As photography has advanced, filters become less and less necessary. The old black and white photographers used a lot of filters. Now we use very few (and some photographers use none at all). The only filters I personally use – or that I really think are useful – are Polarizing filters, Neutral Density filters, and Graduated Neutral Density filters. That’s not to say you need any or all of these, but they are pretty much the limits of what I even consider. A few words about each:
These are usually circular filters. They operate by filtering out light rays coming from certain directions. As you twist them on your lens you will see things get lighter or darker. What does that do for you? Two things: (1) it makes skies a deeper, richer blue, and (2) it cuts down on reflections. These are moderately useful, and some people really like them, but I use them sparingly. Keep in mind they work best in the middle of the day and don’t really work at all near sunrise and sunset (due to the direction of the light rays).
For more about polarizers, check out this article, which explains more about how they work and how and when to use them.
Neutral Density Filters
These are filters that restrict the amount of light that is allowed into your camera. Why would you want to do that? So that you can use a longer shutter speed to offset the lack of light. They are great for blurring water or creating a sense of movement in your skies. They come in different strengths, and 3-stop, 6-stop, and 10-stop filters are pretty common (I have all three). I love these filters and use them all the time, but if you don’t find yourself photographing near water (lakes, rivers, or coasts) you might not need one. You can get them as circular filters that screw on to the front of your lens, or as rectangles that fit in holders in front of your lens (either way is fine, and it is just personal preference).
You may be tempted to get a variable strength neutral density filter, which sounds great in theory. Before doing that, however, check out this article.
Graduated Neutral Density Filters
These are rectangle filters and they all use Cokin-style holders that attach to the front of your lens. The top part of the filter is a neutral density filter and the bottom end of the filter is clear glass. The idea is that you put the neutral density part over the bright part of your image (usually the sky) and the clear glass part over the darker part of the image (usually the ground). The filter allows you to overcome dynamic range problems and even out the exposure. Landscape photographers use these quite a bit. Coastal photographers use them all the time. The utility of these filters can be limited because if you don’t have a straight horizon line in your picture you might end up darkening parts of the picture you don’t want to darken. But occasionally they are worth their weight in gold.
Which Filters Should You Get?
I personally own all of these filters and I keep all of them in my bag. I don’t use them that often though. In particular, I use the polarizer and the graduated neutral density filter very infrequently. I find that I can use Lightroom to do what I want. For example, instead of using the polarizer to create darker blues in the sky, I just reduce the luminance of the blues in Lightroom. Instead of using a graduated neutral density filter, I just pull down the highlights and pull up the shadows (in extreme cases I will bracket and combine the exposures). I still carry them, I just don’t use them very much.
The filters I use the most are the neutral density filters. Of those, I tend to use the 6-stop filter the most. In fact, if I am traveling without by photo backpack and am limited to just what I can fit in my pockets, I will usually only bring the 6-stop neutral density filter. It is great for achieving complete control over the appearance of any water in your photos. If that sounds like a rather limited application, you’ll be surprised how often there is water in your outdoor photos. There are obvious features like coasts, lakes, and rivers, but there are also puddles and fountains to think about. There is water everywhere. And where there isn’t water, there will be clouds, which can also benefit from using these filters.
For this reason, if you have no filters at all, I would start with a 6-stop neutral density filter. It will add a lot to your game. After that, add the other filters mentioned in this article as you see the need.