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Calculating EFL of a Three Lens System 
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#1
May1110, 09:04 AM

P: 1

If I have three lenses, each one 50 mm in diameter and with focal lengths of 500mm, 125mm and 150 mm, What would be the EFL if I combined them all together (assuming zero distance between them).
I have been looking online for focal lengths of three lens systems but cannot find anything. Would I just use the 1/f = 1/f1 + 1/f2 equation twice and basically turn two of the lenses into one focal length then use that focal length again with the third? Thanks. 


#2
May1110, 10:14 PM

P: 4,663

Here is the general matrix solution to a three lens system with arbitrary drifts between the lenses. A drift of length d is represented by a 2 x 2 matrix
1 d 0 1 and a thin lens is represented by 1 0 k 1 where k=1/f. I show the matrix equations, and a solution for three 25cm FL lenses separated by two 2cm drifts. For your case with no spacing between the lenses, the focus is 150 cm beyond the last lens. If the spacing between the lenses is 1 cm, the focal length is reduced to 96 cm. If the spacing is 0.1 cm, the focal length is 141.5 cm. Bob S 


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