Why are old FF lenses so small ? Could modern lenses be smaller ?

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MatLD
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Why are old FF lenses so small ? Could modern lenses be smaller ?
4 weeks ago

Hi, I have the feeling that modern lenses with IS, AF made for digital sensors are much bigger than old film lenses.

Is it just the size of the motor and other component ? Is is related to IQ (digital sensor need higher sharpness)?

The comment when people talk about the A7 lenses or m43 fast telephoto lenses is that "the physics just implies huge lenses". Well it is true in a way, but I have the feeling that there is still room for improvement as we can see when looking at the old FF film lenses.

First the common formulla seems gives rather small numbers: 150mm f3 lens has an entrance pupil of 5cm : that is not that big at all.

Second the f-number does not seem to be so easily calculated for laymen :

Wikipedia:

In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil. [...]

The entrance pupil is usually a virtual image: it lies behind the first optical surface of the system.

Can the entrance pupil be smaller than the actual front end element ?

Sony Alpha 7
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