Why are so many kit zoom lenses f/3.5-5.6?
Asked 9/26/2013
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Many interchangeable-lens cameras ship with a standard kit zoom around 18-55mm (or similar) that has a maximum aperture of f/3.5 at the wide end and f/5.6 at the telephoto end. This pattern appears across brands, mounts, and even different sensor sizes. Is there something inherently special about the f/3.5-5.6 range, or is it mostly a design and cost compromise for entry-level zoom lenses?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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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.
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?
You are referring to the flange to sensor/film distance, also sometimes referred to as the registration distance. I'm referring to the throat diameter of the flange: how wide the hole in the ring at the front of the light box is, not how far in front of the focal plane it is.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
12y ago
0
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There’s nothing magical about f/3.5-5.6. It’s mostly the sweet spot for cheap, compact kit zooms.
Maximum aperture depends on focal length divided by the entrance pupil size. At the wide end, making an 18mm lens f/3.5 doesn’t require a very large opening, so it’s relatively easy and inexpensive. But at the long end, a faster aperture gets much harder: for 55mm at f/5.6 the entrance pupil is about 10mm, while f/2.8 would need about 20mm. That means larger glass, a bigger barrel, more weight, and much higher cost.
Kit lenses are designed to be affordable and small, so manufacturers usually accept a variable aperture. In simple zoom designs, as focal length increases, the effective maximum aperture naturally gets smaller unless the lens is made significantly larger and more complex.
So f/3.5-5.6 is basically a practical compromise: reasonably bright at the wide end, still usable at the long end, and cheap enough to bundle with entry-level cameras. Other ranges are possible, but they usually mean either a slower lens, a larger/more expensive lens, or a more premium product.
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