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The vast majority of the kit lenses sold with entry-level (or even upper entry-level) interchangeable lens cameras, tend to have an aperture range of f/3.5-5.6. This seems to be independent of whether you look at SLR or mirrorless cameras, which manufacturer you look at (Canon, Fujifilm, Nikon, Olympus, Panasonic, Samsung and Sony all make a f/3.5-5.6 kit lens) and also of sensor size (you can find a f/3.5-5.6 kit lens for all of Nikon 1, micro 4/3s and APS-C). About the only counter-examples I can find are the Pentax Q7 and Fujifilm X-E1, both of which have an f/2.8-4.5 lens. However, the Q7 with its relatively tiny sensor isn't exactly typical, and the X-E1 is definitely aimed a bit higher in the market - the cheaper X-M1 ships with a f/3.5-5.6 kit lens.

Is there anything "special" about the f/3.5-5.6 range which means that de facto every kit lens has the same aperture, or is it just a combination of engineering realities and no manufacturer being prepared to take a chance on "something different"?

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up vote 7 down vote accepted

It is mainly about the cost/benefit ratio of making cheap lenses. It doesn't cost a lot more to make a lens f/3.5 than f/8 at an 18mm focal length since the entrance pupil (sometimes referred to as the effective or apparent aperture) is still well within the diameter of the mounting flange used by most interchangeable lens camera systems. As the lens is zoomed out to 55mm, the needed entrance pupil for f/5.6 is just under 10mm while the needed entrance pupil for f/2.8 would be 20mm which is approaching a significant percentage of the diameter of the mounting flange of ≈38mm for the micro 4/3 format or the 44mm of the Nikon F mount. Since most lenses will be made at least the same diameter as the mounting flange, the room for an aperture of the size needed for an f/3.5-5.6 lens in the typical kit lens focal lengths is already inside the lens tube, even with all of the other things that are wedged between the diaphragm and the lens barrel.

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Just to check you're talking about the same thing I think you're talking about - according to Wikipedia, the flange for Nikon F is 46.5 mm (Canon EF/EF-S is 44mm). Just a typo? – Philip Kendall Sep 27 '13 at 7:44
You are referring to the flange to sensor/film distance, also sometimes referred to as the registration distance. I'm referring to the diameter of the flange: how wide the hole in the front of the light box is, not how far in front of the focal plane it is. – Michael Clark Sep 27 '13 at 7:52

Ease of manufacturing and similar zoom ranges. In a fairly simple lens, the f/stop is going to decrease as the lens is extended since they are only as fully open as necessary for the short end and as they lengthen the focal length, the same size opening becomes a slower and slower aperture since it is based on the ratio of focal length to aperture opening.

Since the kit lenses typically don't bother to go to f/3.5 or f/4 is relatively cheap to make compared to f/2.8 and they do it for the wide side of the lens and the 5.6 follows naturally due to the distance to the long end of the focal length range. This is also why the one f/2.8 you mentioned shifts by roughly a stop on both ends. The range is still the same, but they decided to go for a bigger aperture compared to the wide end of the lens.

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As a zoom lens is extended, the relationship of the front elements/groups increases the magnification between the front of the lens and the diaphragm, which causes the entrance pupil (or effective aperture) to increase for the same size diaphragm opening. Otherwise the same 5.14mm opening required for f/3.5 at 18mm would yield an f-number of 10.7 at 55mm focal length. – Michael Clark Sep 27 '13 at 7:15

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