9.3 Interaxial distance and convergence
13 min read•august 20, 2024
Stereoscopic cinematography involves capturing and presenting images that create a three-dimensional effect. Two key concepts in this field are and convergence. These parameters determine how depth is perceived in 3D footage and greatly impact the viewer's experience.
Interaxial distance refers to the space between camera lenses, while convergence is the angle at which they point inward. By adjusting these factors, cinematographers can control the depth and positioning of objects in 3D space, creating immersive visuals while avoiding viewer discomfort.
Interaxial distance
- Interaxial distance is a crucial parameter in stereoscopic cinematography that determines the perceived depth of the 3D image
- It refers to the distance between the centers of the left and right camera lenses in a stereoscopic camera setup
- Understanding and manipulating interaxial distance allows cinematographers to control the depth and visual impact of their stereoscopic footage
Definition of interaxial distance
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- Interaxial distance is the horizontal separation between the left and right camera lenses in a stereoscopic
- It is measured in millimeters or inches and represents the distance between the optical centers of the two lenses
- The interaxial distance mimics the distance between human eyes, which is typically around 63mm (2.5 inches) on average
- Adjusting the interaxial distance affects the perceived depth and three-dimensionality of the captured images
Factors affecting interaxial distance
- The choice of interaxial distance depends on various factors such as the desired depth effect, subject distance, and lens focal length
- Increasing the interaxial distance enhances the depth effect, making objects appear more separated in 3D space
- Decreasing the interaxial distance reduces the depth effect, resulting in a flatter, more subtle 3D appearance
- The distance between the camera and the subject also influences the selection of interaxial distance
- Closer subjects require smaller interaxial distances to avoid excessive parallax and viewer discomfort
- Distant subjects allow for larger interaxial distances to maintain a noticeable depth effect
Relationship between interaxial distance and depth
- Interaxial distance directly affects the perceived depth in stereoscopic images
- A larger interaxial distance creates a greater separation between the left and right views, resulting in increased
- Conversely, a smaller interaxial distance reduces the separation between views, leading to a more subtle depth effect
- The relationship between interaxial distance and depth is not linear; doubling the interaxial distance does not necessarily double the perceived depth
Calculating interaxial distance
- Cinematographers can calculate the appropriate interaxial distance based on the desired depth effect and shooting parameters
- The rule is a common guideline that suggests setting the interaxial distance to of the distance to the closest subject
- More precise calculations involve considering factors such as lens focal length, sensor size, and the desired depth range
- Stereoscopic calculators and charts are available to assist in determining the optimal interaxial distance for specific shooting scenarios
Interaxial distance vs human eye separation
- The interaxial distance in stereoscopic camera setups often differs from the average human eye separation of 63mm
- Smaller interaxial distances (e.g., 25mm or 50mm) are commonly used to create a more natural and comfortable 3D viewing experience
- Using an interaxial distance that matches human eye separation can result in exaggerated depth effects and visual discomfort
- Cinematographers must strike a balance between creating a noticeable and maintaining viewer comfort by selecting an appropriate interaxial distance
Convergence in stereoscopy
- Convergence is another essential parameter in stereoscopic cinematography that influences the positioning of objects in 3D space
- It refers to the angle at which the left and right camera lenses are tilted inward to create a single point of convergence
- Convergence determines where the left and right images align, affecting the placement of objects relative to the screen plane
Definition of convergence
- Convergence is the inward rotation of the left and right camera lenses in a stereoscopic camera setup
- It is measured in degrees or as a distance from the camera to the point where the left and right images converge
- The point of convergence is where objects in the left and right views align perfectly, appearing at the same depth as the screen plane
- Objects in front of the convergence point appear to protrude from the screen (negative parallax), while objects behind it appear to recede into the screen (positive parallax)
Types of convergence
- There are two main types of convergence in stereoscopic cinematography: toe-in and parallel
- Toe-in convergence involves physically tilting the left and right cameras inward to converge at a specific distance
- Toe-in convergence can introduce keystone distortion and vertical misalignment, requiring careful alignment and post-production correction
- Parallel convergence maintains the cameras in a parallel orientation and achieves convergence through horizontal image translation (HIT) in post-production
- Parallel convergence eliminates keystone distortion and simplifies the stereoscopic workflow, making it the preferred method for many cinematographers
Factors affecting convergence
- The choice of convergence depends on the desired 3D positioning of objects relative to the screen plane
- The distance between the camera and the subject influences the convergence setting
- Closer subjects require a closer convergence point to place them at or near the screen plane
- Distant subjects allow for a farther convergence point to maintain a comfortable depth range
- The lens focal length and camera sensor size also affect the convergence calculation
- The desired depth budget, which is the range of depth from the closest to the farthest object, guides the convergence decision
Relationship between convergence and parallax
- Convergence directly affects the parallax, which is the apparent horizontal offset between corresponding points in the left and right images
- Objects at the convergence point have zero parallax, appearing at the same depth as the screen plane
- Objects in front of the convergence point have negative parallax, creating the illusion of protruding from the screen
- Objects behind the convergence point have positive parallax, appearing to recede into the screen
- Excessive negative or positive parallax can cause viewer discomfort and should be avoided by carefully setting the convergence
Calculating convergence
- Cinematographers can calculate the convergence based on the desired depth positioning of objects and the shooting parameters
- The convergence distance is often set to the distance of the main subject or the midpoint of the depth range
- Stereoscopic calculators and charts provide guidance on selecting the appropriate convergence for specific shooting scenarios
- On-set monitoring tools, such as real-time 3D displays and depth analysis software, help cinematographers assess and adjust the convergence during production
Convergence vs interaxial distance
- Convergence and interaxial distance are two distinct but interrelated parameters in stereoscopic cinematography
- Interaxial distance primarily affects the overall depth range and the perceived separation between objects in 3D space
- Convergence determines the positioning of objects relative to the screen plane and the distribution of positive and negative parallax
- Adjusting the interaxial distance changes the depth magnitude, while adjusting the convergence shifts the depth range forward or backward
- Cinematographers must balance both parameters to achieve the desired depth effect while maintaining comfortable viewing conditions for the audience
Stereoscopic camera setup
- Stereoscopic camera setups involve the configuration and alignment of two cameras to capture left and right eye views simultaneously
- The choice of camera setup depends on factors such as the desired depth effect, shooting conditions, and budget
- Proper camera alignment and synchronization are crucial for creating a seamless and comfortable stereoscopic viewing experience
Dual camera configurations
- Dual camera configurations use two identical cameras mounted side by side to capture the left and right eye views
- The cameras are typically mounted on a stereoscopic rig that allows for precise adjustment of the interaxial distance and convergence
- Common dual camera configurations include:
- Parallel: The cameras are mounted parallel to each other, with the lenses perpendicular to the baseline between the cameras
- Toe-in: The cameras are angled inward to converge at a specific distance, creating keystone distortion that requires correction in post-production
- Dual camera setups offer flexibility in adjusting the stereoscopic parameters and can be adapted to various shooting scenarios
Beam splitter rigs
- Beam splitter rigs use a single camera and a mirror assembly to capture the left and right eye views simultaneously
- A semi-transparent mirror placed at a 45-degree angle splits the incoming light into two paths, directing it to the left and right sides of the camera sensor or two separate sensors
- Beam splitter rigs ensure perfect synchronization between the left and right views and eliminate the need for post-production alignment
- They are particularly useful for close-up shots and situations where a compact stereoscopic setup is required
- However, beam splitter rigs have limitations in terms of interaxial distance adjustment and may introduce light loss due to the mirror assembly
Side-by-side rigs
- Side-by-side rigs are the most common and versatile type of stereoscopic camera setup
- Two cameras are mounted side by side on a horizontal bar, with adjustable mounts for setting the interaxial distance and convergence
- Side-by-side rigs allow for a wide range of interaxial distances, making them suitable for various shooting distances and depth effects
- They provide flexibility in camera movement and can be used with different camera models and lens configurations
- Proper alignment and synchronization of the cameras are essential to avoid vertical misalignment and temporal disparities
Alignment of cameras
- Precise alignment of the left and right cameras is crucial for creating a comfortable and visually pleasing stereoscopic image
- Misalignment can cause vertical disparities, keystone distortion, and other visual artifacts that lead to viewer discomfort
- Camera alignment involves ensuring that the cameras are level, have the same height, and are perpendicular to the baseline between them
- Alignment tools, such as laser alignment systems and calibration charts, are used to achieve accurate camera positioning
- Regular checks and adjustments are necessary throughout the shooting process to maintain proper alignment
Synchronization of cameras
- Synchronization ensures that the left and right cameras capture images at the same time, preventing temporal disparities and visual discomfort
- Genlock (generator lock) is a common method for synchronizing cameras, where a master sync signal is sent to both cameras to control their timing
- Timecode synchronization involves using a common timecode signal to ensure that the cameras start and stop recording simultaneously
- Shutter synchronization is crucial to avoid temporal differences between the left and right views, especially in scenes with fast motion or flashing lights
- Monitoring and checking the synchronization throughout the shoot is essential to avoid sync issues that can be difficult or impossible to correct in post-production
Adjusting interaxial distance and convergence
- Adjusting the interaxial distance and convergence allows cinematographers to control the depth perception and visual impact of stereoscopic footage
- These adjustments are made based on creative intent, the desired depth effect, and the need to maintain viewer comfort
- Proper manipulation of interaxial distance and convergence can enhance the storytelling and immersion of stereoscopic content
Creative control of depth
- Cinematographers use interaxial distance and convergence as creative tools to guide the viewer's attention and enhance the emotional impact of a scene
- Increasing the interaxial distance can emphasize the depth and separation between objects, creating a more immersive and dramatic effect
- Decreasing the interaxial distance can flatten the depth, focusing the viewer's attention on specific elements or creating a more intimate feel
- Adjusting the convergence allows for the placement of objects in front of, at, or behind the screen plane, directing the viewer's gaze and creating a sense of depth and space
- Creative depth control can be used to isolate subjects, establish spatial relationships, and create visual hierarchies within the frame
Avoiding excessive negative parallax
- Negative parallax occurs when objects appear to protrude from the screen towards the viewer
- While negative parallax can create an immersive and engaging 3D effect, excessive negative parallax can cause viewer discomfort and eyestrain
- Cinematographers should be cautious when using negative parallax and avoid placing objects too far in front of the screen plane
- The "30mm rule" suggests limiting the negative parallax to 30mm (1.2 inches) or less to maintain a comfortable viewing experience
- Gradual transitions and depth cues can help guide the viewer's eyes and reduce the strain caused by excessive negative parallax
Minimizing viewer discomfort
- Viewer discomfort in stereoscopic content can arise from various factors, including excessive parallax, rapid depth changes, and visual inconsistencies
- Cinematographers should strive to create a comfortable and natural 3D viewing experience by following guidelines and best practices
- Limiting the overall depth budget, which is the range from the closest to the farthest object, can help prevent excessive parallax and depth jumps
- Gradual depth transitions, such as slowly moving from positive to negative parallax or vice versa, can reduce viewer discomfort
- Maintaining consistent depth cues, such as occlusion and relative size, helps the viewer's brain process the depth information more easily
- Avoiding sudden changes in depth, such as cuts between extreme positive and negative parallax, can minimize visual fatigue and disorientation
Interaxial distance and convergence charts
- Interaxial distance and convergence charts are tools that help cinematographers determine the appropriate settings for a given shot
- These charts typically provide recommended interaxial distances and convergence settings based on factors such as subject distance, lens focal length, and desired depth effect
- The charts consider the principles of stereoscopic perception and the limits of viewer comfort to suggest safe and effective parameter ranges
- Cinematographers can use these charts as a starting point and then fine-tune the settings based on the specific requirements of the scene and creative intent
- Digital stereoscopic calculators and apps have largely replaced physical charts, providing more precise and customizable recommendations
Real-time monitoring of stereoscopic image
- Real-time monitoring of the stereoscopic image is essential for ensuring the quality and comfort of the 3D footage during production
- Stereoscopic monitors and 3D viewing systems allow cinematographers to assess the depth, alignment, and overall visual impact of the stereoscopic image
- These monitoring tools often provide features such as parallax displays, depth analysis, and warning indicators for potential issues like excessive parallax or misalignment
- Real-time monitoring enables immediate adjustments to the interaxial distance, convergence, and camera alignment to optimize the stereoscopic effect
- Collaborating with a stereographer or stereoscopic consultant can help ensure the quality and consistency of the 3D footage throughout the production
Common issues in stereoscopic cinematography
- Stereoscopic cinematography presents unique challenges and potential issues that can affect the quality and comfort of the 3D viewing experience
- Cinematographers must be aware of these issues and take steps to prevent or mitigate them during production and post-production
- Addressing these common issues is crucial for creating visually appealing and comfortable stereoscopic content
Vertical misalignment of images
- Vertical misalignment occurs when the left and right images are not perfectly aligned vertically, causing visual discomfort and eyestrain
- Even small vertical disparities can be noticeable and disruptive to the viewer, as the human visual system is more sensitive to vertical misalignment than horizontal disparities
- Causes of vertical misalignment include improper camera alignment, lens mismatches, and mechanical issues with the stereoscopic rig
- Preventing vertical misalignment requires precise camera setup, regular checks, and the use of alignment tools like calibration charts and laser alignment systems
- In post-production, vertical misalignment can be corrected through image shifting and keystone correction, but it is best to minimize the issue during production
Keystone distortion
- Keystone distortion is a perspective distortion that occurs when the left and right cameras are not parallel to each other, typically in toe-in configurations
- It results in the vertical edges of the image appearing to converge or diverge, creating a trapezoidal shape
- Keystone distortion can cause vertical disparities and visual discomfort, especially in scenes with straight lines and geometric shapes
- Preventing keystone distortion involves using parallel camera configurations or correcting the distortion in post-production through image warping and keystone correction algorithms
- The use of beam splitter rigs or parallel camera setups can help minimize keystone distortion during production
Depth budget violations
- The depth budget is the range of depth from the closest to the farthest object in a stereoscopic scene, measured in screen distances or percentages
- Violating the depth budget means exceeding the comfortable range of depth, causing excessive positive or negative parallax
- Depth budget violations can lead to visual discomfort, eyestrain, and difficulty in fusing the left and right images
- Cinematographers should plan the depth budget for each scene and ensure that the interaxial distance and convergence settings stay within the acceptable range
- Monitoring the depth budget during production and making adjustments as needed can help prevent violations and maintain a comfortable viewing experience
Edge violations
- Edge violations occur when objects in negative parallax (appearing in front of the screen) are cut off by the edge of the frame
- When an object is visible in one eye but not the other, it creates a visual conflict that can cause discomfort and break the stereoscopic illusion
- Edge violations are more common in close-up shots and scenes with objects near the edges of the frame
- Preventing edge violations involves careful framing, leaving sufficient space around objects in negative parallax, and using floating window techniques in post-production
- Floating window adjustments can be used to mask the edges of the frame and create a virtual window that accommodates the protruding objects
Ghosting and double images
- Ghosting and double images occur when the left and right images are not perfectly matched in terms of brightness, color, or alignment
- These issues can be caused by factors such as lens flare, reflections, or differences in the optical properties of the left and right camera lenses
- Ghosting appears as a faint, transparent duplicate of an object, while double images show a more distinct separation between the left and right views
- Preventing ghosting and double images requires careful camera and lens selection, matching the optical properties of the lenses, and controlling reflections and lens flare on set
- In post-production, color correction, image alignment, and disparity adjustments can help minimize ghosting and double images, but it is best to
Key Terms to Review (16)
3D Effect: The 3D effect refers to the perception of depth and three-dimensionality in visual imagery, achieved through the use of stereoscopic techniques that create the illusion of spatial relationships. This effect is crucial in cinematography, as it enhances viewer immersion and realism by simulating how human vision perceives depth through the placement of two slightly different images for each eye. Understanding this effect involves grasping concepts such as interaxial distance and convergence, which directly influence how depth is represented on screen.
Binocular disparity: Binocular disparity refers to the slight difference in the images perceived by each eye due to their horizontal separation. This difference is crucial for depth perception and three-dimensional vision, allowing the brain to merge these images into a single, cohesive view that conveys depth. Understanding how binocular disparity works helps in creating effective stereoscopic visuals, using camera alignment techniques, determining interaxial distance, and projecting 3D content accurately.
Camera rig: A camera rig is a device or structure that provides support and stability for a camera, allowing for smooth movement and precise control during filming. It can enhance the cinematographer's ability to achieve dynamic shots, making it easier to execute techniques such as tracking or matchmoving. A well-designed rig can also accommodate various accessories, which helps in capturing high-quality footage in different shooting environments.
Convergence Angle: The convergence angle refers to the angle at which two camera lenses or viewpoints intersect in three-dimensional space to create a stereoscopic effect. This angle is crucial for achieving depth perception in 3D imagery and is influenced by the interaxial distance between the cameras. Properly managing the convergence angle helps ensure that the viewer experiences a realistic and comfortable visual experience.
Depth Perception: Depth perception is the ability to perceive the distance and three-dimensional structure of objects in our environment, enabling us to understand spatial relationships. This skill is crucial in visual storytelling, as it affects how viewers interpret leading lines and visual paths within a frame, as well as how they experience stereoscopic images created by different camera setups. Effective depth perception allows filmmakers to guide audience attention and create a sense of immersion in their narratives.
Interaxial distance: Interaxial distance refers to the spatial separation between the lenses of a stereoscopic camera rig, which plays a crucial role in creating the perception of depth in 3D imagery. This distance affects how viewers perceive three-dimensional space and depth, making it an essential factor in achieving effective stereoscopic alignment and convergence in 3D cinematography.
James Cameron: James Cameron is a renowned Canadian filmmaker, director, and producer, famous for his groundbreaking contributions to the film industry, especially in the realms of science fiction and action genres. His innovative use of technology and storytelling has reshaped cinematic experiences, making significant advancements in visual effects and 3D filmmaking. Cameron's work often emphasizes themes of human resilience and environmentalism, which resonates with audiences worldwide.
Lens separation: Lens separation refers to the distance between the optical centers of two lenses in a stereoscopic camera setup. This distance plays a crucial role in creating depth perception and stereo images, as it determines how converged or diverged the two images will be, influencing the viewer's sense of three-dimensionality.
Martin Scorsese: Martin Scorsese is an acclaimed American film director, producer, screenwriter, and actor, known for his influential work in the world of cinema since the 1960s. His films often explore themes of violence, identity, and morality, while showcasing a unique visual style that incorporates innovative cinematography techniques. Scorsese's contributions to filmmaking have made him a pivotal figure in the industry, shaping the landscape of modern cinema and influencing generations of filmmakers.
Panning: Panning is a camera movement technique where the camera is pivoted horizontally from a fixed position, typically to follow a moving subject or to capture a broader scene. This technique creates a sense of motion and can enhance storytelling by allowing the audience to engage with the environment and the characters within it. Panning plays a significant role in establishing spatial relationships, especially when considering interaxial distance and convergence as well as depth scripting and storyboarding.
Rigging adjustments: Rigging adjustments refer to the modifications made to camera rigs and setups to optimize the placement and alignment of cameras in relation to the subject being filmed. This includes adjusting interaxial distance and convergence, which are crucial for achieving the desired depth perception and stereoscopic effect in cinematography. These adjustments help ensure that the final image captures the intended spatial relationships and enhances viewer immersion.
Stereoscopic depth: Stereoscopic depth refers to the perception of depth that arises from the brain's ability to process visual information from two slightly different perspectives, typically provided by two cameras or lenses in filmmaking. This effect allows viewers to experience a more immersive and realistic representation of three-dimensional space on a two-dimensional screen. The depth perception is influenced by factors such as interaxial distance and convergence, which play key roles in how depth is rendered in stereoscopic images.
Theory of depth: The theory of depth refers to the visual perception and representation of three-dimensional space on a two-dimensional medium, particularly in cinematography. This concept involves how the placement of elements within a frame and the techniques used can create an illusion of depth, enhancing the viewer's experience. Understanding this theory is crucial for manipulating interaxial distance and convergence to achieve desired spatial effects.
Viewer immersion: Viewer immersion refers to the extent to which an audience feels absorbed or engaged in a visual narrative, creating a sense of presence within the story being told. This phenomenon is influenced by various factors such as visual quality, emotional connection, and spatial depth, all of which enhance the viewer's experience. Techniques that elevate viewer immersion can significantly impact how audiences perceive and interpret a film, making them feel as though they are part of the action rather than mere spectators.
Visual acuity: Visual acuity refers to the clarity or sharpness of vision, which is essential for accurately perceiving details in images. In cinematography, it affects how well an audience can discern elements within a frame, impacting composition, depth of field, and overall storytelling. Visual acuity can be influenced by various factors, such as lighting, lens choice, and viewer distance from the screen.
Visual storytelling: Visual storytelling is the art of conveying a narrative through visual media, such as film, photography, or animation, where the imagery plays a crucial role in enhancing and expressing the story. It involves a combination of techniques that guide the viewer's emotions and understanding through carefully chosen visuals, framing, movement, and composition. Effective visual storytelling engages the audience, allowing them to connect deeply with the narrative and characters.