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I am designing a camera for a project. How do I determine its fstop number if I am designing it? The equation I have is:

fstop=f/D where f is the focal length and D is the aperture diameter.

I have calculated my pixel size for the detector and the focal length based on how many pixels I want on a target.

How do I calculate the optimal lens parameter?

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  • If you have calculated focal length, you already know f. What you need to know is D, or if you use an existing lens, it may specify fstop instead. A lens you designed may become unsharp at large values of D, so it may require experimentation to see which D produces the best pictures. If you use an existing lens, it probably says the maximum f-stop in the specs, such as 50mm f/1.8. – juhist Oct 31 '22 at 19:01
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    Note that "aperture diameter" here is not the physical diameter of a diaphragm placed somewhere inside the lens. It is the size of the entrance pupil as observed from the center of the lens' optical axis at a sufficient distance from the lens to be considered infinity (which varies with focal length). More simply put, 'D' is the diameter of the entrance pupil, not of the physical aperture. Due to the magnification provided by lens elements between the front of the lens and the physical aperture, the difference between physical aperture and entrance pupil diameter can be quite large. – Michael C Nov 03 '22 at 08:13

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Assuming you're designing the internals of the camera, it isn't completely unreasonable to simply specify a "reasonable" figure for focal length (to give the desired field of view) and aperture (to provide sufficient light) and call out a commercially available lens with those specs. For instance, if you had an old 1/3 inch (diagonal) 4:3 sensor and wanted a "normal" field of view (30-35 degrees corner to corner) you'd specify a lens in the range around 8-10 mm focal length and f/4 or so (maybe faster if you wish to design in low light capability), then look for commercially available lenses in with those parameters. Given that's a fairly common range for video cameras, you shouldn't have any problem finding a make/model to call out "or equivalent".

Now, if you want/need to design the actual lens, that's a much more complex problem and would require you to spend considerable time with optical design ray tracing software, modifying existing designs to meet your specific needs -- but in that case, your professor should have at least introduced you to the school's copy of such software (it typically costs thousands of dollars for a license, so you probably aren't expected to buy your own) and given a starter on how to use it.

Zeiss Ikon
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