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f/4.5 doesn't mean that the hole in the iris is physically 1/4.5 times the focal length. It means that the size of the image of the hole, as viewed from the front of the lens through the lens elements in front of it, is 1/4.5 times the focal length. The image of the hole is the entrance pupil. If you can, look at the image of the hole from the front as you ...


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If all of the magnification takes place between the front of the lens and the physical aperture when you zoom, then the entrance pupil, which is the effective aperture that is actually used to calculate f-number, also grows by the same ratio as the focal length. As you zoom to longer focal lengths the size of the aperture as it appears through the front of ...


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As you know, the focal ratio (f-number) is calculated by dividing the working focal length by the working iris diameter. Therefore, these fundamental values are intertwined as to their influence on image brightness (exposure settings). The bottom line, as you zoom, image brightness rises an falls significantly. In fact, double or halving the focal length ...


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