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New 1100D/T3, question about max aperture

Started Jan 20, 2012 | Discussions thread
AndrewIC Forum Member • Posts: 85
Re: not that simple ...

Ehsaan Faramosh wrote:

My the question was, if the lens CAN manage a 9.8mm aperture diameter, why does it not use it at the 18 mm end?
That would give it an f-number of 18/9.8=1.8

Zoom lenses are all about compromise. It could be that in theory the optics could provide a better-than-f/3.5 image at the wide end, but it might look like total crap. Also, because the lens elements physically move around relative to each other when changing the focal length, then the aperture the glass supports changes with zooming: to do better we'd need bigger lens elements, and/or a different arrangements of lens elements. Such glass does exist - the Canon 17-55 f/2.8 has a wider aperture across the entire zoom range, but is something like eight times the price.

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