filters; yes, or no?

[cosmonaut]

Take the UV off at night. You can get some back reflection into the lens and back onto the film. But no I don't use one.

 

[Gumby]

This isn't logical to me: they also spend R&D money to put "filter" threads on the end of their lenses and additional R&D money to design their own "filters".

I suppose it is for about hte same reason that GM, or Ford, or Toyota, or Mercedes, or BMW, or &c. don't make their own headlights (or many other parts of the car).