Well with digital a lot of filters can now be done in PS.
Here's the list of what can't, or at least it's MUCH easier with a filter:

CP:Circular Polarizer. This cuts down reflections in glass/water and adds contrast to a scene. Very good for landscape work, or anything where reflections aren't wanted. It will reduce the exposure by about a stop. They can rotate once on the lens to adjust the effect. The 1 stop light loss will always be there, but the amount it reduces glare changes. Cars or water images would be good examples of subjects your want one, but they're good for a lot more than just that.

ND:Neutral Density. These are gray filters. They're used to make exposures longer when you need a long shutter speed, but f32 and ISO 50 aren't enough. Waterfall pictures with blur during the day would be a good example. These come in different stops meaning how much longer they make the exposure. 1, 2 and 3 stops are the typical options. These can be round and thread onto the lens themselves or they can be square/rectangular and use holder.

GND:Gradual Neutral Density. Same as above but they fade from clear to the dark part. They can have a quick/hard fade or a slower/soft fade. These are used when part of the exposure is much brighter than the rest. Think a sunset. The dark part it positioned over the bright portion of the frame. This is similar to merging exposures in PS and HDR gets a similar result if not overdone. These need a filter holder as they need to be adjustable.

UV or Clear Glass Filters--Used to protect the front element of your $$ lenses... UV filters also cut out haze and marginally improve colors/contrast.

M

Just to give you an example, the only filters I have in my bag are:

- Circular Polarizer
- An assortment of Graduated ND Filters
- A 3-stop Regular ND Filter

- Scott

Originally Posted by Markitos

UV or Clear Glass Filters--Used to protect the front element of your $$ lenses... UV filters also cut out haze and marginally improve colors/contrast.

M

that is also highly debated. I don't care to get into that here, but the flip side of the coin is any extra glass hurts IQ not matter the quality. Also if your lens gets dropped some will argue the shattered filter could do more damage than save it. IMHO don't drop your lens and you don't have to worry about it, and insure it. I use hoods for this reason unless I'm in a VERY harsh environment where I know I'll be getting a ton of crap on the front element (salt water spray).

what does a linear polarizer do?

i just picked a used one up for $3.99 at my local camera shop. the same one new, was $50. i figured i would try it.

Originally Posted by jacobsen1

Originally Posted by Markitos

UV or Clear Glass Filters--Used to protect the front element of your $$ lenses... UV filters also cut out haze and marginally improve colors/contrast.

M

that is also highly debated. I don't care to get into that here, but the flip side of the coin is any extra glass hurts IQ not matter the quality. Also if your lens gets dropped some will argue the shattered filter could do more damage than save it. IMHO don't drop your lens and you don't have to worry about it, and insure it. I use hoods for this reason unless I'm in a VERY harsh environment where I know I'll be getting a ton of crap on the front element (salt water spray).

Yes, I have heard that side of the argument as well. I generally use my lens hoods also.

M

CP:Circular Polarizer. This cuts down reflections in glass/water and adds contrast to a scene. Very good for landscape work, or anything where reflections aren't wanted. It will reduce the exposure by about a stop. They can rotate once on the lens to adjust the effect. The 1 stop light loss will always be there, but the amount it reduces glare changes. Cars or water images would be good examples of subjects your want one, but they're good for a lot more than just that.

Also, great for snow shots in bright sun.

--kC