Cinematic Depth of Field

The following depth of field methods provides a cinematic look that closely resembles photography and film for desktop and console platforms using the Deferred Shading renderer and Clustered Forward renderer.

Cinematic

Cinematic DOF closely matches real-world cameras, similarly to Circle and Bokeh DOF, you can see circular-shaped Bokeh (out-of-focus areas) with sharp High Dynamic Range (HDR) content. This method uses a procedural Bokeh simulation that provides dynamic resolution stability and alpha channel support while being faster, scalable, and optimized for projects developing on desktops and consoles.

Focus on the Shot's Subject

The key to achieving an aesthetically pleasing depth of field effect is all in how the subject(s) of the shot is focused. There are three core factors that affect setting up Depth of Field for any given shot:

• Decide on the Lens Focal Length to use.

• Choose an appropriate Aperture (f-stop number).

• Choose a Distance to your Subject(s) from the camera.

Let's break down the elements that make up the camera and scene being captured to understand what's going on when we adjust these settings:

1. Points in the Scene

   1. Red: Out of Focus

   2. Blue: Perfectly in Focus

   3. Green: Marginally in Focus

2. Focus Distance to Shot's Subject (1. ii)

3. Camera Lens

4. Lens Diaphragm (measured in F-Stop)

5. Lens Focal Length

6. Filmback / Image Sensor

7. Final Image Result

Take note that the rendered image on the right is inverted through the camera lens. Red is part of the background and more out of focus, and Green is part of the foreground and marginally out of focus.

Here we have the multiple points (1) representing objects captured by the camera to a defined Focus Distance (2), in this case, the blue subject. The Aperture (4) defines how blurred objects in the foreground and background will be that are not in focus, and, finally, the Focal Length (5) of the lens controls the field of view, or how zoomed in the image will be.

Aperture

The Aperture defines how sharp or blurred the foreground and background are based on the diameter of the Diaphragm which is controlled in f-stop.

1. Camera Lens

2. Lens Aperture Diaphragm measured in F-stop

This diagram demonstrates the Diaphragm (2) blocking light passing through the Lens (1). The size of the Aperture (or f-stop number) sets how much light passes through the Lens in turn controlling how much the foreground and background are blurred based on the Focus Distance of the shot.

Two elements make up the Aperture; the F-stop number and the Diaphragm.

1. The Aperture is the diameter of the opening through which light passes. The Aperture is defined by Focal Length divided by an F-stop number.

2. The Diaphragm is a mechanical structure made up of multiple blades used to block light. It opens and closes based on the supplied F-stop.

The lens Aperture is defined by the diameter of the Diaphragm which decreases as the F-stop number increases controlling how shallow or deep the depth of field effect is. See Focus Distance for a diagram showing this effect.

To demonstrate how the Aperture works, drag the slider to change the F-stop number between 1.4, 2.8, and 5.6. Smaller F-stop numbers create a shallower DOF with more background and foreground blur. Larger F-stops increase the depth of DOF with less blur.

Note that only the F-stop is being changed here, the Focus Distance and Focal Length remain constant at 7m and 75mm, respectively.

Drag the slider to change the Aperture F-stop.

Normally when you would adjust the Aperture settings of a real camera, you'd have to also adjust the Exposure settings at the same time to maintain the same light intensity received by the filmback/image sensor. However, in Unreal Engine this is not a real camera, so adjusting the F-stop and Diaphragm does not control the light intensity.

Focal Length

The Focal Length is the distance from the center of the lens to the Filmback (or image sensor) measured in millimeters (mm).

1. Camera Lens

2. Focal Length

To demonstrate how this works, drag the slider to change the Lens Focal Length to be between 50mm, 75mm, and 100mm. Notice how effectively it changes the shot's Field of View (FOV); as the focal length increases, the FOV decreases. You can think of the focal length as being similar to zooming in with a camera, except when doing so, the out-of-focus areas of the shot in the background and foreground are becoming more pronounced. In the example below, you can see this when changing between a focal length of 50mm and 100mm; the shot appears mostly in focus throughout the foreground but when using a 100mm focal length, it's easier to see that the background is more out of focus and the foreground has a minimal amount of blur too.

Note that the diagram and example here are only changing the lens Focal Length; the Focus Distance and F-stop remain constant at 7m and f/2.8, respectively.

Focus Distance

The Focus Distance is the distance from the center of the camera lens to the subject of the shot that is put into focus, creating a focal plane. The closer the camera is to the subject, the more the background will end up out-of-focus.

1. Focal Points in the Scene

   1. Red: Out of Focus

   2. Blue: Perfectly in Focus

   3. Green: Marginally in Focus

2. Focus Distance to the Subject (1b)

3. Camera Lens

This diagram demonstrates objects (1) that make up the scene; background, shot subject, and foreground. The distance from the Lens (3) to the focal subject (2) - in this case the blue dot - is the Focus Distance. Areas in the background and foreground are blurred depending on the F-stop used and the Lens Focal Length.

We can visualize the DOF Layers by using the Show dropdown in the Viewport to select Advanced \> Depth of Field Layers. In this instance, Green represents the foreground objects, Black is the area where objects are focused, and Blue is part of the background.

To demonstrate how this works, drag the slider to change the Focus Distance between 4m, 7m, and 10m. Adjusting the Focus Distance adjusts the Focal Plane (purple dotted line) in the scene, indicating where the in-focus region of the shot is. The blue reference point in the diagram uses the Character as the focal subject here, so as the camera shifts shot focus (black/gray area above) to different parts of the scene, objects in the foreground and background will shift too. Starting with 4m, the Police car in the foreground is in sharp focus and the character is out of focus in the background. The blue reference in the diagram indicates this with its focal point intersection in front of the filmback. At 7m, the character is completely in focus with background and foreground objects out of focus. At 10m, the construction barriers are in sharp focus and the character and a Police car are out of focus in the foreground. The blue reference in the diagram indicates this since the intersection point extends beyond the filmback. Through all these Focus Distance changes, the DOF effect increases or decreases depending on an object being in the foreground or background.

Note that only the Focus Distance is being changed here; the F-stop and Focal Length remain constant at f/1.4 and 75mm, respectively.

Procedural Diaphragm Simulation

The job of Depth of Field is to apply focal importance to a subject(s) in your scene. Just as importantly, out-of-focus areas can be as important. Unreal Engine 4's depth of field simulates the lens diaphragm by enabling you to aesthetically control the shape of the Bokeh (out-of-focus areas) by adjusting the number and curvature of diaphragm blades.

For performance reasons, the diaphragm simulation is only supported on Engine Scalability settings of Epic and Cinematic. Lower quality settings will fall back to circle Bokeh shapes of equal area to ensure similar behavior with auto exposure functionality, in turn disabling the diaphragm blade simulation.

Number of Diaphragm Blades

The Diaphragm (or Iris) is a mechanism made up of blades that can adjust to aesthetically inform the look of Bokeh shapes. Using a lower number of blades will result in a polygon-shaped diaphragm whereas a higher number of blades improves the curvature of the diaphragm to appear more natural with rounded out-of-focus Bokeh.

The number of diaphragm blades directly influences the shape of the Bokeh when the Maximum Aperture is increased (minimum F-stop number is decreased).

Aperture and Max Aperture

The Aperture is the opening of the lens controlled by the diameter of the Diaphragm which is measured in F-stops. The amount of light allowed to pass through the lens is controlled by the size of the aperture. Adjusting the size of the aperture sets the focal plane to adjust how in-focus or out-of-focus areas outside of the focal region are.

The following chart shows the correlation between aperture size (f-stop number), max aperture (min f-stop number), and depth of field effects.

A larger aperture (lower f-stop number) has a shallow focal region, causing the foreground and background to become more out-of-focus. A small aperture (higher f-stop number) has a deeper focal region that includes more objects in focus in the foreground and background.

Aperture F-stops: 1.4, 2.0, 2.8, 4.0, 5.6, 8.0

While the Aperture can be as small as you'd like, it can only ever be as large as the maximum size of the lens. The Maximum Aperture (measured in f-stop) defines how large the lens can be. This also defines the curvature of the blades that make up the diaphragm by limiting the size that the aperture can open; limiting the size of the aperture shortens the effective focal region which limits how out-of-focus areas can be.

5 Diaphragm Blades with Different Max Aperture Values

In this example, the Number of Diaphragm Blades is set to 5. With a smaller Aperture, the blades of the diaphragm become more visible in the Bokeh shapes. Using a larger Aperture (f-stop is equal to minimal f-stop) the Bokeh shapes will begin to become circular.

In this example, a Cine Camera Actor is being used with the following values:

• Min F-Stop: 1.4

• Max F-Stop: 4.0

• Diaphragm Blade Count: 5

• Current Aperture: 1.4 - 4.0

With these values, the F-stop can only accept values between 1.4 and 4.0. As the depth of field effect is deepened with a larger F-stop (smaller aperture), the diaphragm blades become more visible in the Bokeh shapes.

Keep in mind that the aperture is not controlling the light intensity. This choice has been made so that you don't have to continually adjust the exposure at the same time as with a real camera.

Cinematic DOF Settings

Cinematic Camera

The following settings are specific to the Cinematic Camera Actor. Note that you have access to the Camera and Depth of Field settings as well.

Post Process Volume and Camera Actor

The following settings are available with the Cine Camera Actor, Camera, and Post Process Volumes.

Useful Console Variables for Optimization

The challenge of DOF is that it can vary depending on your content and how much importance has been given to the artistic direction of your game with DOF. With that in mind, the DOF implementation offers a variety of console variables ready to be customized per platform for developers to better control the performance budget allocated. 

Below are some console variables that can be useful for binding Cinematic DOF performance.

• r.DOF.Kernel.MaxBackgroundRadius: The maximum size of the background blurring radius in horizontal screen space.

• r.DOF.Kernel.MaxForegroundRadius: The maximum size of the foreground blurring radius in horizontal screen space.

• r.DOF.Scatter.MaxSpriteRatio: The maximum ratio of scattered pixel quads as sprites. Useful to control DOFs scattering upper bound.

Note that these are a good starting point and additional console variables can be found under r.DOF.*.

Bokeh DOF Method (Legacy)

As of Unreal Engine 4.23, Bokeh DOF has been removed from Unreal Engine and the source code. If using 4.22 and 4.21, Bokeh DOF has been deprecated for the Deferred Desktop Renderer and the Forward Rendering path.

Bokeh DOF refers to the name of the shape that can be seen in photos or movies when objects are out of focus. This method renders a textured quad for each pixel using a user-specified texture to define a shape with which to reproduce the effect of a camera lens.

The implementation only uses half-resolution to perform this intensive effect. It attempts to save render performance in areas where the effect is not noticeable by using Adaptive Depth of Field. Bokeh DOF is more costly than other DOF methods available in Unreal Engine making this a prime candidate for cinematics and showcases where appealing visuals often outweigh any performance concerns in those situations.

Adaptive Depth of Field for Bokeh DoF

Bokeh DOF is rendered at quarter resolution (half resolution in each direction) by default for performance reasons. In most cases, the downsampling is barely noticeable, which is perfectly acceptable; however, in some situations, it can lead to artifacts and unwanted results. Adaptive Depth of Field resolves these types of artifacts where possible.

The characters being blurred in the background appear blocky when using only the downsampled DOF technique. Also, some artifacts appear around the horns of the character in the foreground. Adaptive DOF eliminates these types of artifacts, and the background characters have a much smoother appearance.

Adaptive DOF can be visualized by using the Adaptive Depth of Field to show the flag in the Level Viewport under Show \> Visualize. When enabled, an overlay is shown where the downsampled effect is being used (green) and where the full-resolution effect is being used (red). The regular scene color is shown where no blurring is being applied.

In general, you want the Adaptive DOF to display all green. The more red shown by the visualization, the more expensive the scene is to render each frame.

Bokeh DOF Settings

The following settings are available with the Cine Camera, Camera, and Post Process Volumes, located under the Lens tab in the Depth of Field section.

External Resources

• SIGGRAPH 2018 - "A Life of a Bokeh" by Guillaume Abadie (PowerPoint presentation ~150 Mb)

• Diaphragm (Optics)

• Aperture

• F-Number

• Understanding Factors that affect Depth of Field