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What are gray filters or ND filters and what are they used for?

One of the most important goals in photography is to avoid camera shake. To do this, you will probably try to reduce the exposure time to the smallest possible value. It is not without reason that many photographers spend huge sums of money on expensive lenses that allow even more light to enter thanks to an even larger aperture.

An aperture of f / 2.8 is great. Really good lenses even achieve values ​​of f / 1.4 or even f / 1.2.
In addition to short shutter speeds, this open aperture also results in a buttery soft background and exposes your subject neatly.

 

What does this have to do with the neutral density filter?

Not every photographer shoots his photos in the studio and can specifically adjust the light output.
As soon as you take photos in bright sunshine, but you need an aperture that is as open as possible for design reasons, the camera quickly reaches its limits despite ISO 100 with an exposure time of 1/4000 of a second. In order not to over-expose the picture, you now have to close the aperture further, but this will destroy your planned picture look.

And this is exactly where the gray or ND filter comes in (Neutral density) for use. You can find another use for long exposures. Here, too, you make use of the properties of the gray filter to increase the exposure time significantly (sometimes even to several minutes).

 

How does the ND filter work?

In principle, the ND filter is nothing more than sunglasses for your camera and simply darkens the lens. The amount of incident light is reduced and the exposure time is artificially extended by a specified factor.

Gray filters are available in different strengths with which you can achieve different exposure times.

Even if it mostly appears gray or black, colors and sharpness are not influenced by a good gray filter. That's why he will Neutral density Called a filter. Incidentally, like the polarizing filter, the ND filter cannot be digitally reproduced!

 

What can you do with a neutral density filter?

As already described, the ND filter enables significantly longer exposure times depending on its density. Shutter times of several minutes can be achieved in this way. However, the depth of field can also be influenced by using an ND filter. In addition, it enables the sharpness in the picture to be increased.

 

Sharper images and longer exposure times

Many ambitious photographers know the pictures of velvety streams and snow-white, smooth waterfalls. In most cases, these images were created with an ND filter. The long exposure time makes the water blur and appear evenly soft.
In order to achieve exposure times of several seconds, one would be forced to close the aperture as far as possible without a gray filter. During the day, despite the aperture being closed and ISO100, it is often not possible to expose for a long time without the images becoming too bright. This is where the gray filter comes into play. On the one hand, by using the ND filter, the exposure time can be increased to several seconds, while at the same time the aperture value can be reduced to give the picture more sharpness.
This is related to what is known as diffraction. Every lens has an optimal focus range, which is around half of its aperture value.

How long should I expose for such pictures?

In the case of waterfalls, times between 5 and 30 seconds are absolutely sufficient. When photographing water surfaces such as the sea, which washes stones in waves, an exposure time of over 30 seconds is recommended. Here it makes sense to use the bulb mode in combination with a remote release. Cloud images also benefit from long exposure times.

Here are some tips for better long time exposures.
 

Taking pictures with the aperture open

A second use of the gray filter is to reduce the depth of field in bright surroundings. Most cameras offer shutter speeds of up to 1 / 4000s. On sunny days, strong overexposures can occur, especially when using bright lenses with an open aperture. Correctly exposed images then require a higher f-number. In order to still get a soft and blurred background in portrait photography, for example, a gray filter can be used. This can be used to darken the image and increase the exposure time to an adequate level.
 

 

Here are some photos that were taken with the ND filter


 

 

Which neutral density filter do you need?

The most commonly used type of filter is the so-called Screw filter.

This is screwed onto the filter thread of the lens and cannot be moved and at the same time offers a minimal height. In addition, any number of filters can be screwed in front of each other. Here are manufacturers like Haida, Hoya and B&W
Here you have to pay attention to the correct filter size.
With screw filters there is the option of working with filter adapters. Here the largest filter required is bought and adapted to another smaller lens with adapter rings. This can reduce the cost of filters since only one set of filters is required.
I don't use this method myself because I want to mount my lens hoods as well.

Another design is the Slide-in filter. There are various systems in which darkened glass or plastic panes can be pushed into a frame and the picture can be darkened very flexibly. The best-known system here is probably that of Cokin.

This system is particularly recommended in connection with gray graduated filters, which change from gray to transparent and thus darken parts of the image. I advise against the use of gray graduated filters in the screw variant. Compared to the slide-in system, this lacks the flexibility to rotate and move the filters in order to adapt the gray gradient to the horizon.

Unfortunately, the slide-in filters have one major disadvantage: a lens hood can no longer be used. That's why I only use screw-on filters with the exception of the gray gradient filters.
 

A little theory

There are different “strengths” in ND filters. These are given with different numerical values. There are designations such as ND8, ND64 or the technical density values ​​such as 0.9 or 3.0 ...

Various ND filters are listed in the following table. The most common filters have a gray background. In addition to the simplest designation for the gray filter, the specification ND (x) is also the approximate multiplier for the exposure time. The density is another value that is occasionally given instead of the ND values. If you don't want to remember these values, you can download the cheatsheets as PDFs at the end of the page.

I admit that the calculation of exposure times for the use of ND filters is not immediately obvious.
In the case of weaker ND filters, calculation is also not necessary. Here the camera manages to measure and expose correctly despite the gray filter screwed on. Starting with an ND64, for example, you should calculate the times. Here the camera is often no longer able to focus correctly or to measure correctly. Here is a short calculation example:

If I would normally expose an image at 1 / 50s without a filter, I would have to expose about 1 / 8th of a second using an ND-8 filter. With an ND64 one second and with an ND1000 15 seconds.

How does this value come about?
The exposure time is doubled with each increase in the aperture value by a full aperture. Therefore it makes sense to look at the aperture values ​​for the various neutral density filters.

In the next table you can see a list of the most common neutral density filters and neutral density filter combinations with the corresponding number of f-stops by which the exposure is reduced.
The left column shows the exposure time without a gray filter. The key data for the correct exposure time can be found from these two data.

This table can also be found in the Cheatsheet - PDF.

The cheat sheets can be downloaded as PDF here: