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So I've read when the camera is correctly focused the focal length is the distance between the sensor and lens. But when drawing a ray diagram for the object, lens and sensor the the focal point ends up inbetween the sensor and lens.

Also when considering the object as a point it would make sense that the rays from that point all converge on the the sensor behind the lens to give a sharp image. So if the point is far away the rays will come in parallel and converge at the focalplane. It all makes sense but I just don't understand ray diagrams.

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  • $\begingroup$ FYI: The "focal length" of a camera lens is the distance between an arbitrary "node" within the lens and the plane on which parallel rays entering the lens are converged. (photo.stackexchange.com/a/22197/51833). That is to say, it's the distance from the node to the sensor when the lens is "focused at infinity." In order to focus on a closer subject, the lens must be moved further from the sensor. $\endgroup$ Apr 15 at 14:08

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The pictures you see with the focal point is between the picture or film or sensor and the lens. This picture comes from the time, where you moved the lens according of the distance to the object. For an object near the camera you move the lens away from the film, only for pictures of objects more than about 10 m the focal point ist on the film or sensor. Only in smart phones you have only one distance, and very short focal length, but than you can not take sharp pictures of close by objects. New phones therefor have different lenses for 3 different distances.

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  • $\begingroup$ The more sophisticated smart-phone cameras these days have lenses with internal elements that move for focusing. When you see a smart phone with two or three different lenses (two or three different whole cameras, actually) they generally have different focal lengths/different fields of view. $\endgroup$ Apr 15 at 14:13

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