[Brightcolours]

Valid points, especially the two filters. I imagine a half darkened stripe of optical glass to be moved from left to right. Adding not only one, but two moving surfaces (dust and wear, particles, oil...) in a very small volume of a camera body. Making it close to impossible to focus because of 10 stops amplification (or less, Fujifilm's ND filter coming with the 56/1.2 APD is ND8, so only 3 stops).

 

Sigma DSLR? Or the mirrorless sd quattro types? Anyway, most mirrorless cameras do have a glass element in front of the sensor. 

See Lensrentals Blog:

 

 

All digital cameras have glass in front of the sensor, JoJu.