There are two lines on the wikipedia pages for “cardinal points” and “focal length” that seem to contradict each other, and I would be extremely grateful if someone could explain to me why they do not. In the page for cardinal points, it says:
If the medium surrounding the optical system has a refractive index of 1 (e.g., air or vacuum), then the distance from the principal planes to their corresponding focal points is just the focal length of the system. In the more general case, the distance to the foci is the focal length multiplied by the index of refraction of the medium.
This makes sense to me. I also get that these principal planes can often be located outside of the lens with some clever optics, allowing for lenses that are physically shorter than their focal length. However, on the page for focal length, the page reads:
When a photographic lens is set to “infinity”, its rear nodal point is separated from the sensor or film, at the focal plane, by the lens’s focal length. Objects far away from the camera then produce sharp images on the sensor or film, which is also at the image plane.
I don’t see how these can both be true, because if the focal point, the point as I understand it to be where all the light converges, was on the film plane, an image wouldn’t be rendered, it would just be an indistinguishable point of light. Does the light not have to travel a distance past the focal point to the film plane in order to form an image?
I think it is possible that I am getting my front and rear nodal points confused, or that I have a larger fundamental misunderstanding about how focal length is measured. Thank you so much for your help!