Underwater Depth of Field Calculator
Use your actual focal length, aperture, focus distance, and flat-port spacing to estimate the physical water-side range that will meet a chosen sharpness criterion. The result separates the image distance seen by the lens from the real distance to the subject in water.
Total underwater depth of field (m)0.28
- Underwater near focus limit (m)
- 0.88
- Underwater far focus limit (m)
- 1.16
- Depth in front of subject (m)
- 0.12
- Depth behind subject (m)
- 0.16
- Underwater hyperfocal distance (m)
- 7.13
- Lens-visible apparent focus distance (m)
- 0.76
- DOF change versus air
- -24.23%
Uses a flat-port paraxial apparent-depth model and a circle of confusion based on the selected sensor diagonal.
Very close macro distances exceed the accuracy of thin-lens and paraxial approximations.
How to use this calculator
- Choose the sensor format and sharpness criterion that suit your intended output size and viewing distance.
- Enter the actual lens focal length and working aperture, not their full-frame equivalents.
- Measure the physical focus distance from the lens entrance-pupil area to the intended subject plane.
- Estimate the entrance-pupil-to-port gap, then choose seawater, fresh water, or air for comparison.
- Read the physical near and far limits, and treat very close macro results as a starting estimate.
How flat ports change depth of field
A flat port makes a subject in water appear closer to the lens than it physically is. The effect matters when setting focus and when converting ordinary photographic depth-of-field equations into water-side distances. This calculator applies the usual geometric depth-of-field equations in the apparent object space seen by the lens, then maps the limits back through a centered flat water-to-air interface.
Let d be sensor diagonal, q the selected sharpness divisor, c = d / q the circle of confusion in millimetres, f focal length, N f-number, u physical focus distance, g the entrance-pupil-to-port gap, and n the water refractive index. The apparent focus distance is s = g + (u - g) / n. The apparent hyperfocal distance is H = f² / (N × c) + f.
The apparent near limit is Anear = H × s / (H + s - f). The apparent far limit is Afar = H × s / (H - s + f) when its denominator is positive; otherwise it is infinity. Each finite apparent distance is converted back to a physical water-side distance with U = g + n × (A - g). The displayed front and rear depth are the differences between that result and the entered physical focus distance.
What moves the result most
Actual focal length, aperture, focus distance, and the circle of confusion have the largest photographic effect. Wider apertures, longer focal lengths, closer focus, and a stricter sharpness criterion all reduce the range. The selected refractive index and port gap control how the physical underwater distances correspond to what the lens sees. Seawater and fresh water differ only slightly, while choosing air removes the apparent-depth correction and reproduces ordinary photographic depth of field.
How to use the hyperfocal result
The underwater hyperfocal distance is the physical water-side distance corresponding to the ordinary apparent-space hyperfocal distance. Focus at or beyond it and the far limit becomes infinity in this geometric model. That does not mean every distant detail will look equally good: motion blur, water clarity, lens aberrations, and diffraction can be more important than the circle-of-confusion threshold.
What this calculation leaves out
This model assumes a centered, perpendicular flat port and paraxial rays. It does not describe dome ports, wet lenses, tilted ports, port thickness, strong wide-angle distortion, internal focus movement, or a shifting entrance pupil. It also omits aberrations, diffraction, water turbidity, subject movement, and manufacturing tolerances. At close macro distances, confirm the setup with test images because thin-lens and paraxial approximations become less reliable.
Worked example
With a 35 mm lens at f/8 on a full-frame camera, the standard circle of confusion is 43.27 / 1500 = 0.02885 mm. At a physical focus distance of 1.00 m, a 60 mm entrance-pupil-to-port gap, and seawater with n = 1.339, the lens-visible apparent focus distance is 0.762 m. The apparent hyperfocal distance is about 5.34 m.
The calculator maps the near limit back to about 0.88 m and the far limit to about 1.16 m, for roughly 0.28 m of physical underwater depth of field. The physical underwater hyperfocal distance is about 7.13 m. Selecting air instead makes the apparent focus distance equal the entered 1.00 m and gives the conventional photographic result for the same lens settings.
Common questions
Why does a flat port change underwater depth of field?
A flat water-to-air interface refracts light, so the lens sees a subject at a shorter apparent distance than its physical distance in water. Conventional depth-of-field formulas work in that apparent object space. The calculator converts their limits back to physical water-side distances so they can be used when placing or focusing on a subject.
Is underwater depth of field always 1.33 times the air value?
No. The water refractive index is near 1.33, but depth of field is not a simple distance scale. The change depends on focal length, aperture, focus distance, circle of confusion, and the air gap between the entrance pupil and the port.
Should I enter actual or full-frame-equivalent focal length?
Enter the actual physical focal length marked or reported for the lens. The sensor format separately determines the circle of confusion. A full-frame-equivalent focal length would combine sensor format twice and give the wrong result.
How do I estimate the entrance-pupil-to-port distance?
Measure along the optical axis from the lens entrance-pupil area to the water-facing plane of the flat port. The entrance pupil is usually inside the lens rather than at the front element, so manufacturer drawings or a practical parallax test may help. A rough estimate is still preferable to treating a large housing gap as zero.
Why can the far focus limit become infinity?
In standard geometric depth of field, the far limit goes to infinity when focus reaches the hyperfocal condition. The same condition is tested here in apparent object space. It is a mathematical limit of the chosen circle-of-confusion model, not a promise that all distant underwater detail will appear crisp.
Does this calculator work with dome ports?
No. A dome port forms a virtual image through curved interfaces and needs a different optical model. This calculator is specifically for a centered flat port with a water-to-air apparent-depth correction.