ND Filter

What are ND filters?

The terms “Neutral Density Filter” and “Neutraldichte Filte” refer to ND filters.

Certain filters are designed to block certain wavelengths of light, such as visible light (VIS) or near-infrared light (NIR).
For example, bandpass filters aim to allow certain wavelength ranges (or “bands”) to pass through or block them. These filters usually alter the color of the picture that was taken.

ND filters, on the other hand, aim only to uniformly alter the wavelengths’ density (brightness). They are considered “neutral.”

When are ND filters appropriate to use?

Suppression of background

Why would someone want to, say, make the image eight times or thirty-two times darker? Subsequently, you would need to expose it eight times, or 32 times longer.
This apparent drawback can be benefited from: an ND10 filter, which darkens the entire image ten times, can be used to suppress background objects in a scene.
Excellent! The foreground that interests us has now vanished as well.

However, if you use a strobe for the foreground, the background stays dark and the foreground becomes visible once more. Strobing in a room with regular lighting, on the other hand, would have kept the backdrop intact.

suppressing the highlights

In order to prevent motion blur, a bright or shiny light that moves quickly would actually need to be strobed. Now that would result in a saturated bright object which should be avoided. An ND-Filter helps.

Dimming the image without loss of resolution

If an image is too bright, for example, a very fine, dense stripe pattern directly in front of a lamp, closing the aperture by a large amount would make the fine details invisible because the airy disk grows linearly with the aperture number. An ND filter, on the other hand, maintains the resolution.

Do I achieve the same effect of an ND filter with a larger aperture number?

Short answer: No, the mode of action and side effects are different.

What are the advantages of ND filters over dimming the lens?

Closing the aperture increases the depth of field as a side effect.
So if you want to maintain a small depth of field but have a darker image, ND filters are a good choice.
If you need to dim by a high aperture level, it often comes at the expense of resolution because the size of the Airy disk depends linearly on the aperture number.
If you need a darker image of unchanged resolution, you should use an ND filter.

Attention, trap!

There are two different notations for ND filter specifications:
Specifications like ND8 or ND1000 denote a filter that requires 8 or 1000 times more exposure time.
The same filters are sometimes labeled with ND3.0 and ND10.0, with 3 f-stops or 10 f-stops, or with 3 or 10 “stops.”

With each f-stop of dimming, the aperture diameter becomes smaller by a factor of ( \sqrt{2} ) (1.41), so the area of the aperture becomes smaller by a factor of ( (\sqrt{2})^2 \approx 2 )
To obtain an image of the same brightness, you must expose it twice as long at each step
ND3.0 denotes 3 stops, so an 8-times longer exposure time
ND10.0 denotes 10 stops, so a 1024-times longer exposure time.

Can ND filters be stacked?

Yes, but keep the mentioned trap in mind:

N3 + N3 = N6
3 times longer exposure after 3 times longer exposure = 6 times longer exposure
N2.0 + N2.0 = N4.0
( 2^2 ) = 4 times longer exposure after ( 2^2 ) = 4 times longer exposure = ( 2^4 ) = 16 times longer exposure
Mixing is a bit more difficult with different specifications.
N3.0 + N2 = N8 + N2 = N10
( 2^3 ) = 8 times longer exposure after 2 times longer exposure = 10 times longer exposure.
Here, ND10 = ( 2^{3.32} ) longer exposure = ND3.2


You must always ask what the other person wants to achieve; otherwise, you can easily talk past each other.
When asking for ND filters, it’s best to always say how many times longer you want to expose.