Neutral Density Filters - What, When, and How

ND - Neutral Density. For the beginning landscape photographer this term could just as well stand for Niggling Disorientation. The ND filter is possibly one of the more misunderstood accessory for photographers.

Looking at one for the first time, whether a split-graduated type or a solid one, many may vaguely understand what its purpose is, but few instinctively know how to use it and under what circumstances. Yet, the ND is one of the two filters I consider a “must have” in my photo bag, right there next to my circular polarizer. While this article might not be for the experienced outdoor photographer, I offer this information for those who are starting out in this field and need some solid and basic information on the description, use, and purpose of this very important filter.

 

WHAT ARE ND FILTERS?

A neutral density filter is designed to do one thing – reduce the transmission of light to the photo sensor. Think of it as a sunglass for camera lenses. The better quality filters will filter the intensity of the light without introducing color shifts or imposing a color cast – just like good quality sunglasses do versus cheap ones. The better ones are also carefully manufactured to minimize flaring and ghosting. By reducing the amount of light hitting the camera sensor the photographer is able to make changes to exposure values. This is done to achieve certain effects - such as to take advantage of reduced depth of field, by allowing wider aperture settings, or to photograph subjects in such a way as to project movement or fluidity through longer exposure times. A variation on the solid ND filter is the graduated ND filter, or ND grads. These serve a unique purpose and so are a little more specialized. They allow you to capture the full dynamic range of an outdoor scene within one single exposure. Since they have a clear section and an opposing filtered (darkened) section, they can compress the exposure values of the light areas and dark areas present so that the camera can record both highlights and shadows with a reasonable amount of image detail. A bright sky against a darker foreground is a good example. Neutral density filters come in several forms and configurations. I’ll group them into to three basic types.

Circular Screw-On Solid Neutral Density Filter

• The Solid ND

These filters come in two shapes, the more common circular, screw-on type (just like your polarizer), and the square/rectangular shape. Both accomplish the same thing; they are just designed to attach to the front of your lens differently. Is one better than the other? The end result is the same, but as I’ll point out later, I believe each is better suited based on one’s shooting preferences and budget. The solid ND also comes in varying strengths. The strength of an ND is typically measured in f-stops. The strength of these filters usually range from 1 to 15 f-stops of filtration.

• The Variable ND

This filter is only made in the circular design and screws on to the front of your lens. It has a rotating bezel much like a circular polarizer. As the name implies, these ND’s are designed to give you a varying range of density from about 2 to 8 f-stops of filtration in one package. A great concept, especially if you have limited space in which to carry your gear. While very popular, they do have some limitations, and they also tend to be more expensive. These are not your usual ND filters – they do not use density dyes to darken the filter material. They use opposing polarizing filters, that when rotated on the same axis increase or decrease the light stopping capacity of the filter.

• The Graduated (or Split) ND

The graduated ND filter also comes in both circular screw-on and rectangular types. I’m going to stop here and make a rare, absolute statement. Circular graduated neutral density filters are absolute rubbish. Do not buy one. If you own one, sell it to your worst enemy. Get that pointless thing, designed by evil elves from somewhere south of the North Pole, out of your bag once and for all. Okay, I feel much better. ND grads, as they are sometimes called, also come in varying strengths - from 1 to 5 f-stops. Some makers even offer what is called a reverse ND grad. This unique filter is used when shooting into the sun at sunrise/sunset. The filter has the usual density transition except that it is darker at the center and then fades slightly toward the top. This will give you more filtration where the sun is hanging over the horizon without making the upper sky too dark.

WHEN TO USE ND FILTERS

Blurred Passing Subjects Using ND Filter

Although ND filters are certainly not relegated to use only outdoors, I can safely say I can’t think of a single time I’ve ever used one indoors or under controlled lighting. So, in this article, I’ll focus on outdoor use only. Let’s start with the solid ND.

The ability to use the passage of time, motion, and light to stir emotion and project a message or mood is what I think makes photography a true art. It gives the photographer the ability to present imagery that escapes the bounds of the static capture, going beyond the documentarian style that can at times be limiting, stilted, and lifeless. With a solid ND filter at your disposal you can take a picture of a moving object and use effect to record that movement across the frame. Let’s say you are photographing a bicycle race and you are fortunate to be standing near the road or track. It is sunny outside and the light is so strong that you can’t set the camera exposure so that you can use a shutter speed that is slow enough to record the motion of the passing cyclist. You want to blur him to represent motion and without a filter to hold back the light you won’t be able to get this type of image. But if you attach an ND filter to the lens you can effectively reduce the light to the point where you can now utilize the slower shutter speeds that give you some blur. This usually works better, from an artistic perspective, than a static image where the cyclist seems to be frozen in time; just sitting on his bicycle with the tires stopped and no motion in his legs as he pedals on past you. With the ND filter you can represent the cyclist as a blurred, ghostly object, while the roadway and the background are frozen with sharp details. Or you can reverse this play on motion and pan your camera keeping the cyclist in the middle of the frame as you shoot. With the ND filter the motion effect would show the cyclist relatively sharp while the background would almost be a complete blur – again representing motion in a very effective way.

Using an ND Filter to Soften Cascading Water

Another very popular use of the solid ND filter is to record moving water, like a stream or the surf of the ocean, with a slow shutter speed. Some people don’t care for this effect, but I find this to be one of my favorite techniques when shooting water. The slow shutter speed that is, again, provided by the light filtering capability of the ND filter will produce the moving water as a misty, ethereal , almost cloud-like feature. To me, it has a very soothing effect. In my view recording moving water as a frozen mass of churning froth just isn’t something that gives me a calming effect. I almost don’t desire to be there. By contrast, when I see a beach scene with some large rocks where the surf is coming in and out and it is recorded as a misty veil of softness amid the hard boulders I immediately want to be there.

A third way that a lot of outdoor photographers use to great effect is to record fast moving clouds with the ND filter. This is similar to the water effect except that in this case you have a sharp and detailed foreground with clouds that take on a rushing motion across the frame.

Finally, an ND filter can be used to access wider apertures rather than to lengthen shutter speeds in the exposure equation. Wide apertures (smaller f-stop numbers) will give you less depth of field. This is a desirable feature when you want to isolate a subject by blurring the foreground and background. The Japanese have a name for this out-of-focus effect; it’s called “bokeh”. A single flower isolated in a close-up photograph with the background well out of focus giving a creamy background is a good example of this technique.

Using the Graduated ND Filter

Now let’s move on to the graduated ND filter. This filter has a specialized purpose and is rarely used outside of landscape photography. Simply put, this rectangular filter is used to hold back the light intensity in the sky so that you can set an exposure value that will capture details in the darker terrain or foreground. It really is quite a simple filter to use but the key to success is placing the transition of this split filter so that it is hidden in the scene. This takes a little practice. The other important issue with this type of filter is choosing the correct strength so that the overall exposure remains balanced, a true and natural looking interpretation of the scene you are photographing. I’ve seen many images where the filter is not placed precisely and it either darkens the upper region of the foreground, or it is placed too high in the scene resulting in an unnatural bright halo just above the horizon. Also, if you don’t have the correct amount of density you can get weird looking results. If the filter is not dark enough for the situation, or by contrast, you might have a filter that is too strong the sky will just not look right. For this very reason, and in order to use these filters effectively, you must have a set of filters spanning several densities. There are also two types of transitions – a hard transition for use when the horizon is well defined and level across the frame, and a soft transition where the terrain’s edge is jagged, like a mountain range in the distance. Here is a good place to explain my little rant in the previous section about ditching your circular graduated ND filter. These filters have the transition from dark to light right in the middle of the filter. This means that the only possible way to use this filter is to frame your shot where the horizon runs exactly in the middle of the frame. In almost all cases and situations you want to avoid this placement of the horizon. It is one of the most unpleasing compositions I can think of. Good landscape photography depends on compositional decisions that create pleasing imbalances between light and dark often referred to as “tension” or “positive versus negative space”. Placing the horizon smack-dab in the middle of the frame will never give you this tension. So, like I said, take that circular graduated ND filter and run over it with your car. Many photographers, including myself, often stack two or three, rectangular ND grads to get the strength of filtration we need. This is another big advantage of using rectangular filters.

HOW TO USE ND FILTERS

Using the Reverse ND Grad Filter

Since using neutral density filters usually means you’ll be shooting at slow shutter speeds it is imperative that you shoot from a solid tripod. The exception to this is if you are trying to record action or the movement of fast moving subjects. In this situation you’ll want to hand-hold the camera, and you’ll probably be shooting at no slower than 1/60 of a second. This is still a manageable shutter speed when hand-holding the camera.

Let’s first go through good technique using neutral density filters with a tripod. When using a solid ND filter you’ll have to prepare the camera for the shot before attaching the filter. I find that the more useful filters are those rated at 3-stops and above. At these densities it is almost impossible to see through the viewfinder with the filter on. So focusing, composing, and zooming is a futile effort with the filter attached to the lens. My recommendation is to set up the camera as you usually do using your tripod. Once you have everything perfect and the image in the viewfinder is focused correctly use your camera light meter to give you the correct exposure. You can either use the Manual mode or Aperture Priority mode. With Manual mode simply increase the exposure by 3-stops by adjusting the shutter speed. If using Aperture Priority you set the aperture and adjust the shutter speed to compensate for the ND filter by adding exposure compensation using the Exposure Compensation controls in the camera. If you are using a 3-stop filter simply dial in 3-stops of exposure compensation. With darker filters beyond 5-stops you’ll have to switch to Manual mode and adjust the meter by using the command dial to lower the shutter speed. This is because exposure compensation is usually limited to +/- 5-stops on most DSLR cameras. Once you have biased the exposure to accommodate the ND filter you then screw on the filter very carefully so as not to move the focus of the lens, the camera position, or the focal length setting - if using a zoom. There is one other thing to consider when using the ND filter. If your exposure turns out to be longer than 30 seconds (and this is a real possibility) you’ll need to time the exposure manually using your watch. Or preferably, use a remote shutter release with an intervalometer that has the capability to time the exposure for you and automatically close the shutter when the time expires.

Attaching a screw-on filter is actually a little tricky, especially when you are trying not to move the camera or the lens settings. Add to that; let’s say you are standing in a beautiful stream. You’ve found the perfect cascade to try your flowing water technique, but there are large river stones all around you - a filter’s worst enemy. The light is perfect at the moment but could change any second so you are trying to work as quickly as possible. Suddenly you’ve grown bear paws and you can’t get the darn filter screwed on. If you accidently drop the filter it is sure to shatter when it hits those rocks. And now, with your mind racing you start thinking, “How does that saying go; lefty – loosey, righty – tighty?” Okay, okay. But is that when you are facing the threads or behind them? This is why I prefer to use rectangular filters using a filter holder. These filters can be a bit more expensive, but in my eyes, well worth it. Dealing with a filter that simply slides into an already attached holder is so much easier and faster. I personally use a holder where you can detach the holder from the lens ring attachment for even easier on-and-off action. It simply snaps in place and to remove it you just tug lightly on a release pin and off it comes - with filter and all. Great!

Using ND grads is a little more complicated than simply attaching the filter, adjusting the exposure, and banging away. As I noted earlier there is the need to precisely place the transition of the filter on the horizon so that it becomes virtually undetectable. It also helps when you have the exact amount of density so that you don’t get a sky that is too dark or light. There are several ways to make this placement accurately. One is to use the viewfinder and the other is to use live view on the rear LCD of your camera (if it has it). The former method also requires that you have a DOF (depth of field) preview button to stop down the lens. Stopping down the lens makes it easier to see the transition in the viewfinder. Here’s how to do it using the viewfinder and your DOF Preview button.

Set your camera’s metering mode to Matrix. This mode is the easiest way to get the correct exposure without having to spot meter the sky and the ground to get readings of both exposure values. You can set your camera’s exposure mode to your preferred setting although I recommend Aperture Priority or Manual mode. Once the camera is set to your liking and you have the composition you want, place the ND grad filter in the holder. Now looking through the viewfinder push down the DOF Preview button and move the filter up and down until you notice the transition of the filter. It should be darker at the top than the bottom. Now simply place the filter transition as best you can so that it lines up exactly with the horizon. If using Manual mode adjust the exposure to “0” on the meter. If you are in Aperture Priority you should be good to go. Since you are using Matrix metering the camera’s on-board processor and database of exposure information does all the work for you. You can now take the shot.

Using the rear LCD with live view is pretty similar. In this case you simply activate live view and do the same procedure using the rear LCD as your viewing apparatus. In these brief instructions I am purposely leaving out one important element. For those who usually think ahead of the narrator you’ve probably already figured it out. “How do I know which strength of grad to use and whether I should use a hard transition filter or a soft one?” The answer is – As a beginner just start with the filter in your kit with the least amount of density, take a shot, evaluate using the rear LCD and histogram, and decide if you need more filtration. With time and experience you will be able to make somewhat accurate guesses which filter will work best. Even then you’ll still have to rely on test shots until to nail it. The same goes for whether to use a hard or soft transition filter. Certainly, if you’ve got a straight, even horizon it’s pretty obvious the hard step is going to be your best choice. Back in the old days shooting film you didn’t have this luxury. You had to actually take meter readings of the sky and the ground and calculate the difference in exposure and then select the correct filter strength. To compound this most photographers were using color slide film which has very narrow exposure latitude. This also impacted your exposure decisions since you sometimes had to compromise on the exposure. Do you want to sacrifice some shadow detail to make sure you don’t blow out the highlights? It was rough back then, thank goodness for digital.

Remember when I mentioned stacking ND grads? This technique leads me to another way of using your grads that is especially effective when you have a lot of objects protruding out from the horizon into the sky. Neither a hard step nor a soft step will do the job effectively in these cases. A lot of photographers opt here to throw in the towel and just shoot two or more exposures to cover the range of exposure and simply blend them digitally on their computer at home. That is perfectly acceptable, and I do it as well, but you can still get it done with your “old school” filters.

I’m talking about hand-holding ND grads. I do this frequently and find it somewhat more convenient than having to deal with a filter holder. It also gives me an easy way to stack filters and to use a technique to make the soft transition filters more useful when confronted with those uneven horizon situations. I’ll take my filter, gripping it at the tip of the lower edge, and hold it over the lens. It takes some getting used to and frankly it is a lot easier if you have the larger 4x6 inch filters since these larger ones keep your fingertips out of the frame. Then before I release the shutter I’ll begin to move the filter up and down rather quickly in front of the lens. It doesn’t take much movement, just a fraction of an inch up and down in a steady motion. Once I have a smooth motion going I’ll take the shot. This up and down movement, however slight, as the shutter is open, is enough to eliminate the visible transition of the filter in the final image. I know it sounds a little odd but it works. I have found that when using this technique you tend to lose some amount of filtration so I recommend going with a filter that is one or two stops denser than you would use otherwise.

I hope I’ve covered all the bases in this article. It should be useful to you in helping you to understand neutral density filters and give you the basics in their use. There is nothing quite like using a solid ND to make some magical images. The fun part is that it can’t be simulated in post processing. On the other hand, digital cameras have incredible exposure ranges these days and some would say (rightly so) that graduated neutral density filters will soon be made obsolete. They just might, but I tend to believe they still have their place in the photo bag. When used properly there still is a lot to be said about getting the correct exposure in one frame. And the results, when done right, can be stunning.