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How does focal length affect the size of an image made by a lens?
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In optics, the focal length of a lens is a primary factor in determining the size of the image produced. The relationship between focal length and image scale can be summarized as follows:
- Long-focus lenses: A lens with a longer focal length produces a larger image of a distant object. Because it magnifies the subject more, it covers a smaller portion of the overall scene.12
- Short-focus lenses: A lens with a shorter focal length yields an image on a smaller scale, which allows the lens to capture a wider field of view of the scene.12
Mathematically, the image magnification (m)—defined as the ratio of the image height to the object height—is related to the focal length (f) and the distances of the object and image from the lens. For a distant object, the image size is directly proportional to the focal length. Consequently, photographers and scientists often use lenses of varying focal lengths to adjust the image scale and the field of view depending on the requirements of the task.32