Why is the EM5 so much more popular than the GH3?

The OMD is much smaller and more of the size that most M43 users would want. As is the G5 Panasonic.

The GH3 is a much less compromising camera aiming, with its 2 fast zoom lenses, at a the professional end of the market. That's a good or bad thing depending on your point of view but whereas for me the size is a selling point, for most it will be a minus.

Personally I prefer in body image stabilisation (not for performance reasons, but because it operates on all lenses). On the other hand, I'd be unhappy to work without the electronic shutter and full iPad control that the GH3 offers.

I can see the OMD with its retro style appealing even to inexperienced photographers looking for a great camera. The GH3 has less appeal in that direction. That's the main reason why the Olympus sells more, in my opinion.

It might change (though I doubt it) when the camera is more widely available in the USA but obviously GH3 sales will increase then.

Both, by any standards are very, very good cameras really requiring a jump to FF sensor cameras to make a meaningful jump in quality.

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David
www.dthorpe.net