NoFilmSchool's Robert Hardy presents a primer on lens focal lengths and how it affects an image.
He writes, "Focal length is one of the more straight-forward aspects of lens design. In the simplest sense, focal length is the physical distance between the imaging plane (ie. the sensor) and the optical center of the lens. Focal length values--which are expressed in millimeters--are basic measurements of how wide or zoomed-in a particular lens is. Easy enough, right? Where things start to get more complicated, however, is with all of the different aesthetic implications that the focal length of a lens has on the the final image."
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