There are many types of filters used for a variety of situations but there are three key categories of filters you could consider; Ultra Violet (UV), Polarising & Neutral density (ND).
Ultra Violet (UV) filters really don’t do much other than protect your lens. They do filter some UV light which is said to reduce the blue in some images but this is minimal and offers little to not improvement in image quality. A lot of people (me included) use a high quality UV filters permanently attached to their lens to protect it (I would rather scratch a $250 replaceable UV filter than the glass on my $3000 L lens). The only time I take my UV off is when I am replacing it with a different filter.
Polarising filters (also known as circular polarising filters) are great for outdoor bright conditions when you want to control reflections and improve the colours in your image. If you’re shooting outdoors in bright sunny conditions with lots of reflective light, the polariser will help you manage the bright highlights by up to two stops (lens dependant). A polariser will also saturate colour and give your shot more impact. These are really great if you are taking photos of water in bright conditions. If you are taking photos inside or in dull conditions it would be best to take the polarising filter off.
Neutral density (ND) filters basically reduce the amount of light entering the lens. They come in different strengths and are named in relation to how many stops of light they will effect when exposing the image i.e. a 2stop ND filter will reduce the amount of light entering the lens by 2 stops. This means you can use a slower shutter speed or wider aperture to achieve the equivalent exposure that was achieved without the filter.
They are called ‘neutral’ because they absorb all colours in the visible light spectrum equally and therefor they absorb only light without creating any colour cast (they don’t change the colours in your image).
ND filters are generally used in really bright conditions or for long exposure landscapes with a tripod. You can even get tricky ND filters that have the dark part on only half of the filter. These are used when shooting sunrises/sets where the sky is bright but the land below the horizon is dark. Line the filter up correctly and the image exposure will be more balanced. All filters are available in both ‘slot in’ and ‘screw on’ types.
What do I use? I use Hoya ‘screw-on’ UV as general protection and ‘slot-in’ Lee hard graduated neutral density filters for my landscapes and seascapes. The great thing about the Lee system is you can stack multiple filters and adjust the height of the dark section of the filter to suit the shot. I also use a Lee Coral coloured graduated filter to warm the sky slightly (always good to get it right in camera). In my opinion for high end landscape photography, Lee filters are the best filters on the market (both in quality and price).
So what to buy?
For your purpose I would consider what you want the filter to do.
– A good quality UV is a must in my opinion. I use Hoya (HD) protector filters featuring hardened optical glass that has 4 times the breaking strength in ANSI standardized testing. Expensive, but on my L lenses it’s worth it.
– If you are doing a lot of bright sunny outdoor shoots near lots of water and reflective objects, I would strongly consider buying a polarising filter as I think you would really enjoy the results you will get. For best results you will need to take it off for indoor photography.
– If you are doing more serious landscapes on a tripod with longer shutter speeds I would get yourself a ND. Again you would have to take this off when not shooting landscapes. The Lee slot in style filters are quite expensive so if you are starting out you could consider going for ‘screw-on’ filters until you know if you want something more specialised.
Don’t forget to buy the right size filter for the diameter of your lens (thread size). You can usually find the thread diameter on the front of the lens or in your manual. If you are not sure you should take your lens with you when you buy.
My final tip (and probably the most important), get a good quality microfiber lens cleaning cloth and clean your filter regularly. We all get lazy from time to time but this is really important as the impact on image quality can be huge. All the light entering your camera is going through that filter so keep it clean and get the shot!