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MissScience33

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Hi everyone,

for a lab I need to determine the position of the cardinal points of a telephoto lens. The focal points were determined experimentally, and the focal length given on the lens is 200mm. The principal points should be determined with f=PF.

In all the textbooks, the principal points mostly find themselves back within the thick lens/lens system, or very close to it if it is for instance a meniscus lens. For a telephoto, both principal points should find themselves back in the object space.

But if I draw the principal points such that the focal lengths go from the focal points towards the lens, the rear principal point finds itself closer to the front focal point, and the front principal point is closer to the rear focal point. I think this is strange since the focal length should then be the shorter distance between the (in my sketch) rear, respectively front focal point and the front, respectively rear principal point.

I was wondering if due to the fact that in a telephoto, there are two lenses(in theory, in practice there are apparently more to prevent aberrations I think), with a negative and a positive focal length, in order to find the principal points, one should go in the same direction, towards the object space for both? Or maybe in opposite directions, but not towards the lens? This seems slightly weird to me if I understand the theory correctly, but maybe I missed something with the signs of the radii of curvature, or the focal lengths?

Or is there maybe a problem with the positions of the focal points?

Thanks for any input!

for a lab I need to determine the position of the cardinal points of a telephoto lens. The focal points were determined experimentally, and the focal length given on the lens is 200mm. The principal points should be determined with f=PF.

In all the textbooks, the principal points mostly find themselves back within the thick lens/lens system, or very close to it if it is for instance a meniscus lens. For a telephoto, both principal points should find themselves back in the object space.

But if I draw the principal points such that the focal lengths go from the focal points towards the lens, the rear principal point finds itself closer to the front focal point, and the front principal point is closer to the rear focal point. I think this is strange since the focal length should then be the shorter distance between the (in my sketch) rear, respectively front focal point and the front, respectively rear principal point.

I was wondering if due to the fact that in a telephoto, there are two lenses(in theory, in practice there are apparently more to prevent aberrations I think), with a negative and a positive focal length, in order to find the principal points, one should go in the same direction, towards the object space for both? Or maybe in opposite directions, but not towards the lens? This seems slightly weird to me if I understand the theory correctly, but maybe I missed something with the signs of the radii of curvature, or the focal lengths?

Or is there maybe a problem with the positions of the focal points?

Thanks for any input!

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