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.
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 25-cm FL lenses separated by two 2-cm 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