Most DSLRs and mirrorless cameras don’t have integrated neutral-density (ND) filters. Without ND filters, you might find it difficult to properly expose your images, especially when you are filming on sunny days or in areas with a lot of bright colours (e.g. snow, deserts, etc). A mistake that most beginners make, is that they lower their ISO, adjust their shutter speed, and squeeze their aperture to properly expose their image. This indeed could result in a properly exposed image, but it will negatively affect your overall image quality and look. I will briefly explain how to expose using a variable ND-filter.
But first, let’s talk about the exposure triangle.
ISO
This has become a popular term in the filmmaking world. I personally think it’s overrated. I can technically describe what ISO refers to, but I will keep it simple for now. ISO - also known as ASA or EI - simply refers to the sensitivity of the image sensor. EI (Exposure Index) is essentially the same as ISO, but it involves a few other factors that relate to exposure. I’m not going to dive too deep into that. I’ll stick to the basics and keep it simple.
The higher the ISO, the more sensitive the sensor becomes thus allowing more light to process. It’s good to know that a visual artefact comes along with higher ISOs: noise. Noise can be seen as tiny dots on the image and it could make your image look hazy. Every sensor handles noise differently. Sometimes you’ll notice more magenta in noise, and sometimes more green. This could affect the skin tones of the people in your frame and overall image quality. It’s good to know your sensor and what it does with higher ISOs. Most cameras have a base ISO, and some cameras have dual-native ISO. This means that they have two base ISOs, instead of one. A base ISO is technically the best ISO with the biggest dynamic range and least amount of noise. If you have an overexposed image and your base ISO is 640, I wouldn’t recommend you to lower your ISO. A lower ISO can have more noise than your base ISO.
Shutterspeed
When I started my film studies in 2008, I found it hard to understand what shutter speed exactly was. We can compare the shutter with curtains from a theatre. The object behind the curtain is the image sensor (back in the days, it was film). When the curtains quickly open, light is allowed to fall on the sensor. The longer the curtains stay open, the more light falls on the sensor. Obviously, the shutter from a camera opens and closes very quickly.
The most practical way to understand this is to get an old analogue photo camera and see what happens to the shutter if you, for instance, change it from 1/4 to 1/200. The numbers above always relate to seconds. So shutter 1/4 is a quarter of a second.
With shutter speed, you can control and manipulate your motion blur. You can also expose your images with it, but with filmmaking, we tend to stick with the 180-degree shutter. This is because the 180-degree shutter creates natural motion blur.
The 180-degree shutter incorporates the frame rate. It’s usually twice the amount of the frame rate. So if you are shooting 25 frames per second, your shutter speed will be 1/50. If you are shooting 50 frames per second, your shutter speed will be 1/100. If you are shooting 24 frames per second, your shutter will be 1/48 (technically, this is a 178-degree shutter).
Of course, filmmaking is a creative profession, so you don’t have to stick to these numbers. You can be creative with this. When you film fast motions in front of a green screen, I recommend going for a higher framerate (e.g. 1/200) to avoid green trails of motion blur. If you want to make a character look nervous and unstable, try to film handheld on a higher shutter speed. The creators of Homeland increased the shutter speed to portray Carry Mathison as an unstable person with bipolar disorder.
Aperture
Most beginning filmmakers will change their aperture to expose their images. Especially on a wide shot, you can barely see the difference between F2.8 and F8 on a small screen. Many years ago, I used to do the same, until I became more critical and noticed that some of my shots looked hazy. It is therefore important to familiarise yourself with lens diffraction, and when diffraction occurs with your lens.
With lens diffraction, sharpness decreases at smaller apertures. In essence, diffraction is the concept that (light)waves can interfere with one another. If you want to know how your lens performs at different apertures, you can a website such as DXOmark to look up your lens. I’ve work with all sorts of lenses, from high-end Cooks to consumer lenses, and I find that Sigma lenses deliver outstanding image quality for a reasonable price.
Exposing with a variable ND filter
Ok, now we’ve been through the basics of the exposure triangle, we can talk business. Before you take your shot, I’d recommend setting your camera on its sweet spot.
First step: set your variable ND ring at the lowest setting (letting as much as light in).
Ask yourself the following questions
⇢ At what focal length will I film my shot?
⇢ What is my frame rate?
⇢ Is your camera set at (one of) the base ISO?
⇢ What is your shutter speed? Is it twice your frame rate (180-shutter)? Or do you want to film this shot at a different shutter speed?
⇢ At what aperture will I film this shot?
⇢ Is my white balance set properly?
Ok then! Now you have consciously chosen your settings to create the look you want. The next step is to look at your exposure tools (waveform, histogram, false colour, etc.) to see if you have a properly exposed image. If you are overexposed, twist your variable ND filter and use exposure tools as a reference. You can use the false colour tool to analyse your image on more specific areas (e.g. the face). It might be the case that your character is standing in the shadow and that the background is very bright.
If you are underexposed because you are filming during twilight or at night, consider increasing your ISO, open up your aperture or even better: bring in extra light. Just be aware of the potential artefacts that come along with ISO and aperture adjustments.
Choosing the right variable ND filter
There are many brands on the market that offer variable ND filters. I have worked with multiple brands and types, but I cannot recommend one specific model or brand. I can recommend you to choose high-quality glass and not go for something cheap and flimsy. Remember, a bad piece of glass will highly affect the light rays coming in your lens, and therefore diminish your image quality.
I can also recommend taking a bigger filter size and work with step down rings. My 24-70mm has a filter size of 78mm, but my variable ND filter is 86mm. I use a step down ring to make it fit. If I don’t use step down rings, I have to get a variable ND filter for every lens, and that might become expensive!
I hope this article was easy to read and that you understand the importance of (variable) ND filters. If you have any questions about filmmaking or photography, feel free to contact me. If you want to get fully immersed in filmmaking or photography, you might be interested in one of my (online) courses.