How would the lens on the RX100 need to change in order to remain fast throughout?

Started Jul 24, 2012 | Discussions thread
adhemar Regular Member • Posts: 204
Re: How would the lens on the RX100 need to change in order to remain fast throughout

nosnoop wrote:

losangeles wrote:

Since the announcements of the Samsung EX2F and Panasonic Lumix LX7 I'v been wondering why the Sony RX100 becomes so much slower with larger focal length while the other two cameras almost preserve their aperture at the lowest focal length.

What differences in lens design enable the Panny and Samsung cameras to achieve this? Can anybody with expertise chime in and possibly also explain why doing something similar in the Sony with its larger sensor would have been difficult?

The basic physics behind this is the fact that the aperture (opening in the lens) is related to the focal length of the lens. So for example, a 50mm lens at a f stop of f/2 would have an aperture opening of 50/2 = 25 mm in diameter. As you can see, as the focal length gets longer, the same aperture would be larger (e.g. 600mm f/2 would require an aperture opening of 300mm or 30cm!). That's why as the focal length increases, it gets increasingly difficult to design a lens with wide aperture, and why "fast lens" gets so big for higher focal length lenses. It also becomes very tough to design fast lens in compact dimension as the focal length increases.

And this exactly the point I was trying to make in the other thread. Panasonic, with the FZ200, has a lens that open at F2.8 at the long end of the zoom, 108mm, true focal length.

Why can't Sony make a f2.8 lens at 37mm?

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