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Neutral density, or ND, filters are simply like sunglasses for the camera. They screw onto the front of the camera lens and make the image look darker.
When we are making videos we know there are 3 ways to control the light, ISO, Aperture, and Shutter Speed, but with this tool, we can add one more option!
In this next example, we have the shutter speed at 1/50 because we are recording at 24 frames per second, and everything looks fine.
But what if we want the background to look blurrier, we open the aperture, and that’s it, right?
By opening the aperture, the image is totally overexposed because there is too much light entering the sensor, the iso is already as low as it can go, and we want to leave the speed where it is because if we increase it, the movement does not look natural. So this is where the ND filter comes into play.
When we place it, we can work with the aperture of the camera quite open without touching any other parameter that will damage our video.
Another function of the ND filter that I love is for taking pictures like this one:
See how the water looks like silk? Super cool, isn’t it?
Well, these photos were taken at a very, very slow shutter speed, but the problem is that during the day, a lot of light enters the sensor and not even by closing the lens aperture to the maximum and lowering the iso to the minimum, we can’t avoid an overexposed picture. That’s why we use the ND filter to reduce the amount of light entering the sensor and get the desired results.
There are several types of ND filters, but I will talk here about fixed and variable ND filters.
Fixed filters are those that have a fixed neutral density, as the word says.
For example, if you see an ND filter that has a number 8 or ND8, it darkens the image to 3 stops, which is like closing the diaphragm of the lens 3 points down.
The problem with fixed ND filters is that you need to have several filters with different densities, which can be very expensive, and that is why there are also variable ND filters.
Variable ND filters (which are more practical ; ) These have a ring where we can adjust the filter’s intensity.
For example, with the one in the picture, I can move between 1 stop and five stops; this would be like having 5 ND filters in one.
And you’re probably asking yourself, then why the hell do fixed filters exist if variable filters come out better? And the answer is image quality. Many variable filters change the colors of the pictures, and many generate a strong vignette. There are others that, when set to maximum, create an X in the image.
ND filters come depending on the diameter of the lens. For example, this is a Sony 16-35 lens, which has a diameter of 82mm; we see this number on the edge of the lens or on the cap.
There are many more brands of ND filters on the market, and costs increase or decrease depending on quality.
The variable ND filter that I used for this example is the True Color from NiSi; you can check their products on their website.