The 1/4 strength provides a more noticeable halation around light sources and a greater reduction in contrast than the 1/8, making it the most popular choice for a visible cinematic effect.
No, this filter does not interfere with the phase-detection or contrast-detection autofocus systems of modern mirrorless or DSLR cameras.
Yes, as long as the filter's thread size (e.g., 77mm or 82mm) matches your lens's front diameter. Check the 'ø' symbol on your lens barrel.