I'm searching a simple and cost-less way to remove the fisheye effect by:
- adding a material (additional corrective lens) or
- find the right software (free if possible) to remove the fisheye effect on my pictures
I doubt there's a hardware solution to this, and if there were one, it's likely to cause a decrease in image quality. Software is your solution, here.
A lens profile released by Adobe for DJI Phantom 2 Vision's camera can be used to remove lens distortion
This would require that you use post-processing software that understands adobe lens profiles (e.g., Lightroom), but that would probably be the fastest, easiest way to batch-process a number of photos to remove the distortion of the lens, particularly if you're shooting in DNG.