why fixed aperture lenses?

Started 2 weeks ago | Discussions thread
MediaArchivist
MediaArchivist Veteran Member • Posts: 4,380
Key point

bobn2 wrote:

the aperture is the diameter of the entrance pupil seen through the front element

I think this is the key point. If a zoom lens is designed such that what you describe above changes diameter in direct relationship to the focal length as the lens zooms in and out, the f-number will remain constant. It is not necessary for the physical aperture itself to get smaller at smaller focal lengths, just the appearance of it as viewed through the front.

Side note: are there any zoom lens designs that do, in fact, change the physical aperture as the focal length changes? Perhaps to reduce an aberration that would otherwise be more complex at one end of the zoom's range?

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