Give your camera a rose-tinted view of the world
If you are not the proud owner of a new digital SLR (or SLR-like) camera, then this section is not for you. Most compact digital cameras are not designed to use filters.
Now that half of the readers have departed, the rest of us can continue on and find our more about the useful — and not so useful — filters you can use.
Why You Should Use a Filter
Natural light changes. It alters its color, its intensity and its direction. While it is wonderful to go out into the world and photograph the light as it changes, it can also be a problem.
Sometimes the light does not want to cooperate with the mood you are trying to achieve in a photograph. Harsh light needs to be toned down for a portrait. Reflected light off water needs to be reduced for a landscape. Flower colors on an overcast day need a boost.
Filters will help you do this.
Since lenses come in different sizes, so do filters. Filter size is measured in mm and there is a wide range: anywhere from 37mm to 95mm.
The most important thing to do before you purchase a single filter is to find out what size you need. Since filters screw into threads on the front of the lens, the size of the filter must match the size of the lens. Most lenses will tell you what size they are: just look at the front (where the glass and threads are) and you should see a number for mm. This will be the right filter size for your camera.
Protect Your Lens From Scratches
The simplest filter you can get for your camera is a UV filter. This is a perfectly clear filter. So what's the point?
The point is that a UV filter costs about $ 20 to $ 50. A lens (even a cheap one) costs several hundred. If you scratch the glass on your lens, the whole entire lens must be discarded (unless you want a nice scratch running through all of your photos). If you scratch the glass on your UV filter that is protecting your lens, you just have to go get another one.
This is especially important for the SLR-like cameras that don't have interchangeable lenses. Should you scratch the glass on a lens like that, the whole entire camera has to be replaced (since you can't just swap out the lens). Not good.
Many people attach a UV filter to their lens and leave it on permanently. If it ever gets gunked up, you just have to remove it, clean it, and re-attach it. Pretty simple.
Purists argue that there should be nothing on your lens that doesn't have to be there. The argument is that anything attached to your lens distorts the image in some small way. Being clumsy, I prefer to play it safe and have UV filters attached to every lens I own.
Eliminate Glare and Reflection
If sunlight is reflecting off plant leaves and wet surfaces, you'll need a polarizer to cut through the reflections and tone things down. This is the primary use of a polarizing filter.
This filter is actually two filters stacked on top of one another. One filter attaches to you lens, and the other rotates. The great thing about a polarizing filter is that you can see how it reduces glare as you watch. You can look through your viewfinder and twist the front of the polarizing filter. Presto! Reflections will start to disappear.
Polarizing filters also have an interesting side effect, which can be overdone. When you are taking a photograph at a 90° to the sun (not at it or away from it) a polarizing filter turns the sky a deep, dark blue. While this can have a pleasing effect when you'd like the sky to look more intense (right after a storm or before one), you can also wind up with sky that is almost black.
If you look at enough landscape photographs, you will eventually be able to tell when the photographer used a polarizing filter to "boost" the sky.
Warm Up the Scene
Mother Nature is fickle. Sometimes the light is too bright and harsh. Other times it's not there at all. The day that you pick for you family reunion turns out gray and overcast. With the right filter, you're photos don't have to look bleak.
There are three common filters called "warming" filters, due to their ability to warm up the colors in a scene: 81A, 81B and 81C. The intensity of the warming effect increases with the letters (A is the least and C is the most).
These filters are most useful for portrait photography on days where the light is on the dull side. They give skin tones a golden hue, like soft afternoon light.
Please realize that a filter can only do so much. If you are trying to take photos of your spouse on a rainy cloudy day, not amount of filtering is going to make things look as if you're on a Hawaiian beach at sunset. But if your subject is standing in the shade and you want them to really stand out, warming up the colors can make a difference.
Reduce the light
Since a camera is designed to capture light, you might wonder why you would ever want to reduce it. Before we get too deep into this section, make sure that you are all caught up on your understanding of shutter speed.
Let's say that you are taking photographs in Yosemite Valley (as if no one else has done this before). You have found a stream with a small waterfall and want to use a slow shutter speed so that the water blurs. There's only one problem.
Your stream of choice is in direct sunlight. Even if you set your aperture to f32 (the smallest it can be), your shutter speed is still too fast to blur the water the way you want to.
Your only choice now is to actually reduce the amount of light getting into the lens. Enter the neutral density filter.
The only purpose of a neutral density (ND) filter is to cut back on the light. Neutral density filters appear as varying shades of gray. The darker the gray, the less light gets into the camera.
The amount of light reduction is measured in stops, like exposure. For example, you can get a filter that reduces the light by 1 stop (also called 0.3 exposure adjustment) or a filter that reduces the light by 2 stops (also called 0.6 exposure adjustment) or a filter that reduces the light by 3 stops, etc.
Graduated Neutral Density Filters
While this may sound like a complex technical component they place in an airplane to regulate airflow, it isn't.
A graduated neutral density filter is just a round filter where one half of the filter is clear and the other half is shaded (by any number of stops, just like a regular ND filter). The transition between the clear and the shaded is smooth — there is not a hard line in the middle of the filter where the clear stops and the shade begins.
So what's the point? There are occasions (rare, but they do happen) when you are taking a photograph of a scene where half of it is dark and half is light. Imagine a mountain overlooking a lake. Let's say that the mountain is in direct sunlight, but the lake is in shade. There is too much contrast in the image for your camera to handle. Without the filter, you have one of two options.
You can set the exposure for the moutain, but the lake will wind up looking black. You can choose to properly expose the lake, but the mountain will wind up overexposed. Either way, the scene is not going to look the same way your eye perceives it.
Now attach that graduated neutral density filter. Rotate it so that the clear part is over the lake (which is dark) and so that the shaded section is over the mountain (which is bright). This balances the wide range of contrast. You can now expose correctly for the lake without overexposing the mountain.