What is a neutral density filter

A neutral density filter (ND filter) is a filter that goes in front of your objective lens element. Its purpose is to reduce the amount of light going into your lens and hitting your sensor. On a really sunny day, an ND filter can be an indispensible tool when it comes to keeping your highlights from getting blown out. There are a lot of applications for a filter like this, but they are primarily used by outdoor photographers.

When taking photos outdoors, there are a number of things that you can do to soften or lessen the light hitting your sensor using practical means. For example, you could shoot a flower under the shadow of a tree instead under direct sunlight. However, photographers shouldn’t resign themselves to losing great shots just because it’s too bright.

neutral density filter black and white image of a riverhttp://www.flickr.com/photos/56871825@N00/6863639303/

Here, we’ll focus on some of the different types of ND filters, common ND filter issues, how to select an ND filter, and the most common uses for ND filters.

Types of neutral density filter

There are two types of commercially-available ND filters: screw-on filters and square filters.

Screw-On Filters

A screw-on filter attaches to the front of your lens. You will need to make sure that the diameter of your screw-on filter is the same as the diameter of your lens. You can use a screw-on ND filter larger than the diameter of your lens, if you use a step-up ring. Never use a step-down ring to use a filter with a diameter smaller than that of your lens. Otherwise, you will introduce vignetting to your image.

With a screw-on filter, you have the option to choose between a filter of a specific density or a fader filter. A set of density filters come with various numerical identifiers that correlate to the amount of light they filter out. The higher the number, the less light gets through the filter.

A fader filter is a neutral density filter that has a range of ND stops built into it. You put the filter on your lens and spin it to darken your image. You can often go from ND 0 (essentially a clear filter) to ND8 or higher by spinning the filter to darken the image. These are most commonly used by photographers who are constantly changing between light and dark settings and who don’t want to constantly change ND filters.

Square Filters

Square filters are used in Cokin-style square filter holders. These filters are essentially a 4-inch by 4-inch piece of glass that you can slide into a filter holder. The filter holder mounts to the front of your lens, and it will usually be able to hold more than one filter.

There are a few benefits to using square filters over screw-on filters. First, square filter holders allow you to easily stack different filters at once to get various in-camera effects. For example, you could use both an ND filter and a warming filter together to produce a unique in-camera effect without the need for heavy post-production. Square filters also allow you to use gradient filters. These filters start out as an ND filter on one end and fade into a clear filter on the other end. If you put the ND side on top, you can use these filters to bring down the highlights in the sky while exposing evenly for the ground.

Common issues with neutral density filters

If you have a few hundred dollars to spend on an ND filter, then you may not have to worry about many of the problems covered here. However, people on a budget will find that there are many issues to be aware of when purchasing budget ND filters. This doesn’t mean that it’s not possible to find good, affordable ND filters. It just means that you need to look a little harder to find good ones and be willing to work around their faults.

Purple casting

One of the most common issues faced with ND filters is that they can sometimes create a purplish cast on your images. Expensive filter sets generally don’t have this issue, but many cheaper filters are known for creating a purple cast. Depending on the filter, the cast can be either very noticeable or barely noticeable.

When the cast is only minimal, this is not something that is very difficult to correct in post. However, it’s time consuming and something you should try to avoid if at all possible. When you have a heavy purple cast on your image, it can be very difficult to fix this even with the latitude you get while shooting in RAW.

TiffenND filters are usually a good place to start looking when you want a good filter for a decent price. Some of Tiffen’s filters are expensive, but there are a number of great Tiffen filters that are reasonably priced. Genus and Heliopan are also great options, though often more expensive than their Tiffen counterparts.

Fader Filter X

In most cases, I would recommend that beginners try their luck with cheap standard ND filters. Most of the time, the casting isn’t very pronounced and it can be fixed. However, I would never recommend getting a cheap ND fader filter.

neutral density filter fader filterhttp://www.flickr.com/photos/cometodeal/9004146947

The mechanism that darkens the filter throughout its range can be built poorly in these filters and it can put a big “X” in the center of your image. This X is created as the filter elements cross over one another throughout the range. On cheap filters, this can get stuck like this and render your filter completely useless.

If you can’t afford a good fader filter, I would recommend getting a set of standard ND filters in three different densities to cover a range of scenarios. It is simply not worth the time, expense, and annoyance to take a chance on a cheap fader filter.

Image softening

ND filters are sometimes made out of glass elements that aren’t exactly great. When compared to the glass used in a Canon L-Series lens, the glass used in most ND filters is a joke. Most of the time, using an ND filter will introduce softening to your image even with the best lenses.

When the neutral density filter is made well, it introduces a minimal, almost unnoticeable amount of softening. However, cheap filters can soften your image quite a bit. There are affordable ND filters that do a great job of keeping your images sharp and clear, but you’ll have to research to find filters that produce the best results. Just keep this in mind when you’re shopping around for ND filters.

Selecting an ND Filter

The type of ND filter you select will depend heavily on the type of photography you do the most. Outdoor photographers tend to rely on Cokin-style square filter sets and variable ND filters the most. However, it’s not uncommon for DSLR videographers, portrait photographers, and photojournalists to use an ND set.

In the majority of cases, a good square filter or good standard ND screw-on filter will leave you with the best image quality. The simplicity of the design is a major part of this, and there are less components to mess up your image this way. When you buy filters made of high-quality glass, you’re much more likely to get the best image out of an ND set.

Using a Cokin-style filter holder is not optimal for many videographers and photojournalists who require mobility and ease of packing. In these cases, a standard screw-on filter can produce great images without killing your budget.

Using a fader filter can be a great option for photographers who find themselves being subjected to variable lighting conditions beyond their control. A great example of this would be taking photos on a sunny day under variable tree cover. While taking photos under tree cover, you may find that you don’t even need an ND filter due to the softness of the light. Walking out of tree cover into the sun may suddenly force you into using an ND filter, which is as simple as turning the filter until you get the desired level of darkness.

When selecting a filter, keep in mind that you really do get what you pay for. Cheaper filters are usually cheaper because they’re made of inferior materials using inferior processes. However, that doesn’t mean expensive is always better. You should take the time to weigh your budget against your expected results and shop accordingly.

Conclusion

An ND Filter can be an indispensable tool. Whether you want to get a shallow depth of field for a natural light portrait on a sunny day or tone down highlights on a landscape shot, ND filters are amazing to have around. Knowing how to use these filters to their full effect should definitely be part of your photographic skill-set. You may find that these filters open up a whole new creative world to your shots, allowing you to capture images that would have been impossible to get otherwise.

About The Author

Jared Skye is the owner of Out of Nowhere Media in Eugene, OR and Gainesville, FL. He specializes in commercial video and photography. His creative photography has been licensed by a number of media outlets, including New Scientist magazine.

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