I've been shooting all weekend with the X5 and the stock DJI 15mm lens and I am pretty surprised to see barrel distortion in the footage. With such a narrow FOV I would not have expected that. It can be easily corrected in post, but it is very annoying to have to deal with. Attached is a sample picture showing the distortion. And just like any fisheye lens, the more you angle the camera down, the more pronounced the barrel distortion is. Just to make sure, I went and downloaded DJI's RAW sample footage, most of it was mountains or did not show the horizon, but in one of the RAW images it was definitely obvious where the ocean met the sky. I checked the DJI site and it is interesting that they do not list the edge distortion value for their lens.