To get the 600 Ohm balanced XLR output from the AT835 into my 2500 Ohm camera, I bought a Hosa MIT-156 XLR to 3.5mm line matching transformer for around $20. You can pay more for a Shure A96F, but the Hosa does a great job. Here is a picture of the setup, with a $23 Rainbowimaging pistol grip:
Please watch the video below to see how the $75 used AT835 stacked up against the $80 Azden SMX10 consumer mic, also plugged directly into the GH3. Not to prejudice you before you listen to the test, but this experience tells me that picking up a used pro mic on eBay can be a pretty good investment - especially when compared to the price of camera mounted prosumer mics such as the $229 Rode VideoMic "Pro".