What is a neutral density filter?

An ND filter – also known as a grey filter – is mounted on the lens to reduce the incident light. Their function is similar to that of sunglasses. If one looks directly into the sun with the bare eye, it does not take long until you only see white. The camera’s sensor is comparable to the iris of the human eye and can also be “blinded” if too much light falls on it. As a result, the image becomes too bright or unusable. ND is called “neutral density filter” and is explained in this article.

When are ND filters used?

ND filters are often used in landscape photography when long exposures are required. For example, when photographing a lake where you want to achieve a smoothing effect, or even with flowing water such as a waterfall. The ND filter only becomes necessary, however, if you cannot expose long enough without the image being too bright or even burnt out despite setting the desired maximum aperture. An ND filter can also make sense for portrait photography with a tendency to open aperture (for a blurred background) and short exposure time. Especially during outdoor shootings in the morning or evening in deep sun, the light incidence on the sensor can be extreme. Photography against the sun is made possible with an ND filter. So great moods can be captured. Basically, it is always worthwhile to have a grey filter with you when you take pictures outside in good weather.

Why is it called “neutral density” and “grey filter”?

“Neutral” means that the filter has no effect on color reproduction or intensity. “Grey filter” does not mean that grey tones are filtered out, but that the filters are coloured grey in different gradations / intensities.

Different levels of ND filters

ND filters are available in different strengths and sizes. However, there are also variable ND filters where you can adjust the intensity yourself at any time instead of screwing several ND filters together. The advantage of an adjustable or vario filter is that you only need one filter… on the other hand, with a stepless ND filter it is more difficult to calculate how much exposure time / f-stops you have finally gained. The following information can be found on an ND filter:

• ND value: For example, is on the filter ND2, this is a linear extension factor. The shutter speed can be doubled with this filter. If we would reach the brightness limit with 1 second exposure time without the filter, we could expose twice as long with this filter, i.e. 2 seconds with the same aperture and ISO.

• NDx value: Alternatively, sometimes a value in the form “NDx 3.0” is given. These 3.0 are a logarithmic specification and cannot be interpreted as an extension factor as above. In this case the extension factor is 10 to the power of 3.0, which gives 10 x 10 x 10 = 1000. Thus, the exposure time can be extended by a thousand times.

• Thread size in mm: To ensure that the filter fits on the lens and can be screwed on correctly, the thread size, respectively the diagonal of the lens corresponds to the indication on the filter. The size of the lens is indicated on the lens. Is it necessary to buy an ND filter for each object size? No! The solution is called “Step up” or “Step down” rings, which are explained below.

Using step up rings as an adapter saves money!

Usually a photographer has different lenses of different sizes. To avoid having to buy the same gray filter for every lens, there are the so-called Step Up or Step Down Rings. These are adapter rings for the lenses to reach a certain circumference. So you can save money if you buy the ND filter with the largest circumference and mount it on each lens with the step rings. For example, if you own a KIT lens with a diameter of 58 mm, you can also buy an ND filter with a circumference of 77 mm to mount it with the rings. When buying the Step Rings, please note that the lens size occurs once.