The popularity of this filter has grown significantly over the past several years as more landscape and outdoor photographers begin to realize just how effective it can be in improving photographs taken under certain lighting conditions. By understanding how to best use this filter, you can adopt it as one of your shooting techniques to dramatically improve your outdoor and landscape photographs.


The Problem: Contrasty Light

Excessive contrast of the light in a scene can be one of your most difficult problems to overcome in exposing a successful landscape photograph. If overlooked, it can cause a sky which appears colorfully detailed to your eye to be colorless and washed-out in the final photograph. Certain contrasty subjects, such as "god beams" streaking through the clouds, if uncorrected, will not even appear in your photograph. But if the exposure is properly corrected as discussed below, they appear as you see them, with color and detail.

One way to limit the negative effect of excessive contrast between the sky and foreground in a landscape photograph is to shoot in the early morning and late afternoon, or on overcast days. These shooting conditions may allow you to capture scenes without the excess contrast destroying your desired effect. Even in these situations, however, the contrast may still be too great to allow you to capture detail in both the sky and foreground.

This problem is especially acute when you shoot transparency film; the type of film preferred by almost all professionals and publishers. Transparency film has an effective exposure latitude of approximately five-stops of light, going from black to white (while still retaining some detail in the black and white areas). In terms of the Zone System, this range involves Zone III (darkest) to Zone VII (lightest). As an example, if a scene contains a bright highlight (Zone VII) which you determine has a correct exposure of 1/2 second at f/22, any shadow areas which are four-stops below this exposure will be recorded as black on the final transparency (assuming the exposure is made for the highlight). The illustration shows the top of a Pentax spot meter with a Zone System label. Zone V is the proper exposure for a "medium" tone subject (18% gray), or at least what you want to appear as a medium tone in your final transparency. So if an exposure is set for Zone V, only those objects within the range of Zone III and Zone VII will be within the exposure latitude of the film. Objects at Zone I or II will be black and those at Zone VIII or IX will be white (or clear).

The most common excessive contrast problem faced by landscape photographers involves a sky which is more than two-stops brighter than the desired medium tone (Zone V) in the foreground. Where the sky is more than two-stops brighter, if you expose the photograph for the foreground, the sky will appear colorless and washed-out. If you expose for the sky, the foreground will be black and lack any significant detail. Neither exposure setting produces the desired result of keeping the sky and foreground within the five Zones.

The Solution

There is a simple solution for this common problem in landscape photography, the use of a split neutral-density filter. Although its name is long, the concept is simple: a filter where the top half is a neutral-density filter and the bottom half is a clear filter. If you look through a split ND filter, the top half is dark, it gradually turns lighter and finally becomes clear from halfway down to the bottom. The reference to "neutral density" indicates that the filtration neither adds nor subtracts from any of the naturally occurring color. The only effect the filter has is in decreasing the amount of light which passes through the ND portion of the filter.

The illustrations show the exact same scene taken within one minute of each other at the same exposure. The first is without any filter. The second has stacked a 2-stop and a 3-stop split ND filters, for an effective 5-stop split ND filter. I don't recommend this, but it dramatically shows the effect of the split ND filter!

Types of Split ND Filters

There are two basic types of split ND filters. The first, and least expensive, is a circular filter which attaches to the front of your lens and is divided equally at the middle into the ND and clear portions. The mount containing the filter allows you to rotate the filter so the ND portion can be placed where needed. The second type is a square or rectangular glass or resin filter, generally 3"x3" or 4"x5", which you slide into a holder attached to the front of your lens. This holder allows you to not only rotate the filter, but also to move it up and down. The additional movement provided by this holder allows for more critical placement of the ND portion of the filter, including using only part of it. Most professionals prefer this system.

Both types of filters can be purchased in a variety of densities, but are generally found in one of the following: 1-stop (ND.3), 2-stops (ND.6) and 3-stops (ND.9). The three different densities provide various amounts of ND effect depending upon the strength of the filter (the strongest being the 3-stops variety). Most landscape photographers prefer the 2-stops filter since it covers most photographic situations.

You can also purchase filters which are split 40% ND and 60% clear. Some photographers prefer these since they tend to divide their landscape photographs with 40% sky and 60% foreground. In addition, you can purchase filters having either a "feathered" or "hard" edge. The feathered edge provides a gradual change from the ND portion to the clear portion while the hard edge provides an instantaneous change. Both types have there merits depending upon the situation, however the feathered edge is the most popular with landscape photographers.

Using the Filter

Most filters do not require any "technique," you just attach it to your lens and shot. The split ND does requires a little more thought and technique, but once mastered it is just as easy to use as any other filter. The technique required in using the split ND involves the placement of the "edge line" (the graduated area between the ND and clear portions of the filter).

In order to obtain the best results with this filter, you will need to learn to recognize situations where its use will improve your photograph. These situations will involve your desire to photograph a scene where the contrast ratio exceeds the range of your film. Typical of this type of scene is a sunrise or sunset where you want to maintain the saturated orange color of the clouds and sky, and also retain the detail in the trees in the foreground which are in the shadows. You might also be faced with the situation where a sandy beach is in the foreground and is backed by a darker area of grasses. In the first situation you would use the ND portion of the filter at the top of the image to reduce the brightness of the sky, in the second situation, however, you would use it at the bottom to reduce the brightness of the sand.

The split ND filter will be particularly useful in scenes which have a dark horizontal or diagonal line or area somewhere across the length of the photograph between the two contrast extremes. One example is the common landscape scene where this dark line exists in the area of the photograph where the sky converges with the ground. A dark line is important since it provides the area needed to "hide" the edge line. If you fail to hide the edge line, or place it over a continuous tone area such as the sky or sand, the edge may be visible in your final photograph. If the edge line is visible, the photograph looks unusual and unnatural, distracting the viewer.

Just because a horizon line fails to exist in your composed image, it should not limit your ability to use the filter. In fact, situations where you can use the filter most effectively are those which allow you to place the edge line on a diagonal. As an example, if you visual the face of a clock, the diagonal edge line in the image might run from the two o'clock position to the eight o'clock position. Important Tip --> The technique of placing the edge line at a diagonal is very helpful and should be used even when there is a distinct horizon line. By placing the edge line at a slight diagonal, you will make it significantly less noticeable since the "minds' eye" tends to perceive horizontal and vertical lines fairly well, but does poorly in perceiving diagonal lines.

Correctly placing the edge line is more difficult when you use a circular filter rather than a square or rectangular one. This is because the circular filters are designed with an edge line which falls across the middle of the filter (always splitting the image in half) which limits your ability to compose the scene and still hide the edge. If you purchase the more expensive square or rectangular filter, which is held in place by a filter holder, you will not face this problem because it allows you to place the edge line almost anywhere, thus freeing you to compose without limitation.

Important Tip --> Two of the most frequent mistakes made by photographers in their initial use of the split ND filter is placing the edge line at the "wrong" horizontal line in the image or composing a scene where the edge line dissects a vertical object which protrudes into the upper portion of the frame, such as a large tree.

The illustration shows the "wrong" and "right" places to align the edge line. Can you tell where the edge line was placed in the illustration? It was in the wrong place. How can you tell? Look at the forest between the river bed and the mountain top. Had the edge line been placed correctly, the forest would not have been exposed as black with little detail.

Where you split a tree, the final photograph may look unnatural since the bottom of tree trunk will be much lighter than the top of the tree, even though both parts of the tree are in the same light. Cutting a vertical object in this manner will also make the edge line very obvious. Try to avoid making this mistake by not putting large vertical objects in your images when using the split ND.

After you learn the techniques described above, the placement of the edge line is rather easy since in most situations you will be able to see it while viewing the landscape image in your viewfinder. This may not, however, always be the case. Important Tip --> If you are using large format equipment or viewing through a slower lens (i.e., f/3.5 or slower), you may have to use one or more of the following techniques:

(1) while looking through the camera, "rock" the filter back and forth until you can see the edge line moving, and then place the edge in the proper place;

(2) while rocking the filter, stop the lens down slowly (this will make the edge line more prominent);

(3) take a piece of paper or business card and line it up with the edge line and then rock the filter (be carefule not to get dirt on the filter or scratch it!); or

(4) by looking at the front of the camera, rotating the filter to the proper position (this is generally the best method for predawn and post-sunset images).

Now that you have learned the basic rules about "correctly" placing the edge line, break them! You should experiment with other placements to see how they affect your images. Some of my favorite photographs taken with the split ND break the so called rules. But don't break the rules as indicated in the illustration! Actually, this is good illustration to show you what the sky looked like with and without a 3-stop split ND filter. Had the edge line been placed correctly, the sky would have looked like the top right and the forest would have looked like the left bottom.


Exposure

Setting your exposure using the split ND filter is no different than setting it without the filter. If you have a meter built into your camera, you can start with the exposure setting suggested by the camera, compensating and bracketing as you would normally do for the subject matter and lighting situation. If you are using a spot meter, you can either meter the highlight area and compensate for the strength of the filter (i.e., open 1-1/2 to two stops for the ND.6) or meter the darker foreground and set your exposure for it, again compensating and bracketing as you would normally do for the subject matter and lighting situation. When using transparency film, the preferred method is to meter the highlight and adjust for the filter factor. This method will help ensure that any important highlights will not be washed-out.

Ethical Use of Split Color Filters

Although I highly recommend the use of the split ND filter, I discourage the use of split filters which provide "coloration" to the scene and modify the natural color. The most typical of these are referred to as "sunset" or "tobacco" colored filters. They work much like the split ND, except they significantly alter the natural color of light. As an example, the sunset type typically provides a saturated orange coloration to the top half of the frame and is used to "punch up" an otherwise colorless or gray sky to look as if there is a "beautiful sunset."

If you use these filters, your images will always be questioned, do they represent the true existing light or are they a product of blatant manipulation? The effect provided by these filters is extremely obvious and provide a poor substitute for images shot under more favorable lighting conditions. While I have seen hundreds of published photographs where a split ND had been used, I cannot recall having seen one where a split color filter was used except in commercial advertising.

Manufacturers and Costs

All of the major filter manufacturers make a split ND filter. Circular filters come in sizes to fit almost any lens up to 77mm, while the rectangular and square filters will fit almost any lens if the right holder and adapter are purchased.

Cost is always a factor and the old saying "you get what you pay for" applies here. Several manufacturers make less expensive plastic or resin filters which they define as being "grey." They are not "neutral" density filters because they modify the natural light, generally by adding a slight magenta or brown cast. The Tiffen resin filter, as well as some other manufacturers, have been known for this effect. Unless you like this effect, you should avoid these filters.

For most situations, your best bet is to purchase one of the more expensive rectangular or square filters which can be both rotated and moved in and out in a holder. This allows the best and easiest placement of the edge line.

Important Tip --> The split ND filters I use are high quality resin filters which are made by Lee Filters and sold by Calumet (800/CALUMET) and B&H Photo (NYC) (and probably others). Although they are resin filters, they are true neutral density filters and do not modify the color quality of the light. In my opinion, they are some of the best currently made, but also are one of the most expensive (about $210 including the filter, holder and lens adapter). They also fit lenses with diameters of up to 82mm. Shign-Ray probably makes the finest glass split ND filters. Although expensive, they are worth the cost. You can call them directly at 800/486-5501 or purchase them from Mountain Light. All the leading filter manufactures make excellent ND filters, including Cokin, Tiffen, etc.

The less expensive circular split ND may adequately meet your needs, but its use is limited. These are made of glass and are true neurtral split ND filters. They tend to cost between $15 and $45 depending upon the size purchased. Important Tip -->If you have several lenses, consider purchasing a filter size larger than the largest diameter lens you use (i.e., if your largest lens diameter is a 72mm, purchase a 77mm filter). You can then purchase several step-up rings so you can use the same filter on all of your lenses saving you a considerable amount of money. The larger size is also important if you plan on using the filter on wide-angle lens with the step-up ring. Without the larger filter you may notice vignetting at the edges of your photograph.

Learning how to use the split ND filter in your landscape photography will dramatically improve your photographs. You will notice detail in your foregrounds which previously "blocked-up," lacking all detail. There will also be a much greater amount of detail and color saturation in your skies.


© 1996-1997 Kevin C. O'Neil, Eastern Light Nature Photography, Ltd. All Rights Reserved.

E-Mail: KevinONeil@AOL.Com