When I was at college, I think it was; we clubbed together on one of the bulk-buy offers in one of the mags... it was something daft. price fell from £5 per filter to about 75p! But we had to buy 100 or something stupid! I used the opportunity to fill in some holes and get some creative ones to play with.
Anyway, top choices for colour.
0/ Skylight / UV... one for each lens as a protection element.
1/ Polariser (Doesn't have to be a circular polariser unless you have an AF camera, and then only if meter or focus system get confused by it) MOST used filter on any of my cameras. If you tried taking away my filters, THIS is the one I would be trying to hide from you!
Removing reflections & saturating colours, is something that cant be replicated or mimicked in PP. So still a useful tool, regardless of medium.
Scanning negs though, I have noticed a lot of corner masking.... not present in prints, cropped to fit print paper format, or noticed in slides (too small, or if projected, usually suffering a bit of corner drop out anyway!), so something to be cautiouse of, they can be thick filters especially on a system mount.
2/ Colour Correction set.
2a) Tungsten -> Daylight
2b) Flourescent -> Daylight
Removes orage or green cast from artificial light when using common day-light balenced film... particularly more useful on slide film, where casts cant be corrected by printer; and these days where getting specialist film can be more difficult/expensive. But correction quickly and easily applied in camera on Digital, or during PP if images scanned; so not as useful as they perhaps once were.
3/ Graduates - I think that there are three graduation densities.
3a) Grey Grad (Medium) for holding back skies. Not used anywhere near as often as I expected.
3b) Blue Grad (lowest density)
Boosts blue skies... used more often than I expected.... but a bit of a cheat, and you have to be careful, as can be incredibly obviouse!
3c) Tobaco Grad (middle density)
Boosts sunsets and beaches... can look cheesy. Not used very much.
3d) Green Grad (lowest density)
boosts foliage & grass... didn't think this would get much if any use, BUT actually quite useful. Aparently human eye is not so sensitive to green light, and can tolerate a lot of boost to saturate green. Particularly useful in summer with strong sunlight and faded grass.
Toning & tinting is so much easier in Post-Process digi-dark room these days, so coloured grads possibly less useful than they were, but you cant get a blown out sky back in PP, so, grey-grad at least still pretty useful.
4/ Star-burst
5a) 2-line
5b) 4-line
5c) 6-line
Errmmm... can put nice high-light twinkle on rippling water and stuff.... and it's something you cant replicate very well in Digi-Dark-room.... used considerately, can work nicely, but big risk of mega-cheese.
5/ Centre Spot
I got three, a black, a white & a tobacco.... The wedding/portrait photographer's favourite, aparently. Does a vignette fade from subject to frame edges. Used, without much success, occassionally. I think they probably work better in studio enviroment.. or for other people! Was easy enough to replicate with a vignette in the dark room, during printing, and a two second doddle in modern Digi-Dark-room; so probably no great loss t any-one these days.
Looking through the Cokin catalogue I still have... I think I have a graduated 'mist' filter knocking around some-where, I think I tried it once. Then there were a bunch of clear filters with artificial rainbows.... pure 60's cheese... probably go well with CND symbol bakula mask! Then into prismatic 'fly's eye' multi image filters... more 70's ABBA album cover crap! Speed or stretch streaks, ans stuff. Mostly all 'in-camera' montage sort of stuff. Very little that cant be done or done better in post-process, esp Digi.
So, a lot of old filters are reletively redundant in digi era, but there are a few still worth having; UV's to protect lens element; Polariser definitely; Neutral Density, & Grey Grad; starburst & maybe centre spot. Colour correction for film.
