Is there a lens you can buy for the cannon that you won't have to switch back and forth between different lenses?
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Since you don't specific price, camera mount, weight, quality, or any other requirements, I have two suggestions:
They both have significant trade-offs, such as variable maximum apertures, but both can basically be used in the widest range of focal lengths available. Unfortunately you just can't get anything like this with a very large aperture such as f/2. Another option would be to simply use a nice prime lens such as the 50mm f/1.4 USM Lens, and use it creatively without switching lenses.