A majorly misunderstood item that is almost a must in every photographer and videographer’s kit is the ND, or Neutral Density filter.
If you have a camera such as the Blackmagic Design Pocket Camera, then there is an ND system built in, as it is in the Panasonic X1. But at the other end of the scale, if you have bought a DJI Air2S drone with the Fly More Combo kit, then there is a packet of ND filters supplied and each needs to be added to the camera lens as needed.
So when is an ND filter needed (or desirable)? What does an ND filter do anyway?
The image you get from a camera or camcorder – and this includes smartphones – is reliant on a number of factors to get it “correct”. These factors are primarily aperture, shutter speed and ISO.
As most will be aware, shutter speed is the amount of time the shutter is open to let light on to the image sensor (or film in the old days) and is usually measured in fractions of a second. For the sake of video, common shutter speeds are 1/50th and 1/60th.
As an aside, if you are also playing around with frame rates, your shutter speed should be set to twice the frame rate so, if for example, you are shooting at 25 frames a second, then your shutter speed should be 1/50th of a second. If you are using the “cinematic” option of 24 frames / second, then as most cameras cannot shoot at 1/48th, then still set it to 1/50th. Shooting at 29.97 frames a second, shoot at 1/60th.
Aperture is the setting for the AMOUNT of light reaching the image sensor and is measured in f stops. Annoyingly, the smaller the number, the wider open the aperture is so MORE light gets in. So f/2.8 will let a lot of light in and f/22 a minimal amount.
Aperture is also fundamental in controlling depth of field which is a whole different subject and I have a tutorial on that here.
Finally, ISO is a measure of how much reaction there will be to the amount of light getting in, and again, in the old days of film, was a consequence of the chemical makeup of the film. The higher the ISO number (called ASA back then) the more sensitive to light it was. As a rough rule of thumb, you would shoot in bright sunlight at between 100 – 200 ISO, under cloud at 400 ISO and increase ISO as it gets darker. We used to shoot rally cars in the forests at night using 1000ASA film.
The higher the ISO though, the grainier the image becomes and so you also need to balance your shutter speed against this. So, as another rule of thumb, in photography, as you increase the ISO, increase your shutter speed.
Right, so after all that, where does an ND filter fit in?
An ND filter’s sole role in life is to reduce the amount of light entering the lens. This allows you to select combinations of aperture, exposure time and sensor sensitivity that would otherwise produce overexposed images.
Imagine you want to shoot waves breaking over rocks. This might need a shutter speed of 10 seconds to get the blurriness of the water you want, but of course, on a bright day using standard settings, this would result in a w-a-y over exposed image.
So enter the ND filter which reduces the intensity of the brightness without affecting colour hue. This then allows you to use longer shutter speeds in conjunction with aperture settings, allowing a large chunk of creativity to be used in how you take your shots!
Have a squizz at his images to get an idea of the breadth of possibilities you’ll have.
ND filters are labelled according to their density and in multiples of 4. So an ND4 filter is not as dense (lets in more) than an ND 32 for example.
The filters supplied by DJI for my Air2S are in the range 4, 8, 16, 32, 64, 128, 256 and 512 and the built in ND filters on my Blackmagic Pocket Cinema Camera 6K has built in 2, 4 and 6 stop ND filters.
(If you want a really good deeper technical explanation on ND filters and f-stops etc, have a look at https://en.wikipedia.org/wiki/Neutral-density_filter by the way.
Like almost everything in video / photography, having the technical knowledge is no substitute for getting out there and experimenting to see what different settings do.
To this end, as I have said many times, take a small notebook with you and write down each shot and the settings used so that later, you can compare them in post-production and see what suits best (and therefore what settings to use in a particular circumstance).
If you are shooting in RAW, this also helps of course.
The attached samples are taken using my DJI Air2S at midday in bright summer sun over water. Note the difference – one without the ND filter and the second with a ND8 on it.