The Special Filter That Most Cinematographers Use

INTRODUCTION

One of the great things about working as a camera assistant for a number of years was that it exposed me, if you’ll excuse the pun, to the camera gear that was selected by a number of international cinematographers. 

In the past, these selections were often shielded by DPs, who were hesitant to share their exact recipe and formula for creating images.

One of the biggest takeaways from my time as an assistant was that many, if not almost all productions which were shot on digital cameras, used what are called diffusion filters.     

WHAT ARE DIFFUSION FILTERS?

Diffusion filters are pieces of glass which can be put in front of the camera’s lens that provide a glowing halation to the highlights, soften the image, or reduce contrast.

These filters can either be placed in a tray inside a mattebox which is clipped onto the lens, or screwed directly onto the front filter thread of the lens. 

When light hits a digital camera sensor with enough intensity to overexpose it, a number of pixels will turn to pure white, while the surrounding pixels that absorb less light won’t clip. 

However, when a diffusion filter is added in front of the lens, the light will get scattered. This means that there will be more of a gradient between the pure white, overexposed pixels in the highlights and the correctly exposed pixels. This gives the bright highlights a glowing effect called halation.

Some secondary effects which these diffusion filters may also have is that they add a layer of softness or blur over the image, thus reducing the lens’ sharpness. This decreases the harshness of how details are captured, which means wrinkles, pores and imperfections on the skin will be softened.

Some diffusion filters are also used to reduce contrast. So instead of capturing punchy, high contrast images with bright highlights and deep shadows, some diffusion filters will decrease contrast, retaining and lifting the information in darker parts of the image and crushing the intensity of the brightest parts of the frame.

In the past, before the rise of using glass softening filters in front of the lens, another old school technique which achieved a similar look was to place nets or stockings behind the back of the lens and between the film plane or sensor so that they didn’t come into focus, but they softened the frame. In old Hollywood this technique was particularly popular when photographing the close ups of lead actresses.     

CLEAN VS DIFFUSED LOOK

Now that we know what some of these diffusion filters do, why is it that they are used so often by cinematographers?

In a previous video I’ve mentioned the idea of creating a look that is very clean versus a look that is more textured.

Although some filmmakers do indeed lean into a clean, sharp, digital look, more often than not, cinematographers like to take a little bit of the digital edge off to make images look a little more filmic, soft and textured. They do this by using a low strength diffusion filter that is not too heavy handed.

FILM VS. DIGITAL

There’s a bit of an interesting dichotomy to finding a middle ground between a clean look versus a textured look when it comes to working with film or digital. Film, by its very nature, possesses many of the qualities that diffusion filters provide. Film doesn't have a sharp clipping threshold like a digital sensor does, meaning the highlights are more muted with a natural halation effect.

It also resolves images less sharply than high resolution digital cameras, having a natural, subtle softness.

Therefore, to counteract the natural ‘softness’ of film, many DPs who shoot on film like to do so with the sharpest, modern lenses possible and without using diffusion filters. Whereas those who shoot digitally try to fight the ‘overly-sharp’ feeling of digital by choosing vintage lenses, shooting through diffusion filters, or doing both.    

TYPES OF DIFFUSION FILTERS

Different diffusion filters come in different strengths of intensity: all the way from full 80s music video softening to a very subtle blooming of highlights.  

Likewise, different kinds of diffusion filters do different things. I’d say that there are three main categories: halation expansion and softening filters, contrast reduction filters, and atmospheric filters. 

Halation and softening filters target the highlights and create a blooming effect around them, while at the same time softening the overall sharpness of the image. Probably the most popular example of this filter is the Black Pro-Mist from Tiffen, which can comes in variations all the way from the very subtle 1/8th filter which subtly halates highlights to a 2 and above, which heavily blooms lights, washes out the contrast in the shadows and gives the entire frame a soft look.

Some other popular softening filters are Black Satins, Glimmer Glass, Hollywood Black Magic, Pearlescent and Classic Softs. These all have similar but subtly different ways of spreading the highlights and softening the image.

The second category are contrast reduction filters - which, as you would imagine, decrease the picture’s overall contrast by lifting the detail in the dark parts of the frame and reducing the thickness of darker hues, to produce a flatter look.

The Ultra Con and Low Con filters are probably the most popular varieties. These lift the shadows and wash out the blacks and dark areas so they aren’t as deep and heavy. 

Finally, there are atmospheric filters, which give the illusion that there is smoke or haze in a space. This is a useful tool for sets where it’s not possible to bring a smoke machine, or for wide exteriors where adding haze is difficult or impossible.

It’s also been used on shows like The Marvellous Mrs. Maisel to increase and add to the thickness of the haze in the room. 

Rather than focusing on only blooming the peaking highlights, like regular softening filters do, these atmospheric filters give a halation effect across the midtones and shadows in the entire frame, also lifting the detail in darker areas - just like smoke does. 

Some examples of these atmospheric filters are Fog and Smoque filters from Tiffen.

DIGITAL DIFFUSION

Most DPs tend to air on choosing low strength diffusion filters which have a subtle effect on the image, since this decision bakes the diffused look in and is not reversible in post.

If, later on in the grade, the amount of diffusion is not sufficient, it can always be accentuated and added to with software. Some filmmakers even favour adding all diffusion effects during post, rather than with filters, as it gives them complete control over the intensity of the effect that they want.

For example, Bruno Delbonnel changed his workflow from using physical diffusion filters in his early work, to shooting his later work as clean as possible, without effects filters, and adding a digital bloom effect in post that mimicked a softening filter. 

Other DPs, like Erik Messerschmidt, have also mentioned their preference for shooting a clean image and then imposing a filtered effect during the colour grade, such as on The Killer where he used a Da Vinci Resolve plugin called Scatter to get a Pro-Mist like halation effect which he could carefully control to his liking. 

WHEN NOT TO USE DIFFUSION

Although, on average, cinematographers like to shoot with a low strength halation softening filter like an 1/8th Pro Mist, 1/8th Glimmer Glass or a Black Satin 1, there are some occasions where they may want to remove it altogether.

This is often done when shooting in bright environments with a lot of reflected light - for example on a white, sandy beach or in the snow. Adding diffusion may push highlights into blowing out and therefore cause a loss of detail.

When light is shined directly through a filter and into a lens, it may flare in undesirable ways, or create a ghosting effect where a distracting outline of the filter itself is superimposed over the image.

Although the current trend is to pair digital cameras with a light diffusion filter, ultimately the decision comes down to the taste of the filmmaker and the story they are trying to tell - where some may be better suited to a hard, sharp, clean look, while others may benefit from a glossy, soft, blooming haze.