Under certain circumstances a
polarizer can be a great help. If you shoot a lot in very bright conditions, getting an
ND filter might be advised (if you know what you need it for). If you shoot a lot of landscapes, an
ND Grad filter can be of great value.
You should only use filters if you really really need them though.
Reflections. Everything that sits in the light path will contribute something to the picture, be it in a positive or in a negative way. If you put a $50 protective filter (substitute UV filter, Skylight filter, Haze filter, etc.) in front of your $900 lens, each and every single one of your pictures will be degraded thanks to reflections that the filters produce between themselves and the front element of the lens. Every year around Christmas time I receive requests from people who can't understand why their shots of the Christmas tree with all its lights look so weird. What they experience is ghosting: reflections off of filters that create ghost images.
Scratch protection? Using a filter for scratch protection is so deeply engrained in many people's minds that they don't even question it anymore. Which turns it into a really easy sell for every camera salesman. They really love selling you protective filters. "You obviously don't want to buy a lens without a protective filter, right? One little scratch and your investment will be down the drain." Well, not so far.
Here's a video for you to watch. The answer is actually simple: don't spoil the majority of your pictures while you could get the same or an even better level of protection using a lens hood. That's right, there are many reasons to get a lens hood, and one is that it will give your lens protection from bumps. The other reason for a using a lens hood is that it which will increase the overall contrast and clarity of each and every picture you take.
Color correction? These are a staple of film photography. We don't need color correction filters with digital photography. All color correction can be done in post processing or by setting the proper white balance before the shot.
Effect filters? Today star effects, cross filter effects, diffusion filter effects etc. can be much more effectively produced during post processing. There's no undo if you shoot with a filter and you get it wrong.
So what filters are okay? The few reasons where I agree with using filters are these:
- a few Canon L lenses require a filter to complete the weather sealing.
- the ND grad filter will help you control dynamics in certain scenes
- the ND filter will help you shoot very bright scenes at wider open apertures
- the polarizer lets you control non-metallic reflections, increase color saturation and contrast and get darker blue skies (but take it off indoors, as it also steals some light)
- some specialized filters will allow you to do things that would be hard to do any other way in a single shot (example: split diopter)