What are your most useful square filters?

[John the Greek]

I just bought a Cokin filter system for my Nikon N80 and a ND.8 graduated filter. I'm excited to purchase other filters to use with the Cokin filter mount but I'm not sure what's out there and which ones are the most helpful.
Would it be worth it to buy a lighter ND filter... like a .3 or .4?
and how about the color filters... tobacco? warming? same thing?

 

[micatlady]

I have several solid ND filters in different darknesses, helps out quite a bit. I also have many colors of graduated ones, I don't use them often but have had some really cool results when I have. I also have a polarizer that fits into my cokin holder (I have the P series). I have UV filters on all my lenes, mostly to protect them, and it's cheaper (broke college student) just to have the one polarizing cokin than to buy one for each lens. I guess you have to decide what you want to shoot and go from there, there are tons of different fitlers. If you shoot B&W get red, yellow, blue and green, one of the little cokin booklets helps tell you which filter to use for what application/effect.

 

[Diarmuid]

I wouldn't get too carried away buying filters. There are a few basic filters but there are lots of Cokin filters that you will only ever use once or twice. Here is what I believe to be the more useful filters

Graduated; The grey or ND is best and get a strong one for most effectiveness
Warm filters; These are yellow/orange and give a nice warm evening glow to portraits and landscapes.
Polariser; although I prefer the round type even though it means buying one for each lens.
Starburst; A speciality filter that wouldn't be used that much but a four point star can be handy to add a shimmer to highlights.
Contrast filters (colored filters); these would be handy when shooting on Black & white film. Red and green are the most useful as red darkens blue skies and green lightens foliage.

One word of warning regarding Cokin filters, they scratch very very easily.

If you want to get the most from your camera, including how best to use filters. Why not join the School of Photography Basic Photography Course. See http://www.schoolofphotography.com/basic_photography_course.html