Which voltage to replace a battery?
A 1s 1350 mAh Lion cell provides ~= 5 Wh of energy.
A 3S 5200 mAh battery gives ~= 55 Wh.
So the small battery is about 10% of the capacity of the large one and probably runs the camera for longer than the large battery runs the copter so not all the 1350 mAh is needed per flight.  

So a buck converter with reasonable efficiency to operate the camera from the main battery will add relatively minimal load and the slight reduction in weight will partially make up for this. 

SO using one battery for both seems sound. 

Note that 


A camera MAY have current peaks rather larger than its average draw - a converter must support these. 
The buck converter needs to be well behaved and NEVER output excess voltage, that your camera's days may be long on the face of the land. If you do not trust the converter 110% you may wish to add a good capacity zener (overvoltage protection) diode across Vin_camera fed by a fuse from Vout_converter so that the fuse blows if Vout is excessive.