Filters: Neutral Density

Neutral Density Filters. What exactly are they?

ND Neutral Density Filters ND Filters or Neutral Density filters are, in their simplest form, darkened glass that you place between your subject and your camera sensor and in the majority of cases they screw into your lens. They are called neutral density because the filter reduces all light wavelengths reaching your cameras sensor meaning colour reproduction is not affected. There are also graduated ND filters available but in this article we will cover the standard screw in type ND filter.

ND filters are available in 2x, 4x, 8x, 16x, 32x etc all the way up to 8192x (and beyond in fact). Each increase in multiple signifies a 1 f-stop, or 1 EV difference in light registered by your cameras sensor. In photography terms, ND filters are quantified by their lens opening percentage, f-stop reduction or optical density. So in terms of lens opening percentage, a 2x filter represents 50% of the lens area opening or 1EV (1 f-stop) reduction, 4x represents 25% or 2EV (2 f-stops), 8x represents 12.5% or 3EV and so on.

ND Filter Type Optical Density f-stop Reduction EV Reduction % Original Lens Opening Area
ND2x 0.3 1 1 50.0%
ND4x 0.6 2 2 25.0%
ND8x 0.9 3 3 12.5%
ND16x 1.2 4 4 6.25%
ND32x 1.5 5 5 3.13%
ND64x 1.8 6 6 1.56%
ND128x 2.1 7 7 0.78%
ND256x 2.4 8 8 0.39%

To explain by example let us pretend we have a 4x (2 f-stop) ND filter at hand. If we set up our camera in manual mode and our settings are ISO 100, f/2.8 and a shutter speed of 1/250s without an ND filter and we are showing perfect exposure, when we place the 4x ND filter on the lens you should notice that your cameras built in light meter is now showing a 2 stop (2EV) underexposure for the same scene. So to ensure we get the same exposure we had without the ND filter we need to modify the lens aperture (open it up letting more light in), the ISO (increase it making the sensor more sensitive) or the shutter speed (slow it down letting more light in) or a combination of the three. Usually however it is the aperture or the shutter speed that is modified as the prime purpose of an ND filter is to give you more flexibility with these two camera settings. Using ND filters will allow a photographer open up their lens aperture fully in midday sun for a range of shutter speeds. It will also allow a photographer take 30 second long exposures in bright light without overexposing. If daylight portraits are your photo of choice then check out why you would use an ND filter to give you more flexibility with your aperture. Or if you are a landscape or long exposure photographer then check out why ND filters are used to control shutter speed choice.

ND Neutral Density FiltersOne disadvantage of using an ND filter is that it darkens what you see through the viewfinder rather dramatically and depending on what type of filter you are using (16x, 32x etc) your camera may not be able to focus at all because it is so dark. Where aperture considerations are concerned this is not so much an issue as you are mainly using the ND filter in bright daylight conditions and should never have an issue focusing. However if you are using an ND filter for long exposures in poorly lit conditions then this can cause problems. Check out our other article on ND filters and shutter speed for ideas on dealing with problems like this.

Another thing to bear in mind is the type of ND filter you choose. One word of advice, buy a good one. Cheap ones may lead to ghosting whereby the light reflected by your cameras sensor is re-reflected off the inside of your cheap ND filter, causing what can only be described as ghost effects on your final image. This is totally avoided by buying a pro quality filter.

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