Get More Control Over Light with Neutral Density Filters
What It Does
Wonder how photographers get flowing water to look like a stream of fog? They use a strong Neutral Density (ND for short). The easiest explanation for what an ND filter does is reduce the amount of light entering your camera by set amounts. ND filters are dark with no color to them at all, hence the “Neutral” name.
The filter can be used in a variety of situations to create effects that are simply not possible to get any other way.
The two main uses of Neutral Density filters are:
The best way to get these dreamy blurred motion effects is to use a strong neutral density filter like the Hoya SOLAS IRND 3.0. This dark filter reduces the amount of light entering the lens by a full 10 stops. In real world terms that means if on a sunny day the camera shows an exposure of 1/250 of a second shutter speed and an F/8 aperture, the IRND 3.0 will reduce the shutter speed 10 stops so with the same F/8 aperture the shutter speed would be 2 full seconds!
In darker situations, such as a stream in a forest on a cloudy day, shutter speeds as slow as 30 seconds can be obtained!
Always use a tripod when using strong ND filters to get long exposures so only the motion is blurred, not the things that are not moving!
Some will say that you can always stop down to a very small aperture such as F/16 or F/22 but keep in mind that most lenses will not be as sharp due to refraction issues at these small apertures. So unless you need the depth of field for an effect, apertures smaller than F/11 are not recommended for most lenses.
Allow wider apertures to be used
The other main use of Neutral Density filters is to keep shutter speeds constant but use wider apertures for very shallow depth of field effects. Let’s say you want to photograph someone in bright light but the area you are in has a busy or distracting backgrounds. A wide aperture lens like a 50mm F/1.4 or even 1.8 shot wide open can yield a nice smooth very out of focus background to minimize any distraction and focus all the attention on the subject in focus. The camera is showing an exposure of 1/2000 of a second at F/8. A neutral density filter like a IRND 1.2 will reduce that exposure by 4 stops so that an aperture of F/1.8 can be set to yield a very out of focus background so all attention is on the subject.
Reducing exposure for flash
There are cameras with HSS (high-Speed Sync) flash capabilities but the majority of DSLRs and Mirrorless cameras are limited to a flash sync shutter speed of 1/250 of a second or sometimes slower. Let’s say you want to take a photo of someone outside in beautiful late afternoon sun and in the setting you want the person is badly backlit. The camera exposure setting read shutter speed 1/1000 of a second at F/11 and when you take the shot the person is just a dark shadow in the picture.
A flash will lite up the subject and balance the natural sunlight if the exposure can be brought down into the cameras flash sync range. A moderate neutral density filter like the SOLAS IRND 1.2 has a 4 stop light reduction allowing the camera to set a shutter speed of 1/125 of a second with an aperture of f/8 or 1/250 of a second at F/5.6 for more shallow depth of field. A strong filter can be used for an even wider aperture but focusing will become difficult.
Before delving into how a neutral density filter works let’s talk about the numbers. Looking at neutral density filters online or in a any given store will yield results with a lot many different numbers, 0.3, 64, 1.0, 1000, 3.0. The reason for this is there are two different rating systems for neutral density filters. One is an Optical Density rating and the other is a Filter Factor rating. Different companies use one or the other to show the light reduction of the different filters. Without getting into the math, the optical density of a filter is a logarithmic ratio of the mount of light entering the filter vs the amount of light passing through it. The Filter Factor refers to the multiplicative amount of light the filter passes, and its easier to think of that number as the bottom number of a fraction. A neutral density with a Filter Factor of 2 is passing 1/2 (50%) the amount of light entering in which equals a one stop reduction in light.
Here is how the two systems stack up. One “stop” of light equals one full step in aperture (say from F/5.6 to F/4) or shutter speed (say from 1/125 to 1/60 of a second).
|Light Reduction||Optical Density||Filter Factor|
A strong neutral density filter with an Optical Density of 3.0 (Filter Factor 1000) yields a 10 stop light reduction which means very slow shutter speeds of several seconds are possible in full sunlight. A quick example; if the exposure setting on a bright sunny day are 1/250 of a second at F/11 a NE filter with a rating of 3.0 will allow the shutter speed to be reduced 10 stop from 1/250 of a second to 4 full seconds using the same aperture of F/11.
How it works
Traditional Neutral Density filters such as the Hoya HMC filter use a dyed glass process where a formula of elements are added to clear optical glass in is molten state to add color. In the case of ND filters, that color is a very neutral gray of a set density, of for a light reduction of 1, 2 or 3 stops. The glass ND glass is then polished and multicoated before being assembled into the aluminum frame.
Traditional filters are excellent at blocking a set amount of visible light but most CCD and CMOS camera sensors are very sensitive to Infrared light as well. When stronger neutral density filters are used to achieve very long exposures during the day the filter blocks visible light but does not block as much IR leading to a unwanted color shift in the photo also know as Infrared contamination. IRND filters such as the SOLAS IRND control the amount of IR light is passing as much as it controls the amount of visible light it is passing. Photos taken with strong IRND filters such as a 3.0 (10 stop light reduction) have the most neutral and natural color balance.
Variable Neutral Density Filters
Most filters reduce the amount of light by one set amount which is anywhere from 1 to 10 stops. If another strength is desired the filter in front of the lens needs to be swapped out for another one.
Like the name implies, a Variable Neutral Density filter is able to produce a range of light reduction in one filter. That range is usually from 3-9 stops. To change the amount of light reduction the filter is rotated between “Min” and “Max” markings on the filter’s frame.
The advantages of Variable Neutral Density filters is cost and convenience. Only one filter needs to be bought and carried when shooting. The disadvantage is sharpness and color shifting. Most Variable ND filters are not quite as sharp as a single strength Neutral density filter and all Variable ND filters have color shifts that change depending on the density used. This color shift can be difficult to correct later. Also, it is impossible to accurately tell if the filter is set exactly to a particular density.
A Variable ND filter uses two circular polarizing screens places in opposition to each other, one is attached to the back of the filter frame and the other is attached to a rotating ring at the front of the filter frame. The when the filter is rotated the two polarizing screens have the effect of blocking more and more light as they come into alignment.
The reason for the Min-Max on the filter frame is that as the filters come into alignment they begin to interfere with each other causing a exposure variation pattern in the form of X, also known as the “Cross effect”.