Third party LP-E6 batteries for Canon 5D Mark III and firmware 1.2.1
Compatibility, you might find that majority of third party batteries are compatible with few exceptions.
Capacity is entirely different matter. Most are 75% of OEM at less than 1500Mah. Only Phottix is at close to 1600 Mah. OEM is rated 1800mah. Most third party cost 66% less though. If you are willing to pay 66% less for 75% capacity, go for it.
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