5volt from 2 Li Ion

I want to supply 5volt regulated from 2 seriesed Li Ion cells ( 8.4v max)

While I could use a simple 5volt regulator this would burn off the extra 3.4volt as heat. Is there a more efficient way to get 5 volt? Perhaps boosted from 4.2 volt?

Either way solution needs to be small.

use a switching regulator rather than a linear one

model...link.... please

Oh, this is to power a REplay XD 1080 camera so 0.5 amp is probably heaps.