Lens Size, Front Element Size and Focal Length - Science Behind This?

Started 1 week ago | Discussions thread
MediaArchivist
MediaArchivist Veteran Member • Posts: 4,329
Rule of thumb

One rule of thumb that can quickly give you an idea of a proposed lens' internal width:

(focal length) / (max f) = max diameter

So a (theoretical) 300mm f/1.2 lens would need to be 300/1.2 = 250mm wide somewhere on the inside. For a zoom lens with a constant f/number, compute at the maximum focal length. For a zoom with a varying f/number, compute at both ends and use the larger result.

The f/number is the focal length divided by the width of the entrance pupil. The focal length is longer than the actual length of the lens for telephoto lenses, and shorter for retrofocal lenses. The entrance pupil width is how big the actual aperture appears when viewed from the front of the lens. There is some oversimplification is what I wrote above, but my initial formula is still a pretty good rule of thumb to give you at least a ballpark diameter. The front element might be much larger to reduce vignetting, but almost certainly won't be smaller.

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