Dual Underwater Strobe Aiming Calculator
Enter the actual underwater field of view, strobe placement, and beam angle to see whether the two beams cover the frame or leave a dark center. The result also reverses the geometry into an aiming angle and a minimum useful beam width.
Illuminated frame coverage100%
- Frame width at subject (m)
- 2
- Center overlap or gap
- 0.978 m overlap
- Recommended outward aiming angle (degrees)
- 6.87
- Minimum beam angle for center-to-edge coverage (degrees)
- 52.31
- Beam-angle margin (degrees)
- 47.69
- Estimated aperture at frame center
- 21.36
Assumes equal strobes, symmetric horizontal placement, sharp-edged beams, and a flat subject plane perpendicular to the camera.
Use the actual underwater field of view; an in-air lens specification can overstate the photographed width.
How to use this calculator
- Enter the horizontal field of view your camera actually has underwater, including any port or wet lens.
- Measure the camera-to-subject distance and the horizontal offset of one strobe from the camera centerline.
- Enter the strobe’s stated underwater beam angle and its current outward or inward aiming angle.
- Add guide number, ISO, and power when you want a starting aperture estimate at the center of the frame.
How dual-strobe coverage is calculated
The calculator treats the subject as a flat vertical plane and the two strobes as mirror images. It starts with the photographed frame, not the strobe arm: if camera-to-subject distance is d and underwater horizontal field of view is φ, the frame half-width is F = d × tan(φ / 2). The full frame width is W = 2F. This is the same field-of-view footprint relationship used for underwater camera coverage.
For the right strobe, let s be its horizontal offset, β its full beam angle, and α its outward aim. Its two beam-edge intersections with the subject plane are:
a = s + d × tan(α − β / 2); b = s + d × tan(α + β / 2)
The left beam is the mirror interval [−b, −a]. Each interval is clipped to the photographed frame [−F, F]; the calculator takes the length of their union and divides by W for frame coverage. It also reports the shared width where the two intervals intersect, or their separation when they leave a center gap.
Finding an aiming angle and beam width
For one strobe to cover its half of the frame, the relevant rays run to the frame center and to the same-side edge. Their bearings from the right strobe are θ0 = atan(−s / d) and θ1 = atan((F − s) / d). The midpoint gives the recommended aim, αrec = (θ0 + θ1) / 2, while their separation gives the minimum beam angle, βmin = θ1 − θ0. A negative recommended angle means the geometry calls for the strobes to point inward rather than outward.
Distance and field of view move the result most. A wider lens makes the frame wider at the same distance and needs more beam coverage. Moving closer makes the same strobe separation proportionally larger, which can create a dark center unless you bring the strobes in, aim inward, or use a wider beam. Beam-angle margin is the stated beam angle minus the calculated minimum; a negative margin means one strobe cannot reach from center to its same-side edge in this sharp-cone model.
Guide number and the limits of the model
When both beams reach the frame center, the exposure estimate uses G = G100 × √(ISO / 100) × √(power / 100) for one strobe, combines two equal strobes as G2 = √(G² + G²), then estimates f-number = G2 / √(d² + s²). It is withheld when either beam misses the center or the guide number is zero.
Real beams have soft falloff, hotspots, diffusers, and output differences, rather than sharp uniform edges. The calculation also leaves out water clarity, absorption, backscatter, ambient light, subject reflectivity, vertical arm placement, port geometry, and non-flat subjects. Near a 90-degree beam edge, the tangent projection becomes unstable; the calculator warns instead of claiming a precise coverage result. Use test frames or TTL metering to refine an actual dive setup.
Worked example
Suppose the underwater horizontal field of view is 90 degrees, the subject is 1 m away, each strobe is 0.35 m from center, the beam angle is 100 degrees, and each strobe points 10 degrees outward. The frame half-width is 1 × tan(45°) = 1 m, so the photographed frame is 2.00 m wide.
The right beam runs from 0.35 + tan(−40°) = −0.489 m to 0.35 + tan(60°) = 2.082 m. Its mirrored partner gives full frame coverage and a center overlap of about 0.98 m. The reversed geometry recommends 6.87 degrees outward, needs a 52.31-degree beam for one strobe to cover center to edge, and leaves a 47.69-degree beam-angle margin. With guide number 16 at ISO 100 and full power, the two-strobe center estimate is about f/21.36.
Common questions
How should dual underwater strobes be aimed?
Start with each strobe off the lens axis and aim the softer edge of each beam toward the photographed area. This calculator finds the midpoint between the rays from one strobe to the frame center and to its same-side edge. Treat that angle as a starting position, then inspect test images for backscatter, hotspots, and uneven falloff.
Why do two strobes leave a dark area in the center?
A center gap occurs when the inner edges of the two beams do not meet on the subject plane. It becomes more likely when strobes are widely separated, the subject is close, the beam is narrow, or both strobes are aimed too far outward. Bring the strobes closer, aim them inward, increase beam width, or change distance as the image and conditions allow.
How wide a strobe beam is needed for a fisheye lens?
There is no fixed number because required coverage depends on the underwater field of view, subject distance, and strobe offset. Enter the actual underwater horizontal field of view and your arm placement to calculate the minimum beam angle from center to edge. A diffuser can broaden coverage, but it may also reduce guide number and alter the beam’s falloff.
Should underwater strobes point inward or outward?
Outward aiming often keeps the brightest part of a beam away from water directly in front of the port, which can help limit backscatter. At close distances or with a wide strobe spread, the geometry can instead require inward aim to avoid a center gap. The recommended value can be negative when inward toe-in is needed for the simplified frame-coverage target.
How is underwater guide number used to estimate aperture?
Guide number divided by flash-to-subject distance gives a starting f-number at a stated ISO and power. For two equal strobes reaching the center, this calculator combines their guide numbers by the square-root-of-squares rule before dividing by distance. Water conditions, beam falloff, and subject reflectivity can make the photographed result differ substantially, so verify it with test images or TTL.