by the lux target and the area to be lit — a 24 m² stage at 600 lx needs roughly 36,000 lumens leaving the fixtures at a realistic utilisation factor, whether they leave in one beam or seventy-six. Beam angle does not change how much light you need. It changes how many fixtures you need to deliver it, and that is where the money is.
| Angle at a 6 m throw | Pool | Fixtures for 6 m | What it is for |
|---|---|---|---|
| 1.5° – 5° | 0.16 – 0.52 m | Boxed: 76+ | Aerial shafts, haze work, beam effects. Not a coverage optic |
| 10° – 15° | 1.05 – 1.58 m | 11 – 7 | Tight keys, solo isolation, modelling light on a performer |
| 25° | 2.66 m | 4 | The general-purpose workhorse — a stage section from a typical trim height |
| 40° – 45° | 4.37 – 4.97 m | 2 | Washing a small stage, colour on a backdrop, upstage fill |
| 60° – 67° | 6.93 – 7.94 m | 1 | Even coverage from a single position, front fill, low trim heights |
A spotlight and a wash can cost the same, have the same output and the same build quality, and differ by a factor of seventy in how many of them a stage needs. That is not a defect in either product. It is the reason the discussion should start with coverage and end with output, and never the other way round.
A narrow beam earns its place in two situations, and both are about distance. The first is a long throw: from a 20 m front-of-house position, a 25° fixture paints a pool over 8 m across, far wider than a performer, and spreads its output thin doing it. The same fixture zoom-narrowed to 5° paints a 1.7 m pool at that distance — the same output, concentrated onto the person it is aimed at.
The second is atmosphere. A beam narrow enough to stay visible as a shaft in haze is doing something no wider fixture can do at any output, because the effect depends on the beam having an identifiable shape. This is why beam fixtures retain their own category, independent of how much light they produce.
The practical consequence for a rig plan is that narrow optics are usually paired with long throws and short ones with short. Buying a narrow beam fixture for a 4 m trim height on a small stage produces 16 cm pools that read as unexplained bright spots rather than as light on anyone.
VERSATILIS 300BSW
$1,999
VERSATILIS 700BSWF
$2,079
VERSATILIS 480CMY
$1,469
HUEWAVE MOVING HEAD 1915Z
$259
A fixed-angle fixture costs less than a zoom of comparable build. If a rig will only ever be used at one trim height, aiming at one size of stage, fixed optics are the rational choice and buying zoom is buying capability you will not use.
Zoom starts paying for itself the moment the throw distance is not constant. That happens more often than a first rig plan suggests: a fixture that washes a 6 m stage at a 5 m trim becomes a tight key when the same stage is narrowed to a 3 m runway with the truss moved up, and a fixed 25° optic cannot follow that change. A zoom rides a DMX channel, so the coverage decision becomes a cue rather than a purchase.
| Fixture | Narrowest angle | Widest angle | Range | Where the range fits |
|---|---|---|---|---|
| VERSATILIS 480CMY | 3.5° | 23.8° | 20.3° | Punch-biased. Long throws and beam work, up to a stage section |
| VERSATILIS 300BSW | 4° | 34° | 30° | Beam through wash in a single head, for mid-length throws |
| VERSATILIS 700BSWF | 3° | 42° | 39° | The widest coverage range in the group, and IP65 for outdoor use |
| HUEWAVE 1915Z | 10° | 60° | 50° | Coverage-biased. This is the head you buy to wash a stage, not to punch holes in haze |
The lumen figure is the same at every zoom position; what changes is the pool size and therefore the illuminance at any given distance. This is why a zoom chart that quotes a single lux figure without a stated angle and distance is not usable for planning. Look for an illuminance chart with both axes, or at minimum a lux figure at the narrow end and another at the wide end.
Circular pools are convenient for a datasheet and inconvenient for a stage. The shapes that actually need lighting are wide and short: a backdrop, a cyclorama, a row of performers standing shoulder to shoulder, a runway.
Two approaches address this. The first is an elliptical beam, produced either by an asymmetric reflector or by a lens that compresses one axis, giving an oval pool that covers a wide, shallow area from a single fixture. The second is an asymmetric or offset reflector, which throws an uneven, wall-washing pattern designed to sit flat on a vertical surface rather than as a cone.
Most stage rigs solve the same problem with geometry instead. A wide-angle wash fixture positioned close to the surface produces a broad, shallow pool without any specialist optic, and a line of them spaced at half-pool intervals covers a wall evenly. When the surface is unusually proportioned — a very wide cyc, a long runway — that is the point at which a specialist elliptical or wall-wash optic starts to earn its cost against the alternative of buying more wide fixtures.
For a vertical surface, the useful height a fixture covers is roughly the same as the pool it produces at that distance — a fixture making a 2.5 m circle covers about 2.5 m of wall height, with the top and bottom edges falling off softly. Plan the vertical spacing from the pool size the same way you plan horizontal spacing, and use the 50% overlap rule on both axes.
Angle selection is really position selection, and the two have to be decided together. The table below is the shorthand most designers carry in their heads.
Beam angle is measured where intensity falls to 50% of its peak value and describes the bright core of the pool. Field angle is measured where intensity falls to 10% and describes the full extent of the light, including the soft rim. The field angle is typically 1.4 to 1.6 times the beam angle. When a coverage chart and your on-site measurement disagree, this gap is usually the reason.
Multiply two by the throw distance by the tangent of half the beam angle: pool diameter ≈ 2 × distance × tan(angle ÷ 2). At 6 m, a 25° beam gives about 2.66 m. The reverse also works when you know the pool you need and want to find the angle: angle ≈ 2 × arctan(pool diameter ÷ (2 × distance)).
Because the same total light output is concentrated into a smaller area, so the illuminance in that area is higher. The lumen figure does not change at all with beam angle. What changes is how many square metres the light is spread across, and illuminance is inversely related to that area.
A common working figure is 50% of the pool diameter, which means spacing fixtures half a pool apart. That lifts the level in the seam roughly to the level at the centre of each pool and produces a smooth wash. Less overlap leaves visible dark bands; considerably more overlap means you have bought more fixtures than the coverage requires.
For coverage, a wider beam needs fewer fixtures and delivers more even results. But wider beams are harder to keep off scenery, off the audience and off surfaces you want dark, and they deliver lower illuminance at any given distance. A wash is a compromise between evenness, controllability and intensity, and the angle is where that compromise is set.
Two jobs that depend on the beam having a visible shape rather than on covering an area: aerial shafts in haze, and tight punches onto a single performer from a long throw. Done well, both are effects no wide fixture can reproduce at any output, which is why beam heads remain a separate category in a rig.
The total output is broadly the same at every zoom position. What changes is the solid angle the light occupies, and therefore the illuminance at a given distance. At the narrow end the light is concentrated and the lux is high; at the wide end the same light covers a far larger area and the lux drops accordingly.
At a 6 m throw with the 50% overlap rule, a 25° beam needs four fixtures across the width, a 40° beam needs two, and a 60° or wider beam needs one. A 15° beam would need roughly seven. The fixture count is a function of the beam angle and the throw distance, not of how much output each fixture produces.
Covering shapes that are wider than they are tall — a backdrop, a cyclorama, a runway, a line of performers — with a single fixture producing an oval pool instead of a circular one. It saves fixtures on unusually proportioned surfaces, and on ordinary ones a wide circular optic usually does the same job more cheaply.
This article explains the standard photometric definitions of beam angle and field angle and describes general rig-planning practice. Published angles, illuminance figures and prices for products sold on sanyilights.us are quoted from the current datasheet at the time of writing and vary with operating mode, zoom position, colour mix and firmware version. Coverage and fixture-count figures in this article are calculated from the stated geometry and assume ideal conditions; real installations require allowance for aiming tolerance, optical losses and the shape of the area to be lit. Verify on the real rig where the result matters. Product availability and pricing are subject to change.
