ND filter is a very useful tool in the filmmaker’s toolkit. But, as I see on the internet, there are some problems and myths in what those filters are giving us. I’ve seen a bunch of times some weird pieces of advice, like you should use ND filter to avoid blown out sun or flicker in timelapse. So, from this post you’ll learn and hopefully understand how ND filters work and what they give us.
This is very basic tutorial, for more advance stuff you can check out Should you use ND filters? Motion Blur in Timelapse.
What is an ND filter?
ND filter is a piece of glass or plastic that you put in front of your lens to block some part of light that’s hitting the sensor. ND stands for Neutral Density, because the color of the filter should be neutral, I mean that it shouldn’t change any colors on your video, just exposure. I said shouldn’t, because ND filters usually cause some color shifts, usually green or magenta, sometimes blue. The cheaper the filter the stronger the tint.
There are different density filters. The table below shows some of them. The higher the ND number the more light it blocks. For more in depth description including different types of ND filters and this table explanation check out my post ND filters in Timelapse
The main misconception
ND filters change the exposure, so the brightness of the whole image. Sometimes people think that when they use the ND everything will keep the same exposure but for example they’ll get darker sun that was previously blown out. That’s very far from true. ND can’t change the exposure of some part of the image and keep untouched the other. That’s just not how that filters work. When it comes to image brightness, using the ND gives you exactly the same result that using shorter shutter speed or closing down the aperture (we’re talking just about exposure, not motion blur or depth of field). So, if getting the shorter shutter speed doesn’t help the sun look better on your image neither, does ND filter.
Of course there is something like gradual ND filters, that are used to darken just a part of the image, but they have quite limited usage in video. The darkened part of the filter is a square or of perpendicular shape. You can rotate it on the lens but there should always be a fairly straight line between the part which should be darkened and which shouldn’t. It can be used for static camera shots. It’s more popular in photography, because of the camera movement limitations.
Why we use NDs?
There are two basic reasons for using ND filters – to lengthen the shutter speed (for example we use NDs for that in timelapse) or to keep the aperture open (or both).
For the shutter speed – there is something like the 180 degrees rule in filmmaking, which says that you should use 1/2xfps to get a natural motion blur in your film. So, for 25fps use shutter speed of 1/50 s. Of course on a bright sunny day it’s hard to keep such shutter. There are 3 basic exposure parameters, so settings responsible for how bright your shot is. It’s aperture, shutter speed and ISO. During daytime you keep your ISO low to avoid noise. Let’s assume that you choose the aperture and you don’t want to change that. Then you should adjust the shutter to the light conditions. However if you want to keep it according to the 180 degrees rule, you should add 4th exposure parameter – ND filter. The stronger the filter is, the longer shutter you’ll be able to set.
Let’s say you set all the exposure parameters as described above and you get shutter speed of 1/800 s. To keep it at 1/50s you should block 4 EV of light, so decrease the light hitting the sensor by 16 times – 2^4 (check out the table above to understand relationship between amount of blocked light and exposure values). To do that just use the ND4 filter.
In timelapse we use lower fps values (when it comes to shooting, not displaying) so according to 180 degrees rule we need to get longer shutter speed. For example 1 second shutter speed for 2 seconds interval between pictures (so between each frame in final video). That’s why we use very strong ND filters.
NDs for drones
I feel like the most of the misconceptions come from drone pilots. ND filters for drones are popular, lots of people use them and not everyone knows why. The main reason is to lengthen the shutter speed. We get a natural motion blur then. Also, when shooting against the sun there are sometimes propellers shadows visible on the video when shot with the short shutter speed. Then also NDs are helpful.
The second reason is the aperture. To get nice cinematic shots we usually want to keep the aperture open, so low F value, like F1.4, F2 etc. This gives us this lovely shallow depth of field – in other words we get this lovely background blur. If we have too much light in the scene we could shorten the shutter speed and keep the aperture open, but for the reasons described above we don’t want to do that. So, the easiest way is to use an ND filter.
We have ISO100, shutter speed at desired 1/50 sec, but to keep the right exposure we need to stop down the lens to F8. We would like to use F2 to blur the background behind our subject. The difference between F2 and F5.6 is 3 EV (F2 > F2.8; F2.8 > F4; F4 > F5.6 – each one is 1 EV), so we need 6 times less light. We simply put the ND3 in front of our lens and adjust the aperture.
Let’s assume we don’t have ND3, just ND4 (and we don’t have any variable filter). Then we use ND4 and change ISO for 200 or lengthen the shutter speed to 1/25. Usually ISO200 is very good quality so of course I would choose that.
Pretty much those are the reasons we have NDs in film cameras like Sony FS5, Canon C100 etc. Thanks for reading the whole article 🙂 I hope you’ve learned something. Maybe you’ll be interested in watching my latest hyperlapse project, made just with a smartphone? 😉 >> Krakow Main Square – a Hyperlapse Walk + Making Of
Great explanation of ND filters! thank you!