Question

What is the Relation between intensity ans focal length

Answer 1

The intensity of the light at the image is proportional to the area of the lens: if the lens has twice as much area, it collects twice as much light. For a given lens area, the intensity of the light is inversely proportional to the area of the image: if you take a given amount of light and deposit it onto half the sensor area, you get twice the intensity. Usually, with a camera, the object distance (from the lens) is much longer than the focal length, so the image distance very nearly equals the focal length. In this situation, the height and width of the image are very nearly proportional to the focal length. Therefore the intensity of the light at the image is very nearly inversely proportional to the square of the focal length.

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