It's odd that this should come up .... I mean 'now' that I have taken off all of my filters.
About 2 or 3 months ago I did a very simple an unscientific test with my 28-300 VR which I have been complaining about as being too soft at the long end especially (mostly my fault I think). My thought was that if I am being particular with the sharpness of my images and using a D800 and $1,000 to $2,500 lenses with great glass....why am I putting 'anything' over the lens? It seems to me that anything other than naked glass could only detract from image sharpness. To my eyes, the tests were clearer and sharper w/o filters.
Yes, polarizers etc. I understand. But it just doesn't make sense to me that Nikon would make a lens that needed another piece of glass on the front for $100 or they would have put it there. It should perfrom best naked...and I think DXO tests them that way. As far as protecting the lens from scratches and bumps, well I am not a paparazzi or journalist, only an amateur, and I try to be carefull. If I am taking my D800 out in a croud or dense shrubs I put on a filter just in case. And If I come upon the picture opportunity of a lifetime...I take it off before I shoot. Otherwise I leave the filters off all the time now. (I know many will disagree).
Dan (Nikon D800,V2,Sony HX400V,Lumix ZS40) "I don't read, I just look at pictures" - Andy Warhol