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Calculations for underwater fixed focus, hyperfocal distance, dome/flat port

focus hyperfocal dome flat port circle of confusion

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#1 JesperChristensen89


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Posted 01 November 2018 - 02:00 AM

Hi all,


I am brand new here and recently found this forum searching for information on dome/flat port theory for underwater applications.


For my application I am trying to achieve a "one-focus-fits-all" solution - this is both for domes and flat ports. I have to manually adjust focus on land and once submerged in water it cannot be adjusted. The lenses I will be using are fixed focal lengths, most like from 3-8 mm used on small industrial cameras.


What I have found out till now is first to adjust the focus to the hyperfocal distance. This distance can be calculated as:

H = f^2/N*c+f, where
f = focal length
N = is f-number (f/D for aperture diameter D) - use D
c is circle of confusion
for finding the circle of confusion the following can be used:
c = D/1500, where
D is sensor diameter and d/1500 is used - I read somewhere that d/1500 is commonly used.
Now H gives me the hyperfocal distance. Adjusting to this distance should give me an acceptable sharpness from H/2 to infinity - correct?
Next up we need to translate this to our underwater ports to achieve the same functionality. It is here I find the difficulties being introduced.
Flat ports create a virtual image approximately at the refraction ratio of water and air - correct?
This yields 3/4 given that the refraction index of air is 1 and water is 1.333. Would this be as simple as then focusing the lens at 3/4 of hyperfocal distance for a flat dome?
Dome ports I find to be more difficult. I have read that as a rule of thumb the infinity distance can be found as:
Virtual Image Infinity = (4 * Dome Radius) - (4 * Dome Thickness)
However, how does this translate to a hyperfocal distance? 
I hope that some of you in here can help me clarify this. It would be very useful to have a standard sheet of which one could plug in camera and lens parameters, and then achieve at what distance in air it should be focused to achieve infinity focus using both flat and dome ports.

Edited by JesperChristensen89, 01 November 2018 - 02:01 AM.

Also tagged with one or more of these keywords: focus, hyperfocal, dome, flat port, circle of confusion