Underwater Flat-Port Field of View Calculator
Enter your sensor, lens, distance, and water type to estimate what a rectilinear lens sees through a flat port. The reverse calculation finds the focal length that frames a known subject width at that distance.
Underwater horizontal angle of view (degrees)24.79
- Underwater vertical angle of view (degrees)
- 16.85
- Underwater diagonal angle of view (degrees)
- 29.36
- Underwater frame width (m)
- 0.44
- Underwater frame height (m)
- 0.3
- Width lost versus shooting in air
- 26.74%
- Equivalent in-air focal length (mm)
- 81.9
- Required focal length for target width (mm)
- 52.44
- Flat-port feasibility warning
- The requested frame width is feasible with a flat port in this simplified model.
Assumes a rectilinear lens and a flat, parallel port centered perpendicular to the optical axis.
Port thickness, entrance-pupil position, distortion, focusing extension, wet optics, vignetting, and corner aberrations are not modeled.
How to use this calculator
- Enter the active sensor width and height in millimetres, not a crop factor.
- Enter the actual focal length marked on the lens and the approximate port-to-subject distance.
- Choose seawater, fresh water, or a custom refractive index.
- Enter the subject width you want to fill the frame, then compare the coverage and required focal length.
How flat-port field of view is calculated
A flat port refracts the ray as it passes from water into the air space in front of the lens. That narrows the water-side angle accepted by the lens, so the same lens records less of the scene than it would in air. The effect is often described as a 1.33× magnification, but the exact change depends slightly on the lens angle and the water’s refractive index.
For a sensor dimension x, focal length f, air index na = 1.0003, and water index nw, the calculator first finds the in-air half-angle and then applies Snell’s law:
a = atan(x / (2f)); u = asin((na / nw) × sin(a)); A = 2u
a is the lens’s in-air half-angle, u is the corresponding water-side half-angle, and A is the reported full underwater angle of view. The calculation repeats with sensor width, height, and diagonal. At a port-to-subject distance D, horizontal or vertical coverage is C = 2D × tan(u).
Choosing a focal length for a known subject
To frame a target width T, the desired water-side half-angle is ut = atan(T / (2D)). The reverse calculation uses at = asin((nw / na) × sin(ut)), then returns f = w / (2 × tan(at)), where w is sensor width. A target can become impossible through a planar port when the reverse sine term reaches 1; the calculator reports that limit instead of inventing a focal length.
What changes the result most
Focal length and sensor width set the angle before refraction. Subject distance does not change the angle, but it changes the frame width in direct proportion: doubling distance doubles the scene width covered. Seawater has a slightly higher refractive index than fresh water, so it produces a marginally narrower field. Use the real focal length and the active physical sensor dimensions; full-frame-equivalent numbers would apply crop twice.
What this model leaves out
This is a centered, paraxial composition estimate for a rectilinear lens behind a flat, parallel port. It does not model port thickness, the lens entrance pupil, focus extension, distortion, vignetting, wet corrective optics, or corner aberrations. Very wide lenses can be especially different in practice, and refractive index shifts modestly with salinity, temperature, pressure, and wavelength. A dome port uses a different optical model and should not be evaluated with these results.
Worked example
With a 36 mm by 24 mm sensor, a 60 mm lens, a 1 m port-to-subject distance, and seawater at n = 1.339, the horizontal in-air half-angle is atan(36 / 120), or 16.70 degrees. Snell’s law gives an underwater horizontal angle of about 24.79 degrees, so the frame is about 0.44 m wide. The same lens would cover 0.60 m in air, making the modeled width loss about 26.7%.
For a 0.50 m-wide subject at the same distance, the reverse calculation calls for about a 52.44 mm lens. This is a geometric estimate: select the nearest practical focal length, then allow room for focus distance, port placement, and composition.
Common questions
Why does a flat port make a lens appear more telephoto underwater?
Light bends at the water-to-air boundary before reaching the lens. The lens therefore accepts a smaller water-side angle than it does in air, so it covers a narrower scene. The apparent increase in focal length is a field-of-view comparison, not a change to the lens’s physical focal length.
Is flat-port magnification always exactly 1.33×?
No. About 1.33× is a useful small-angle approximation based on water’s refractive index. The exact field change varies with lens angle, sensor dimension, and whether the water is fresh or salt water, which is why this calculator uses Snell’s law instead of a fixed multiplier.
Should I enter actual or full-frame-equivalent focal length?
Enter the actual focal length marked on the lens and the sensor’s physical dimensions. A full-frame-equivalent focal length already accounts for sensor size, so using it with the actual sensor would give the wrong angle.
What is different about a dome port?
A properly positioned dome port is designed to preserve a much wider field of view by placing the entrance pupil near the dome’s centre of curvature. It creates a virtual image and has focus and corner-sharpness considerations of its own. The planar-interface equations on this page do not describe a dome port.
Does seawater produce a different field of view from fresh water?
Yes, but the difference is small for most compositions. Seawater’s slightly higher refractive index bends rays a little more strongly, producing a slightly narrower field than fresh water. Choose the water type that best matches the dive; the larger practical uncertainty is usually port and lens geometry.