Arguably the biggest downside to DSLR filmmaking is the issue you can run into with moiré & aliasing. Mosaic Engineering has developed a unique answer to the problem, the VAF-5D2 Optical Anti-Aliasing Filter. The filter slides inside the body over the sensor. Best I can tell anyway.
It’s for the 5D MarkII and you can check it out here.
If anyone has tried this I would love to know what you think of its effectiveness.