Simply put, neutral density (ND) filters are a piece of dark glass or resin that inhibits light from entering a lens. The “neutral” in neutral density refers to the fact that, theoretically, the filter is colorless. Basically, an ND filter is sunglasses for your lens. In this guide, we will be explaining the different kinds of neutral density filters and how to choose the best one to fit your photography needs.
Neutral Density Filter Strength
The strength of a neutral density filter refers to its ability to hinder light. This is measured in stops. Each whole number increase in stop inhibits 50% of the light.
For example, a one-stop ND filter would let in ½ as much light. A two-stop ND filter would let in ¼ the amount compared to no filter and ½ as much compared to a one-stop ND filter. Thinking of it temporally, if the exposure time without a filter is one second, then one-stop ND filter would prolong that time to two seconds. A two-stop ND filter would prolong it to four seconds. A three-stop ND filter would prolong it to 8 seconds, and so on. The chart below offers other examples to illustrate this concept.
| Base Exposure (No Filter) | 3 stop ND filter | 10 stop ND filter |
| 1/8000 | 1/1000 | 1/8 |
| 1/4000 | 1/500 | ¼ |
| 1/2000 | 1/250 | ½ |
| 1/1000 | 1/125 | 1 |
| 1/500 | 1/60 | 2 |
| 1/250 | 1/30 | 4 |
| 1/125 | 1/15 | 8 |
| 1/60 | 1/8 | 15 |
| 1/30 | ¼ | 30 |
| 1/15 | ½ | 60 (1 min) |
| 1/8 | 1 | 120 (2 min) |
Some popular ND strengths are three, six, and ten stops. It is also possible to combine (stack) filters to increase the overall strength. For example, combining three- and six-stop neutral density filters will result in nine stops of light inhibition. This works great in a pinch but should be avoided, if possible. Each layer of glass or resin in front of the lens will degrade overall image quality. This could be the softening of details, color shifts, chromatic aberration to some degree. The general principle is to use as few filters as possible to achieve the desired effect.
When Should You Use ND Filters?
Prolong Exposure Time
One of the most popular uses for an ND filter is to deliberately increase exposure time. You can “smear” the appearance of clouds and make flowing water look silky smooth. It has the effect of smoothing out moving textures. Another interesting use of a strong ND filter is to “get rid of” moving objects like people and traffic from a scene. If an object is in motion and a strong ND filter is used, the object doesn’t occupy a particular space long enough to give off the requisite amount of light to be captured by a camera. Try this in a mall or street corner and you’ll be amazed by the results. Remember, you’ll need an ND filter strong enough to prolong the exposure to at least four minutes.
Shallow Depth of Field
Neutral density filters are also popular with portrait photographers. To achieve a beautifully creamy bokeh and a very shallow depth of field, the aperture needs to be wide open. Some lenses can open to f/0.95. Even at the lowest ISO and fastest shutter speed, there might still be an overabundance of light to achieve proper exposure. An ND filter can help mitigate this problem.
Solar/Solar Eclipse Photography
Very strong (16 stops or more) neutral density filters should be used to photograph the sun and solar eclipses. In addition, you should never view the sun through an optical viewfinder — even with strong ND filters in place. While ND filters inhibit visible light, most do nothing to stop harmful IR and UV radiation from damaging your eyes. You should always use live view or an electronic viewfinder. It’s also recommended that you place the ND filter on your lens before pointing it at the sun with a large magnification lens because the concentrated light may damage the camera’s image sensor.
Round vs. Quadrilateral (Square or Rectangular) Filter
ND filters come in round and quadrilateral formats. Generally, round filters screw on directly to the front of your lens — if your lens has filter threads. I say “generally” because sometimes the front element is so big that it would be impractical to have a filter with such a large diameter. Instead, “drop in” filters are used at the back of the lens, near the camera’s lens mount.
If you decide to use round screw-on filters, I recommend purchasing one (or a set) that fits your largest diameter lens. If you want to use these filters on other smaller lenses than you only need to purchase a step-up ring instead of purchasing a new filter or set of filters.
Step-up rings are considerably less expensive than optical filters. For example, if you own lenses with filter thread sizes of 67mm, 72mm, and 82mm, I recommend buying 82mm filters along with 67-82 and 72-82 mm step-up rings.
Quadrilateral filters are used with specialized adapters/holders. They are often more cumbersome to carry than circular filters, but also offer more variety and versatility. For example, there are adapters available that will allow you to attach these filters to lenses with bulbous front elements where circular filters would be impossible to place.
ND Filter Variations
Variable Neutral Density Filter (VND)
A VND filter allows you to change the strength of the filter based on how much it is turned. Videographers find this filter very helpful for achieving a “cinematic” look that is dependent on certain shutter speeds based on frames per second. In varying light, it’s more convenient to simply turn a filter in front of a lens than replace it periodically. Personally, I do not recommend it for photography. I’ve seen cross-type and patchy artifacts when doing long exposure photography.
Graduated Neutral Density Filter (GND)
The vast majority of GND filters are rectangular in shape. They are darkest at the top and gradually become clear near the midpoint of the filter. The strength of the GND filter is determined by its darkest portion.
For example, a three-stop GND filter only inhibits three stops of light at the very top (darkest part) of the filter. As we travel down the filter, there is a gradual transition to 0 stops (clear). It is used during late afternoon or early morning when the sky is much brighter than the foreground. This filter preferentially inhibits light from the bright sky, allowing the foreground to appear brighter. The overall effect is to even out the exposure throughout the frame.
GND filters are available in soft, medium, and hard varieties. On a “hard” GND filter, the transition between the darkest and clearest portion of the filter is abrupt. With a soft GND, the transition is much more subtle and spread out. With a medium GND, the transition rate is somewhere in the middle.
Reverse Graduated Neutral Density Filter (RGND)
An RGND filter is a slight but significant variation on the GND. A RGND is darkest about 1/3 of the way from the top of the filter. It gradually gets less dark towards the top but not completely clear. It also gets gradually dark towards the bottom and does become completely clear. This filter is best used when the brightest part of the sky is near the horizon.
Polarizer
You may not think of a polarizer as an ND filter, but it does inhibit about 1.5–2 stops of light from entering the lens. A polarizer must be engaged (turned) to harness its “polarizing” properties (removing glare from reflective surfaces thus increasing saturation and contrast). However, this is not necessarily needed to utilize its ND property.
Center Graduated Neutral Density Filter (CGND)
Perhaps the least used variation of the ND filter is the CGND filter. It is dark in the center and gradually becomes clear at its periphery. This filter is used to counteract the peripheral darkening (vignetting) that can occur when certain lenses are shot at very large apertures.
Taken without a Neutral Density Filter:
Taken with a Neutral Density Filter:
Recommendations if You’re Just Starting Out
If you’re a photographer trying filters for the first time, I recommend carrying a polarizer and a six-stop ND filter. The polarizer will help you minimize glare from reflective surfaces. Be careful, however, when using it with ultra-wide angle lenses. If you include the sky in your field of view, a polarizer may cause some uneven color saturation.
A six-stop neutral density filter is good to have with you for bright conditions when you want to blur out flowing water or convey a sense of motion to clouds. If you need a little more stopping power, you can always stack it with the polarizer.
Budding portrait photographer who want to maintain a wide aperture on fast lens during bright days should start with a three-stop ND filter. If you’re a videographer, a variable ND makes the most sense.
