Sometimes, you can look at a scene and feel that it’s so sharp, so perfect, that it’s verging on the harsh. It can be overwhelming, and you might want to tone it down a bit, but not at the expense of making your shots blurry. The solution to a softer but not blurry scene might well be to use a diffusion filter. If you’ve never heard of a diffusion filter among the array of camera filters out there and aren’t sure where to start, start here. We’ll explain what diffusion filters are, how they work, and how best to use them.
What is a diffusion filter?
Diffusion filters produce hazy, dream-like or even romantic looks to your images, but, very importantly, they do so without a loss of contrast or sharpness. It’s entirely different from a blurred or out-of-focus image; instead, a diffusion filter will give you a soft haze around the bright objects in your scene and help to lift the shadows a little, too.
A diffusion filter has an etched surface. As light rays meet the filter, these etched imperfections bend some but not all of the rays off of their original path, defocusing them as they spread across the uneven surface. All this creates the filter’s typical hazy look. You will also find that the patterns etched onto the filter will give you different looking bokeh to the usual shape formed by your lens.
When to use a diffusion filter
It’s quite common to hear that a diffusion filter makes your work look “cinematic”. That probably has a lot to do with it replicating the feel that you get from old-time movies, with their lenses, lighting, film and even the use of nets over lenses. Rather than using a diffusion filter generally for that cinematic look, try to keep your uses more focused and specific.
A diffusion filter can certainly signal to your audience that you are stepping back in time, first with that old-time movie feel. But also because the softer feel from a diffusion filter can make something feel further away or like a memory.
Of course, not every film set in the past will need a diffusion filter, but it is a useful tool if you want it.
The hazy feel you get with a diffusion filter is great for creating dream scenes in your movies. It’s soft, floaty and light.
Modern lenses can be so sharp that they do very little to flatter your talent’s skin, giving definition to every pore. By using a diffusion filter, you can smooth out their skin and provide a gentle anti-wrinkle effect. Be careful not to overdo it, though. You don’t want plastic-looking skin, either.
Diffusion filters also work to lift the shadows, making them a little less dark. That helps with those dreamlike scenes but can also suggest a bit of romance or gentleness.
How do you use a diffusion filter?
There are plenty of diffusion filters on the market, made by manufacturers such as Tiffen, Lee, Hoya and Moment. Diffusion filters come in different strengths, and manufacturers tend to use their own grading systems, a bit like ND filters.
The Tiffen Pro-Mist filter is probably the most widely used diffusion filter on the market. It ranges in density from 1/8 to 5, with 5 being the strongest.
There’s also the Black Pro-Mist, which is a slightly darker version of the Pro-Mist. Moment has the CineBloom diffusion filters. Moment doesn’t have the same range of densities as the Tiffen Pro-Mist and Black Pro-Mist filters, though. These come as 10% density (about ½) and 20% density (1).
When it comes to using your diffusion filter, first and foremost, make sure that your scene, and in particular your subjects, are well-lit. The filter works with the light in the scene, so if there isn’t enough, you won’t enjoy much of the effect. However, aim for soft light rather than very strong or harsh light. That can result in things just looking washed out and drab.
Your focal length will affect the level of diffusion offered by your filter. The longer the focal length, the greater the impact a filter will have on the scene. At the same time, it’s easy to lose too much detail when you use a diffusion filter with a wide-angle lens - the diffusion pattern overwhelms the details. To prevent this, use either the lowest level of diffusion available or tighten up your shots.
Drawbacks of a diffusion filter
We’ve already mentioned the “cinematic look” that a diffusion filter can give your work. But cinematic looks come from a great deal more than just diffused light and hazy glow. You don’t want to become overreliant on diffusion filtering. Instead, make sure that using one really contributes to the story that you’re telling.
It’s also impossible to recreate the halation effect associated with celluloid using a digital camera. Halation occurs when light is reflected through the film emulsion from the back surface of the pressure plate. It shows up as the edges of highlights looking blurred, but it doesn’t impact the shadows. Using a diffusion filter can bring you close to this digitally, but it won’t be the same. In particular, it will lift your shadows so your blacks won’t look as deep and dark as they should if you’re trying for halation rather than diffusion.
How to get the glowing effect without a diffusion filter
While there really isn’t anything quite like a diffusion filter, you can simulate the effect in other ways, too. The first is a practical method rather than a post-production process: use a net or silk stocking across your lens. It’s very old-school, but you can’t guarantee consistency. This was the softening technique that I was taught for portraits when I first picked up a camera as a child.
Instead of using a filter like the Tiffen SoftNet–or indeed a net–to give your talent flawless-looking skin, you can do it with post-processing using something like After Effects. Magic Bullet Looks is a plug-in that can give you a look that resembles having used a diffusion filter, too.
The look of a diffusion filter is unique and brings a refined quality to your films. But do be careful not to overuse it. It’s the same with anything from depth of field to gimbal shots and timelapse video; you need to ask yourself why you’re using it and what it brings to your story.