How to determine optical lens parameters in a camera ?

[andre_luis]

Hi friends,



I have a set of lens extracted from various damaged Webcams.
Then I wish to get their optical parameters m ( magnification ) and f( focal distance ).

However, I dont´t know a procedure to do that.
Does anybody could sugest some ?

My first idea was to put a lens with known parameters as reference, but I can´t develop a closed formula.

Follows a model of ideal lens :

Geometrical :

Algebraical :


Thanks in advance.


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[FvM]

Magnification isn't a lens specific parameter, it only depends on the object and image distance (do and di) ratio. Basically a lens can be specified by it's focal length. Lenses can be asymmetrical, multi lense optics mostly are, thus the optical center plane isn't necessary identical with it's mechanical center. This should be considered when determing focal length.

The most simple method to determine focal length is to focus the image of a distant object e.g. on a piece of paper and measure di. With do >> f, di is very near to f. By turning the lens, you can check for asymmetrical center plane.

The method works only for positive or converging lenses, also called convex lenses according to their shape. Camera lenses always are of this type.

 

[andre_luis]

hi FvM,


I should to say apperture, and not magnification. Sorry about mistake.
Please, see that :


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[FvM]

I think, it's difficult to determine the parameters exactly for lenses with short focal length, at least without special optical equipment.

 

[milind.a.kulkarni]

Generally Lens parameter are measured using coherent light.....

Here is process that you may need to follow...
1) Develop the coherent light beam, expanded to the lens aperature using something like beam expander and laser
2) Expose the lens to such beam...try to find out focal point where such beam converges in to ver small point
3) For magnification you need to know the focal lenght of lens as accurate you can....then you can use your formula...

Good Luck

 

[andre_luis]

milind.a.kulkarni,


Nice sugestion you provide.
Thanks a lot.

However, I didn´t realized howto determine the apperture parameter.
It is given as a ratio factor ( 1:n ).

Do you know ?

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[milind.a.kulkarni]

I mean lens aperture diameter ....lens aperature generally the lens light collection area diameter ....i.e. if lens is circular is it diameter of circular section of lens....if you are planning for cylinderical lens then the method is differnt...

Good Luck

 

[FvM]

The calculation of numerical aperture is explained in the appendix of the Sensation catalog.

 

[andre_luis]

Hi FvM,


Could provide a link ?
I searched for this keyword on web, but didn´t find.


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[FvM]

I meant the catalog from lensation.de, the lens vendor from your link.

 

[andre_luis]

Ok....now I got.

You wrote sensation, instead lensation.

:wink:

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[ark5230]

Most of the lenses from cameras are compound lenses, present age lenses are still more complex.
These include combination of convex, concave and miniscus lenses.
Not going into complicated details summary of parameter of interest is like this.
Focal length is the distance at which a parallel beam of light will be brought to a focus.
Example I hold a convex lens in front of sunlight, if it is brought to a sharp point at 10 cm (0.1m) the focal length of the lens is 10 cm or 0.1m.
Power of a lens (in diopters) is the reciprocal of focal length expressed in meters, so the power of the above lens is D = 1/f = 1/0.1 = 10 diopters.
If lens of my spects is 2.5 it means 1/f is 2.5 or f = 1/2.5 =0.4 meter or 40 cm (focal length is 40 cm).
Aperture is the opening of the lens (may be like available size) relative to focal length.
1:2 means ration of opening (max) to focal length is 1:2
1:1.2 has similar meaning indicating a wider aperture.
A smaller number for aparture means a wider aperture. 1.2 is supposed to be better lens than 2.0.

Now it is important that when we talk of distance in relation to focal length, if the size of a combination of lenses is comparable to the focal length it becomes difficult to assert as to where from the distance is to be measured.
Either one can go by element to element of the lens or a very crude thumb rule I frequently use is this:
Take the lens to an optician or optical shop and ask him to find the number. He will use -ve lenses (concave) and try combination that will neutralize the lens and tell you the number in a minute. The number he is giving is the power of the lens in diopters. Reciprocal of this power is the focal length in meters.

 

[andre_luis]

Develop the coherent light beam, expanded to the lens aperature using something like beam expander and laser

Hi all,

Sorry for re-open this thread started by myself, but I wish to take more detais about how to develop that aparatus.
Could anybody give to me a sugestion ?


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[ark5230]

Andre,
Milind.a.kulkarnis description has few issues.
Please attend to the points one by one.
If you wish to find focal length, just hold it in front of a parallel beam of light, say in the sunlight (it is fairly parallel). The point where the beam is brought to a focus or a point is found. Distance of the focus or this point from the lens is the focal length. Magnification that can be produced by this lens is given by M = D/F
D is the least distance of distinct vision (about 25 cm or 0.25m), F is the focal length (in same units as D).
If there is something of specific interest that too can be discussed.

Magnification - Wikipedia, the free encyclopedia

A Raoof Khan

 

[andre_luis]

ark5230,

It is clear to me that Sunlight is an natural source of colimed light beam.
However my question is regarding howto build the mentioned aparatus.

Thanks for reply.


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[ark5230]

The apparatus for measuring focal length is going to be repeatedly used or occasionally. Approximately what precision in focal length is expected ( like on mm or less)? Crude method is to just hold the lens in front of sunlight and measure distance.

 

[FvM]

Referring to the imaging diagram in the previously linked lensation catalog, you see, that the focal length has to be determined as distance betwen focal and principal point. Only for thin and symmetrical lenses, both principal points can be assumed to be located in the center plane. This causes a problem in determining the focal length. You need more than two focal point measurents (with turning the lens) to determine it exactly.