What is neutral density filter (ND)? And how do I use it?

Neutral density filter or ND filter is a piece of dark glass or resin used in photography to reduce the amount of light entering the camera lens without affecting the colors or tones of the image. The term “neutral” in neutral density means that the filter is designed to be colorless, ensuring that it does not introduce any color cast into the image. By limiting the amount of light reaching the camera sensor, ND filters enable photographers to achieve certain creative effects and control exposure parameters. Common scenarios in which ND filters are useful include capturing long-exposure shots in daylight, obtaining a shallow depth of field in well-lit conditions, or capturing motion blur on moving subjects.

Neutral density filter strength

The effectiveness of a neutral density filter is determined by its capacity to block light, which is quantified in stops. With each whole number increment in stops, the filter prevents 50% of the incoming light.

As an illustration, a one-stop ND filter permits half the amount of light to pass through. A two-stop ND filter allows only a quarter of the light compared to no filter, and half as much as a one-stop ND filter. To think of it in terms of time, if the exposure time without a filter is one second, a one-stop ND filter would extend it to two seconds. Similarly, a two-stop ND filter would extend it to four seconds. The extension continues, where a three-stop ND filter would result in an 8-second exposure, and so forth.

Common strengthsfor ND filters include three, six, and ten stops. It’s feasible to enhance overall strength by combining or stacking filters. For instance, combining three- and six-stop ND filters yields a total inhibition of nine stops of light. While this can be a practical solution, it’s advisable to avoid it when possible. Each additional layer of glass or resin in front of the lens has the potential to compromise overall image quality, introducing issues like softened details, color shifts, and to some extent, chromatic aberration.

When should you use ND filters?

Prolong Exposure Time

An ND filter is frequently employed to intentionally extend exposure time, providing the ability to create a “smearing” effect on clouds or achieve a silky smooth appearance for flowing water. This has the impact of smoothing out textures in motion. Another intriguing application of a powerful ND filter is to effectively “remove” moving elements like people or traffic from a scene. When a strong ND filter is used, rapidly moving objects don’t occupy a specific space long enough to emit the necessary amount of light for the camera to capture. Experimenting with this technique in settings like a mall or street corner can yield astonishing results. Keep in mind that a sufficiently potent ND filter is essential to extend the exposure to at least four minutes.

Shallow depth of field

Portrait photographers often favor neutral density filters for specific reasons. When aiming for a gorgeously creamy bokeh and an exceptionally shallow depth of field, it’s necessary to have a wide-open aperture. Some lenses can even reach an aperture as wide as f/0.95. However, even with the lowest ISO and the fastest shutter speed settings, there might still be an excess of light, making it challenging to achieve the correct exposure. In this case, an ND filter is a very useful tool for solving this problem.

Solar/Solar eclipse photography

For photographing the sun and solar eclipses, it’s crucial to employ very potent neutral density filters, typically those with 16 stops or more. Importantly, never attempt to view the sun through an optical viewfinder, even if strong ND filters are in place. While these filters effectively block visible light, they often offer little protection against harmful infrared (IR) and ultraviolet (UV) radiation that can damage your eyes. Instead, it’s recommended to use live view or an electronic viewfinder for safer observation. Additionally, it’s advisable to attach the ND filter to your lens before pointing it at the sun, especially with a magnifying lens, as the concentrated light could potentially harm the camera’s image sensor.

Round or quadrilateral filter (square or rectangular)

ND filters are available in both circular and square formats. Typically, circular filters are directly screwed onto the front of your lens, provided your lens has filter threads. However, it’s important to note that in some cases, the front element of the lens may be too large, making it impractical to use a filter with such a large diameter. In such instances, photographers resort to “drop-in” filters, which are inserted at the back of the lens, near the camera’s lens mount.

Opting for round screw-on filters is a good choice, and I suggest acquiring one or a set that matches the largest diameter of your lens. This way, if you wish to use these filters on smaller lenses, you can simply buy a step-up ring instead of investing in a new set of filters or individual ones.

Step-up rings are a cost-effective alternative compared to optical filters. For instance, if you have lenses with filter thread sizes of 67mm, 72mm, and 82mm, a practical approach would be to purchase 82mm filters and complement them with 67-82mm and 72-82mm step-up rings.

Quadrilateral filters are utilized in conjunction with specialized adapters or holders. While they may be bulkier to carry compared to circular filters, they provide greater diversity and flexibility. Notably, adapters are designed to enable the attachment of these filters to lenses with convex or bulbous front elements, where placing circular filters would be impractical or impossible.

ND filter variations

Variable Neutral Density Filter (VND)

A Variable Neutral Density (VND) filter offers the flexibility to adjust its strength by rotating it. Videographers often find this filter valuable in achieving a “cinematic” appearance, particularly when adhering to specific shutter speeds corresponding to frames per second. When dealing with changing light conditions, it’s more convenient to adjust a filter’s strength by turning it rather than frequently replacing it.

Graduated Neutral Density Filter (GND)

The majority of Graduated Neutral Density (GND) filters come in a rectangular shape, featuring a gradient from darkest at the top to clear near the midpoint. The filter’s strength is determined by the intensity of its darkest section.
These filters are useful in scenarios where the sky is much brighter than the foreground, such as during late afternoon or early morning. They balance exposure by selectively reducing light from the bright sky, making the foreground appear brighter. GND filters come in soft, medium, and hard types, with the transition between dark and clear being abrupt in hard filters, subtle in soft filters, and moderate in medium filters.

Reverse Graduated Neutral Density Filter (RGND)

An RGND (Reverse Graduated Neutral Density) filter is a nuanced but notable deviation from the standard GND. It achieves its darkest point approximately one-third down from the top of the filter, gradually becoming less dark towards the top without reaching complete clarity. Simultaneously, it darkens gradually towards the bottom, eventually becoming entirely clear. This filter is most effective when the brightest section of the sky is near the horizon.

Polarizer

While you might not typically consider a polarizer as an ND filter, it does block approximately 1.5–2 stops of light. To unlock its polarizing capabilities, the polarizer needs to be adjusted (turned) to eliminate glare from reflective surfaces, thereby enhancing saturation and contrast. However, engaging its polarizing properties is not mandatory to utilize its ND characteristics.

Center Graduated Neutral Density Filter (CGND)

The least commonly employed type of ND filter is the CGND (Center Graduated Neutral Density) filter. It features darkness at the center, gradually transitioning to clarity at the edges. This filter serves to mitigate the peripheral darkening, known as vignetting, which can occur when shooting with certain lenses at very wide apertures.

What can we conclude?

For those unaccustomed to using filters in photography, I suggest bringing a polarizer and a six-stop ND filter. The polarizer helps reduce glare from reflective surfaces, but caution is needed with ultra-wide-angle lenses as it may lead to uneven color saturation when the sky is included in the frame.
A six-stop ND filter proves useful in bright conditions when you want to create a blurring effect on flowing water or convey movement in clouds. If additional stopping power is required, it can be paired with the polarizer.

Aspiring portrait photographers seeking to maintain a wide aperture on a fast lens in bright conditions can start with a three-stop ND filter. For videographers, a variable ND filter is often the most convenient choice.