Physics of lens size (r.e. NEX lenses)

Started Aug 31, 2011 | Discussions thread
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Gakuranman Senior Member • Posts: 1,194
Physics of lens size (r.e. NEX lenses)

Hey all,

After a thoroughly illuminating thread last week on sensor size and noise ( ), I'd like to quiz the community brain once again on the physics of lens size. My understanding of the subject is very limited.

What factors determine how small a lens can really go? I'm aware that the basic physics involve the equation D = f / N. That is, the diameter of a lens is equal to the focal length divided by the f number. ( )

A 50mm focal length lens with a widest aperture of f1.4 would need a entrance pupil diameter of 35mm. (For a full frame camera).

However, due to the nature of crop factors, a lens that gives the equivalent field of view on m43 would only be half the size (a crop factor of 2) in order to cover the sensor with an image.

25mm focal length lens at f1.4 would need an entrance pupil of at least 18mm.

On APS-C, with a crop factor of 1.5:

33mm focal length at f1.4 would mean an entrance pupil of 24mm.

However, this would be the minimum of the glass alone.

My question then: what other factors are there to take into count when determining lens size? How would the AF electronics change the design? Also, how does the flange distance affect the ultimate lens size? What about retrofocal designs?

In short, the question is: why are NEX lenses so darn big? Is the difference between m43 crop factor 2 and APS-C crop factor 1.5 that much of a difference?

I'm aware the lens design may vary according to the focal length and this a very big and complicated topic, but any insight or links to clear and well-explained articles online would be greatly appreciated.

I look forward to hearing from some of our more knowledgeable members! Thank you in advance.


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