Is a lens’s focal point located on the film or sensor?

Asked 12/15/2014

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I’m confused about focal length and focal point. If focal length is described as the distance between the center of the lens and the film/sensor, does that mean the lens’s focal point is actually on the film or sensor plane? How are focal length and focal point related in a camera lens?

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Not exactly. The focal point is the point where incoming parallel light rays would converge after passing through the lens. For a simple lens focused at infinity, that point lies one focal length behind the lens.

In a camera, the film or sensor is placed at the image plane, which must coincide with where the lens forms a sharp image for the chosen focus distance. When the lens is focused at infinity, the sensor/film sits at the focal plane, so in that special case the focal point for distant subjects is on the sensor plane.

But for closer subjects, the lens-to-sensor distance changes slightly, and the image is formed behind the lens at a different position. Also, with real multi-element lenses, focal length is measured from principal planes inside the lens, not simply from the physical center of the lens barrel.

So: focal length is a property of the lens, while the focal point is a geometric point where light converges under specific conditions.

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