Filters: What you Need and Don't Need

Trees with Circular Polarizer Filter
(to remove reflections on leaves)

One of the great benefits of digital photography is that you no longer have to carry around a vast array of filters.  Digital cameras have the ability to let you set your white balance for way your scene is lit, and many issues can be solved in Photoshop better than with filters.  Those of you starting out in digital don't know how good  you have it.

I should begin by explaining what a filter is.  A filter is a thin piece of glass that is designed to modify the direction, amount or color of light entering the lens.  Usually this piece of glass is circular with threads on the edges so you can screw the filter right on the front of your lens.  This means that one filter will fit only one diameter lens.  Check the front of your lenses to find their sizes so that you can know what size filter to use with each lens you own.  I have a couple bits of advice about filters in general:

1. Because all filters add a layer of glass between your subject and sensor, you want to buy high quality filters--usually you will want coated filters that preserve the quality of light entering the lens.  These high quality filters are expensive, and they become more expensive the larger they are.  A 77mm filter will be much more expensive than a 58mm filter, even if they do the exact same thing.  But cheap filters will cause you to lose sharpness.
2. Buy filters for the largest lens you own, and use "step up rings" to use it on smaller lenses.  A step up ring simply has threads for one filter size on the back end and threads for a larger filter size on the front end.  That way you can screw larger filters onto smaller lenses.  This may interfere with your lens shade, but it will save you money and space in your camera bag.
3. You can stack filters together--filters of the same size will screw onto each other, which can make storing them easier if you don't mind having to undo the stack to get to the filter in the middle.  You can also stack them on your camera, but don't if you can can help it.
4. I don't understand the motivation to use a UV filter to "protect your lens."  These cheap filters do little if any good for your photograph.  I use a lens cap and a lens shade to protect my lens.  
5. On wide angle lenses (wider than 35mm), you may find that some filters cause "vignetting"--a darkening of the image in the corners of your image.  If it's not too bad this can be solved easily with software, but it's something to watch out for.  You can buy thinner filters to use with wide angle lenses to account for this.  If you are using a "crop sensor" camera with standard lenses made for full frame sensors, this will not be a problem, since the "crop sensor" effectively crops the outside of your image anyway.

I don't generally use filters that don't help my camera take pictures that look natural--that is, I don't personally care for filters that function like "tricks."  And to save space and money, I don't carry filters that do what I can do just as well or better with software.  But there a few filters that can be very useful.  These filters produce effects either 1) that can't be duplicated with software or 2) more effectively than software:

1. Circular Polarizer (CP) filter: Because of the great advances in software and camera technologies, there is only one filter that is simply essential for outdoor photography, and that's a circular polarizer (CP) filter.  A CP filter cuts down on reflections on leaves, flowers and, well, anything else that reflects light.  It also can give the sky a deeper blue color.  To save space here, I have posted another blog entry with the specifics of using this filter.  But it's a necessity in the field.
2. Neutral Density (ND) Filter: ND filters are simply neutral grey glass that cut down on light entering the lens.   This does not darken your photograph unless you're shooting in manual mode.  In aperture priority mode, this filter will slow your shutter speed.  If you want motion blur on waterfalls, grasses, etc, you may need a filter like this on more brightly lit days.  They come in varying strengths.  I have -1, -2 and -5 stop filters.  If  without a filter you have a shutter speed of 1/100sec, adding a -1 stop filter will make it 1/50sec. A -2 stop filter will make it 1/25sec.  A -5 stop filter will make your shutter speed 1/3sec.  As a rule of thumb, a CP filter can sometimes double as a -1 stop ND filter.
3. Split ND Filter: These filters can be helpful when shooting sunrises and sunsets.  A split ND filter is half grey and half clear.  This allows you to decrease the contrast caused by an extremely bright sky relative to the darker ground.  For these I do not recommend a screw on filter.  You can get filter holder that screws onto your lens.  Then you can use a rectangular split ND filter that you can slide up and down to match the horizon.  With HDR photography, this filter is unnecessary, but if you're shooting a flat horizon (such as the ocean), this approach can be faster than messing with HDR software. 
4. Infrared Filter:  My one exception to the general rule of using filters that help my photos look more natural, I love infrared (IR) photography.  IR filters cut out all light but infrared (IR) light.  These photos look strange and surreal, but I like the effect a lot.  Digital cameras all have an IR cut filter in front of the sensor that cuts out most IR light, so putting an IR Filter on your lens will cause you to have very slow shutter speeds.  When I replace my current camera, I'm hoping to convert it to an IR camera--that is, I want to replace the IR-cut filter with an IR filter.  This way it will only shoot IR photos.  One day...

Trees and Ferns with IR Filter
(the filter makes the leaves look near white)

Those are all the filters I use, and I honestly don't have a desire to use any others.  If you don't have a CP filter, put it on the top of your list.  You won't regret it.  The others you can get if/when you ever feel the need.