If you want to have smokey water or level water like the sea or a river then you will need an N.D filter [ Neutral Density filter ]. Trying to find the one that you need can leave you very confused, there are a lot of different kinds, some darker than others, and some that are like the one I use which is a HD Multi-Coated Variable Range Neutral Density filter. The filters come in different shades which will allow you to have longer exposure times. You can get effects of the clouds smoothing out, water looking like smoke, and you can even level the sea if it is not too rough. I use a Variable Range N.D filter which means I can turn the filter ring on the outside of the filter, and the filter will get darker or lighter depending on which way I turn it. The darker the filter the longer the exposure. I have heard people say these kinds of filter don't give a sharp image, which is true to some extent meaning that any filter you use can affect the picture to some degree, Unless you are prepared to spend hundreds of pounds on a single filter witch you may have to do if you are a professional Photographer. I am only a beginner, and on a strict budget so I went the cheaper way. The pictures below are ones I took with the variable filter, and I am very happy with the way they turned out.
You can use the filters on a very bright sunny day which will allow you to have longer exposure times. It is just the same as putting some sunglasses on your lens. Not being taught how to use a camera, a lens, and filters means I am experimenting with different settings and filters to see what kind of photo comes out at the end of the day. I managed to flatten the sea on the pier photo above using a neutral density 3.0 Filter and an exposure time of 30 seconds. Although I must admit that I have not been able to get any more pictures yet that are that good, but I am still trying.
There are many filters on sale that do many things, like enhancing colours of the picture, a red filter will enhance the red and orange colours of the image and orange filters are good for sunsets. The only ones I use are ND Filter ( Neutral Density Filter ) and the Polarizing filter. With the ND filter you can buy single filters with different strengths or buy one like I use which is a ( HD Multi Coated Variable Range Neutral Density Filter ). This means it will cover all the different strengths of the filter, you just screw the filter to the camera lens then rotate the outer side of the filter to get the desired strength. There is one thing to remember if you use the darker settings is that you must put your camera lens into manual mode, then find you image you want, then carefully screw the filter back on because the camera lens will keep trying to focus on a dark image.
The image above was taken on a sunny day with no filter fitted. You can see that the clouds are a little washed out now that the sun is out. The Polarizing Filter works by stopping all reflections, like reflections on water, on glass Windows, and on reflections from metal.
The image above was taken with the polarizing filter on the lens. You can clearly see the clouds standing out more, and the image has darkened. I should have opened the aperture slightly to lighten the image. But Remember that The Polarizing filter can only work properly if the sun is along side you or behind you, if the sun is hitting the filter it won’t work. Also be careful about the corners of your pictures.