Difference between these two Sigma 12-24mm lens?

Sorry cannot help as I do not have those lenses but you should post this on the Sigma forum - many there will be able to explain any other difference to you than the following.

The main reason they are different in price is the more expensive one is designed for a full-frame (35mm size) sensor, so whilst the less expensive one will work fine with most APS or similar sized sensors fitted to many DSLRs, it would not cover a full frame sensor, such as for the later Nikon D800.  Hope that clarifies.  Basically, you would opt for the dearer one, even if you have the smaller sensored camera now, IF you were thinking of getting the full frame DSLR in the near future.

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