Originally posted by Ash The 12-24 and 10-20 are both MUCH bigger (bulkier) and heavier than the little 15. But as for better - that depends on what you want to capture. Architecture might demand a wider angle, but the 15 is a fine lens if you can zoom with your feet. I have the 12-24 and wouldn't be without it, but it is a hard lens to fit in the camera bag...
Plus: the Sigma is f5.6 @ 20mm, not particularly helpful indoors. The 12-24mm might be more useful, but, as Ash said, it's long and significantly heavier than the prime. If you plan to have two bodies, having the right lenses ready for the various contexts shouldn't be a problem--so, the advantage of a UWA zoom diminishes even more. Moreover, I'd expect to use the FA 31mm & DA 50-135mm most of the time. (Never played with these lenses, but I'm happy to imagine using them
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