Constant aperture zooms, can they technically be faster?

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ACPhotos Junior Member • Posts: 26
Constant aperture zooms, can they technically be faster?
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Hey so I've been mulling this question over in my head for a while and I didn't see any answers googling it, hope you all could ponder it with me.

It started when I was looking at my Tamron 28-70 F2.8 and how the aperture blades don't open fully when the lens is at 28mm and F2.8 as opposed to when it is at 70mm/F2.8.

Since F-number is simply the focal length divided by aperture opening, I'm reasoning that in order for the constant aperture zoom to maintain a constant F2.8 the lens is built to give F2.8 when the blades are fully open at the telephoto end (70mm), and gradually closing down when brought to the wide end (28mm). If that is true, are we actually losing out on extra speed on the wider end of the lens? What would a photo with the blades forced all the way open at the wide end even look like? Would it all just be lousy with vignetting?

 ACPhotos's gear list:ACPhotos's gear list
Fujifilm X100S Sony a7 III Rokinon 14mm F2.8 IF ED MC Sony FE 50mm F1.8 Sony FE 16-35mm F2.8 +4 more
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