# Thread: focal length in multi-element lenses

1. ## focal length in multi-element lenses

Hi,
I want to know how focal length in case of multi-element lenses(like Zeiss tessar) is computed?

2. ## Re: focal length in multi-element lenses

Hello.

What are you looking for?. A mathematical formula to calculate the focal length of a lens based on the curvature of its elements?. A physical procedure to measure it for a specific specimen of a lens design?. Or yet something else?.

Bear in mind rarely a real lens behaves as an ideal one of its type would. The focus plane can be curved. Sometimes there is a noticeable difference for the plane of best sagittal focus to that of tangential focus, especially when the lens is operated far from the focusing distance it was designed to. For a given lens the plane of best focus can vary with aperture, that phenomenon is called focus shift. In any case but very rough measurements you will need a definition of focal length (Especially important for mathematical approaches). "the distance to where the rays converge" is useless. To quote some defects in this definition for a rigorous approach: First, the wave nature of light (Rather than rays) is noticeable in the photography as diffraction. There are also aberrations, which in geometrical optics means rays only approximately converge. We also have to account for dispersion for all but optical systems meant to be used with light within a narrow enough band (So it is designated monochromatic); that's generally not the case for photographic systems.

Optics is a board and deep enough branch of physics so that there are specific books. Maybe you would want to read a book about Optics. It's a very interesting topic to me at lest. Another, online resource (Albeit there are online libraries like http://gen.lib.rus.ec/) is http://toothwalker.org/optics.html.

Regards and good luck.

3. ## Re: focal length in multi-element lenses

Hi,
I want to know the mathematical formula to compute focal length of multielement lenses.
My actual purpose is to build an error model for a 3D scanning system.

4. ## Re: focal length in multi-element lenses

Unfortunately the answer is not strait forward. An effective focal length is defined for a complex imaging system but you will have to do the maths.

5. ## Re: focal length in multi-element lenses

Originally Posted by MilT0s
Unfortunately the answer is not strait forward. An effective focal length is defined for a complex imaging system but you will have to do the maths.
Yes, and I might add the math and physics are extremely complex. Probably requires an advanced degree in optics (a branch of science combing advanced math and physics).

Glenn

6. ## Re: focal length in multi-element lenses

Possibly this website will have the equations you are looking for.

http://www.pierretoscani.com/annexeGB.html

7. ## Re: focal length in multi-element lenses

@John C
Thanks for link to wonderful explanation on working of multi-element lens cameras!!

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