The world of filters is expansive and overwhelming - and I'm not talking about Instagram or Lightroom filters, I'm talking about real pieces of glass that you put in front of your lens.  Filters are a mystery to a lot of photographers I know, but they're fantastic tools that help create the best landscape photographs.  In this post, we'll cover just three of the essentials: neutral density, graduated neutral density, and circular polarizing filters.

Neutral Density Filters

The first - and typically most confusing - filter to cover is the neutral density (ND) filter.  ND filters are dark-tinted pieces of optical glass or resin (depending on manufacturer and price).  They are used to reduce the amount of light that enters the lens, and this amount of light reduction is graded in terms of "stops."  Stops are measured in halves, or doubles, which makes the math fairly easy.  Let's say you're taking a photo at a shutter speed of 1 second, and you decide to add a 3-stop ND filter.  Here's the math:

1 second x2 (1 stop) x2 (2 stops) x2 (3 stops) = 8 seconds

At an original shutter speed of 1 second, a 3-stop filter will increase your shutter speed to 8 seconds - provided you don't change any other settings, of course.  Similarly, a 10-stop ND filter will change a 1/100 second shutter speed to 10 seconds.  It does get difficult to keep track of, and luckily there are a number of exposure calculating apps available to make these conversions easy.

The obvious question is, why would we want to stop light from entering the lens?  In landscape photography, long exposures are utilized mostly for manipulating movement.  Think of what would happen when a moving element that is usually still-captured (like an ocean wave, waterfall, or a cloud) is allowed to move through time while the camera sensor is recording.  The resulting image will have some sort of stretch or smoothness to it.  Below is a comparison between a 1/6s exposure and a 171s exposure (check the math - that's using a 10-stop ND).  You can see that the ripples in the water are no longer visible, and the clouds have created a zooming effect as they moved through the sky.


While dark filters like a 10-stop ND often completely erase detail in water, other filters such as 6-stop or 3-stop NDs may capture more detailed movement.  Consider the idea of using a less aggressive ND filter for a waterfall or incoming tide:


Cold Brook Falls in New Hampshire
A 3-stop ND filter allowed me to use a shutter speed of 3.2s to smooth this flowing waterfall, while retaining some of the water's detail.
Sunset at winthrop beach in massachusetts
The same 3-stop ND filter gave me the 0.8s shutter speed I needed to freeze the incoming tide with just enough motion blur to add some drama to the scene.

Incorporating movement into your photographs can add layers of intrigue and expression to your work.  But, remember that long exposures record everything that moves, not just the elements you want.  If you're shooting an image with trees, flowers, grass, or anything else that moves, you'll have a bad day when you try out a 10-stop ND filter on a windy day - those things will be blurry.  Don't let that deter you, though, because practice makes perfect and long exposures are well worth the learning process!

Graduated Neutral Density Filters

Perhaps an even more "life-changing" filter is the graduated neutral density filter (grad-ND).  If you're looking for the quickest and easiest way to balance your exposures, look no further than the grad-ND.  To the human eye, a sky and a foreground may look uniformly bright.  But as you probably know, our visual system is capable of interpreting light in a vastly better dynamic range than a camera sensor.  This is the reason why some people love HDR processing - it is purported to create an image that simulates what the human eye can see (unless you overdo it, but we won't go there...).  An better alternative to HDR is the use of a grad-ND filter, which uses a gradient to darken a portion of your photograph.  I commonly use this type of filter to balance a bright sky with my darker foreground.  My go-to filter is a 3-stop soft grad-ND, which I find balances exposures nicely without leaving a harsh edge.  Below is an example of a photograph before and after a 3-stop soft grad-ND.


It's easy to see that without the use of the filter, the sky would have been completely blown out.  Take a look at the next photo, where I used a hard-edge grad-ND (and tilted it slightly so you can see its effect).  While hard-edge grads work nicely on a straight horizon, they can ruin an image by giving an awkward dark line at the filter gradient.

You may be wondering why it's wise to spend money on a grad-ND when you can just apply a negative exposure gradient filter to your sky in Lightroom or Photoshop.  To be perfectly honest, that sometimes works, but you run a few risks.  If you don't know how to use the histogram, you'll probably blow your highlights permanently lose detail in the sky.  Some people like to make sure they don't clip their highlights, and then they just bring up their shadows.  This creates a ton of luminance and color noise, reducing sharpness and introducing halos to contrasting edges.  We'll cover much more about these concepts in my next series called The Digital Darkroom, but for now I'll finish this topic by saying DO NOT rely on post-processing gradients to balance your exposures.  Save your image quality and invest in a grad-ND!

Circular Polarizing Filters

When light hits a reflective surface, individual rays reflect in different directions.  This causes the bright spots you're familiar with when sunlight hits a car window at just the right angle.  Though you can't see the color/detail below the reflection, it does obviously still exist.  To quickly summarize, circular polarizers (CPLs) are filters that only allow light rays moving in a certain direction to pass through, and they subsequently cut down reflections to bring back some of that detail your eyes can't see.  Alternatively, CPLs give you the ability to control reflection and use it to your advantage.  The filter can be rotated to achieve the desired effect, and can be found in round or square versions.

Common uses of a CPL include reducing glare (such as on water or glass) and increasing cloud detail.  You may not think of clouds as reflective objects, but they are clusters of water droplets, which are highly reflective.  It's incredible to see the difference in cloud texture when you rotate a CPL through its positions.

Below are two photographs of my favorite abandoned house.  On the left, I used my CPL to cut out the glare of the windows because I wanted the sun to come through the large front window, and the color of the sky to come through the cupola.  Hint: I also used my 10-stop ND and 3-stop grad-ND to achieve a long exposure.  On the right, I used my CPL to improve the reflection of the sun off the house's three upstairs windows.

pink house on plum island at sunrise pink house on plum island at sunset

Filter Holders

There are endless brands and types of filters out there.  In my opinion, it's not worth spending money on circular filters that screw on to the front of your lens.  These are not usually best suited for landscape photography (a better use would be adding a 1-stop screw-on filter to a 70-200mm f/2.8 lens when a wedding photographer wants to maintain a f/2.8 aperture on an extremely sunny day).  Instead, I recommend you purchase the three filters mentioned as square filters (or rectangular, depending on the brand).  You'll also need a filter holder, but the advantage is that you can stack filters - you can add a 3-stop grad-ND to a 10-stop ND so your long exposure also has a balanced sky.

The two brands I recommend are Cokin and LEE.  Cokin makes two lines of filters for consumers and professionals.  The consumer line, the P-series, is affordable but provides significantly better image quality than any other similarly-priced filters I've tried.  The Z-Pro series is far more expensive, but the quality is worth the cost.  On the other hand, Lee Filters are the industry standard, and they offer a seemingly endless catalog - but be prepared to pay for the name!  For convenience, I've linked to each brand's respective filter holder.  From there, you can find the filters that suit your personal needs.

Do you use filters?  Are you going to start learning?  Ask me any questions in the comments, and feel free to share some photos as you experiment!

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