Last Updated 2 days ago
Beam spread describes how widely a light fixture distributes its output. On set, the phrase often refers to the difference between a narrow spot and a broad flood, or to the width and shape of the illuminated area. In specifications, that distribution may be expressed as an angle in degrees.
A narrow beam can isolate a small part of a scene. A wider beam can cover more of a wall, subject or diffusion frame. The useful choice depends on the shot, the distance and the fixture's actual light distribution, rather than on a wide or narrow label alone.
Angle and Footprint
The angular spread and the physical size of the pool of light are related but different measurements. If a fixture keeps the same beam setting and moves farther from a perpendicular surface, its footprint becomes larger. Moving it closer makes the footprint smaller.
As a simple geometric example, a symmetrical 30-degree cone would be about 1.61 metres wide at a distance of three metres. At six metres it would be about 3.22 metres wide. These are approximate cone dimensions, not a promise that every point inside the circle receives equal light. Real fixtures have intensity patterns and edges that need to be assessed separately.
An angled surface changes the shape again. A beam that makes a roughly circular pool when aimed straight at a wall can produce an elongated footprint when it strikes obliquely. This matters when a lighting plan calls for coverage across a floor or a wall that is not square to the fixture.
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Beam Angle and Field Angle
Many manufacturers define beam angle using the points on opposite sides of the beam where intensity falls to half the peak value. This is often called full width at half maximum, or FWHM. Field angle commonly uses the wider points where intensity falls to ten percent of the peak.
These figures describe two different parts of the same distribution. Light does not suddenly stop at the beam-angle boundary. The field measurement helps describe the weaker outer area, and there can still be light beyond it. A broad field may matter when spill onto a background is unwanted.
The general phrase beam spread is less precise than a stated measurement convention. Check what a specification means before comparing fixtures. Two numbers labelled as angles are not necessarily measuring the same intensity threshold. A manufacturer's distribution graph and photometric table are more informative than a single headline figure.
What Changes the Spread
The available adjustment depends on the fixture. A focusable Fresnel can move between spot and flood settings. Some fixtures use interchangeable lenses or reflectors. Other optical systems have a fixed spread, so changing coverage requires a different accessory, fixture or position.
Traditional PAR lamps can produce oval patterns, and different lamp or lens arrangements give different distributions. The direction of that oval can matter as much as its width. A wide horizontal wash and a tall vertical pool may come from the same general type of light but serve very different compositions.
Flags and barn doors control where unwanted light lands. They do not turn every fixture into a precisely adjustable optical system. Likewise, dimming changes output rather than serving as a dependable way to select a narrower beam. Judge the actual pattern after any adjustment.
Spread Is Different From Softness
A broad spread does not automatically create soft shadows. Shadow softness depends strongly on the apparent size of the emitting or illuminated source as seen from the subject. A relatively small source can distribute light widely while still producing a clear shadow edge.
Diffusion and bounce can change both the effective source and the distribution. If a lamp illuminates only the middle of a large diffusion frame, the frame's full dimensions do not necessarily describe the area doing most of the lighting. Check how much of it is actually illuminated and how evenly.
Example: Lighting a Background
Imagine a two-person interview with a textured wall behind the chairs. A narrow beam produces a bright patch that looks attractive in a close-up but becomes distracting in the wide shot. Widening the spread may cover the visible wall more gracefully, although the resulting level and spill must be checked again.
If the wider setting also brightens the interviewees from an unwanted direction, the crew may reconsider position or add appropriate light control. Simply choosing the widest available setting does not solve every part of the shot. Coverage, exposure and separation need to work together.
Useful preparation includes checking photometric data at the intended distance, then testing the real fixture and accessories. Note the setting and position when continuity matters. Beam spread is a planning tool, but the camera's view of the scene remains the practical test of whether the light is doing the intended job.
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