In my view, if you want to get the most out of the E-M1, go with one or more of the excellent fast primes available for Micro Four Thirds from Olympus, Panasonic and a number of fine third-party lens makers. Just keep in mind that, while most APS-C sensor cameras have a "crop factor" relative to full-frame of 1.5, the crop factor for Micro Four Thirds cameras is 2x, so "normal" field of view for the E-M1 is 25mm. The Panasonic Leica 25mm f1.4 seems to be the cream of the crop, but the Olympus 25mm f1.8 is less expensive and almost as good. But there are lots of options —over fifty lenses in all. Look for the "Micro" in Micro Four Thirds. Older Four Thirds lenses (many of them excellent) will work on the E-M1, too, but require an adapter.
If you really need need the much larger files, slightly better exposure range or low-light performance of a full-frame camera, or if you can't live without 1024p or 1060p video, then the E-M1 may not be for you. By the same token, if you just want to take good snapshots of your pets or your kids or your vacations, the E-M1 may be more camera than you need or want. (Look at the similar but much less expensive Olympus E-M10 instead.) Otherwise, whether you're coming from an older Four Thirds camera or earlier model Micro Four Thirds, or whether you're upgrading (as I like to put it) from a bigger, heavier DSLR, SLT or mirrorless camera, I recommend the E-M1 highly.
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