Neutral Density Filter – What is it?
Simply put, a neutral density (ND) filter is 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.
Neutral Density Filter Strength
The strength of an ND filter refers to its ability to hinder light, and this is measured in stops. Each whole number increase in stop inhibits 50% of the light. For example, a 1 stop ND filter would let in ½ as much light. A 2 stop ND filter would let in ¼ the amount compared to no filter and ½ as much compared to a 1 stop ND filter. Thinking of it temporally, if the exposure time without a filter is 1 second, a 1 stop ND filter would prolong that time to 2 seconds. A 2 stop ND filter would prolong it to 4 seconds; a 3 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|
|1/15||½||60 (1 min)|
|1/8||1||120 (2 min)|
Some popular ND strengths are 3, 6, and 10 stops. It is also possible to combine (stack) filters to increase the overall strength. For example, combining 3 and 6 stop ND filters will result in 9 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 (softening of the 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. You’ll be amazed by the results! Remember, you’ll need an ND filter strong enough to prolong the exposure to at least 4 minutes.
Shallow depth of field – ND 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) ND 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, because 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 if 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, then 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 (more below). 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, because 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 3 stop GND filter only inhibits 3 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 less 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.
Recommendations, if you’re just starting out with ND filters
If you’re a landscape photographer and trying filters for the first time, I recommend carrying a polarizer and a 6 stop ND. The polarizer will help you minimize glare from reflective surfaces. Be careful, however, when using it with ultrawide angle lenses. If you include the sky in your field of view, a polarizer may cause some uneven color saturation. A 6 stop ND 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.
If you’re a budding portrait photographer and want to maintain a wide aperture on your fast lens on bright days, start with a 3 stop ND filter.
If you’re a videographer, a variable ND makes the most sense.