Neutral Density (ND) filters are a bit like sunglasses for your lens. Except that sunglasses typically tint the light coming through in one form or another. As true color rendition is often important to photographers, neutral density filters strive to lessen the amount of light passing through them without coloring or changing the nature of the light.
The effects of using these filters can be drastic. Some block out almost all light and thus allow for results that weren't previously possible. Others only block a portion of the light, allowing individual expression of what is found in front of the camera. All filters help in a way that is currently superior to standard computer manipulation techniques as they help preserve the data being recording in the camera, as compared to post-production which is often attempting to recover data that may not exist when a filter is not used.
There are four basic types of neutral density filters we will go over in this article: standard, variable, graduated and reverse graduated. All have a preferable time and use, resulting in different effects on the end product.
Standard Neutral Density
A standard neutral density filter is just that, standard. They come in either round versions that screw right onto a lens' threads or in a square/rectangular version to work with filter holders. As neutral density filters are the same darkness across their surface, screw-in versions are often popular for their simplicity of use.
Neutral density filters come in a variety of darknesses as measured by standard stops. A stop is the halving or doubling of the amount of light available and these filters list their potency to block light in either the fashion “1 stop" or 0.3 EV" or “"x2". As the filter numbers increase, the filter becomes darker and darker by a predictable amount, resulting in the need to adjust either shutter speed, ISO or aperture to compensate for a properly exposed image. Most often shutter speed is adjusted to help increase motion blur in an image.
While these three means of listing filters look different, they are all conveying the same thing. The “x2" or sometimes “2x" filter is halving the shutter speed. (e.g. 1/60 to 1/30) 4x will be quartering the shutter speed (e.g. 1/60 to 1/15) and so on. This directly relates to the number of stops needed to compensate for the lessening of the light available. The “.3" marking is simply a measure of the filters optical density and for each .3 increase, the amount of light decreases one stop.
Standard ND filters can be used in a variety of situations. For instance, shooting a waterfall in full sun can often result in a shutter speed only dropping as far as 1/30 of a second. This is not slow enough to get the veil action in the water many people desire to show movement. If a ND filter were to offer three stops less light, this could make the shutter speed 1/4 of a second, giving the water ample time to blur in motion.
There are also times in bright situations when wide aperture is desired but the shutter speed of the camera is already maxed out. In these case, when shallow depth of field is the ideal, two stops less light will be the key to shooting as desired. Lastly, with longer shutter speeds comes the ability to blur objects out of existence. I will show this in the Variable ND section but it can be achieved with any ND filter that blocks sufficient light so as to allow for very faint blur trails.
Variable Neutral Density
Taking the neutral density filter one step further, variable neutral density (VND) filters are an option for those seeking even more latitude. The advantage of these filters is the ability to cover a range of darkening, up to seven stops in some cases. The disadvantage is typically the cost and possibly the starting point in terms of stops.
Variable neutral density filters, by their nature, need to be circular. They function by having two layers with very small perforations and when juxtaposed with each other can allow or deny light to pass through. Think of stacking two screen on top of the each other. Requiring the filters to be circular often creates other limits. For instance, very wide angle lenses will start to show noted vignetting depending on the depth of the filter. Also, fisheye lenses will not work with this filter as a flat front element is needed.
The main advantage of the variable ND filter is the ability to dial in just the right amount of light for a desired effect. Let me demonstrate with the help of a variable ND lent to me by Singh-Ray, which has a range of 2 1/3 stops to 8 stops of density (darkening).
This first image is taken without a filter at the slowest shutter speed. Settings of f/13 and ISO 100 would allow 1/25 of a second.
Now, with the variable ND attached, the shutter speed drops to 1/5 when dialed to “minimum" mark on the side of the filter.
Notice the white streak in the foreground? That is the blurring of a car in the near lane. Much fainter than the truck in the first picture. The exposure has been reduced 2 1/3 stops. Now to move the VND to just before “maximum" and the result is shown below.
This shot has a shutter speed of 3.2 second and, unless you are looking carefully, you will not notice the very faint streaks of a car in the near lane. The streaks can be seen just above the guardrail in front of the tree on the right. Lastly, with the VND on maximum, a shutter speed of 10 seconds is achieved and no cars are visible, as well as a greater streaking to the steam from the stack in the distance.
Graduated Neutral Density
Graduated Neutral Density (GND) filters work well in a number of circumstances. The idea with a GND is to transition from an area of dark to light in a smooth manner. The amount of stops at the top of the filter varies from typically one stop to four stops and the transition can be either hard (nearly a solid line) to soft (fuzzier). The filters can be circular, square or rectangular and sizes will vary so they may be used with larger size lenses.
Often a GND is used to hold back a section of the frame that is brightly lit, like the sky. To our human eyes, which can see about 14 stops of light at one time, a range from brightly lit hills to darker valleys that are hard to make out. But modern cameras can only register 8-9 stops of light. In situations where the sky is bright and foreground is dark, the filter is held up to bring the sky (or mountains or any bright object) down closer the exposure of the other elements. This can help in evening scenes to a get more in line with the camera's over all dynamic range.
By way of example, here are two shots of the same beach. I wanted to make sure the driftwood in the foreground was not too underexposed so as to be unusable while editing later. This shot was taken on a Canon 7D with an EF 10mm-22mm lens, ISO 100, 18mm, f/16, 1/50th of a second.
Notice the atomic sun? It's hard to miss and ruins the shot. A look at the histogram, however, shows that the shadows do not clip, which is what I was planning on.
While the highlights clip, as shown with the white triangle in the upper right corner, (and there is far more information to the right that can't be seen because of the clipping) the shadow detail is fine and within the sensor's dynamic range. To bring the exposure of the sun down, I place a two stop GND so the soft transition ends where the foreground driftwood begins.
And the corresponding histogram.
The GND pulled a lot of the highlight information to the left. While there is now a bit of shadow clipping (mostly due to the silhouettes), far more information is retained for a better start in editing.
One key to GND filters (as well as the Reverse Graduated Neutral Density Filters) is to meter without the filter in place. Also, meter for the foreground in most cases and pick a filter that will bring the highlights down into range befitting of the sensor. Otherwise the camera will take the filtered area into consideration while metering and an overexposed picture will typically occur.
Reverse Graduated Neutral Density
Last is the Reverse Graduated Neutral Density Filter (RGND). This filter has a very special application, although with imagination, more can exist. Its primary function is to be used at sunrise and sunset in order to block out the intense light along the horizon, but then quickly transition lower to allow the foreground mountain or ocean or object which the sun is setting over to be seen. From the darkest point and up, the filter graduates to about one stop less near the top in order to hold the sky back.
Without a RGND filter, and just using a GND filter, the sun would be brighter than the sky above it, which would be held back with the GND. The RGND allows the sun itself to be held back, typically three stops. Again, metering for the foreground is what works best. Here is an example.
The shots have nearly identical exposures (the one on the right is only 1/3 of a stop darker than the left) and as can be seen in the histograms. The effect is more retained data with only the yellow highlights becoming slightly clipped.
Neutral Density filters of different types have varied and useful purposes when it comes to digital photography. Because of the limited dynamic range of most sensors, the filters help in a variety of situations where there is no better option than to tone down the brightness of a scene to accomplish what is needed. Be it straight ND or Variable, Graduated or Reverse Graduated, investing in ND filters will help advance most photographers' abilities to present beautiful reproductions of the world around us.
If you've used a Neutral Density filter before, please post links to your results below in the comments. Share any tips or tricks you've picked up along the way as well.