I'd recommend multicoated screw-in glass filters for field use. Hoya is probably the best value out there, but for a little more money you can get heavier brass-mounted B&W or Heliopan. About all I
ever seem to need is a 25 Red and either a yellow-green or a true green. Color photography is a more involved subject. I never use filters just to protect the lens, except on small cameras I might
be using in the rain. With a view camera I protect the lens with some kind of compendium or other
lens shade. Filters almost always have a slight negative effect on image sharpness, though that fact
is often offset by helping cut thru haze. UV and skylight filters can be helpful in color photog, esp
at high altitudes, but I never use them for black and white. I think most people get carried away and
buy more filters than they actually need. I'd experiment just with the basics first.