How do I accurately measure the size of an object using CCTV lenses?

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Understanding CCTV lenses involves recognizing that nodal points align with principle planes in air due to a constant refractive index. The focal length of a 12mm CCTV lens is typically measured from the mounting surface, not the last lens element. Accurately measuring an object's size from its pixel dimensions in an image requires a clear understanding of lens optics. The discussion indicates a learning process, with the original poster expressing confidence in their conclusions. The thread concludes with a suggestion for closure, indicating resolution of the initial query.
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[SOLVED] Understanding CCTV lenses

Hey guys, I'm trying to get my head around the way a lens works...

I've found the nodal points, and since the medium surrounding the lens is air constant refractive index (air) then they nodal points coincide with the principle planes, right?

Also the Focal length of the lens is 12mm, which I assume since it's a CCTV lens, is measured from the mounting surface rather than the last element of the lens... is that a fair assumption?

I've drawn a diagram of what I think represents the path of a ray going through the lens. (see attached) What I'm trying to do is accurately measure the size of something from it's size in pixels in the image.
 

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  • Lens Principle Planes.png
    Lens Principle Planes.png
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damn, I talk some rubbish sometimes! *think* I've figured it all out now... ignore everything above! If an admin want's to close/delete this thread, please feel free.
 

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