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Pinhole camera model

The pinhole camera model is the simplest theoretical framework in optics and computer vision that describes how light rays from points in a three-dimensional scene pass through an infinitesimally small aperture (pinhole) to form an inverted, perspective-projected image on a two-dimensional plane behind it, establishing a one-to-one mapping between 3D points and their 2D projections without the distortions introduced by lenses.^[1] This model idealizes the imaging process by assuming straight-line propagation of light rays and no scattering or refraction, mimicking the basic principle of the human eye and early optical devices.^[2] The concept traces its roots to ancient observations but was first systematically studied by the 11th-century Arab physicist Ibn al-Haytham (Alhazen), who used pinhole projections in his Book of Optics to demonstrate that light travels in straight lines and to analyze image formation during solar eclipses, laying foundational principles for modern optics.^[3] Later refinements occurred in the Renaissance, with Leonardo da Vinci documenting the camera obscura—a practical pinhole device—in the early 16th century to observe light behavior and natural phenomena.^[4] By the 17th century, astronomers like Johannes Kepler employed pinhole setups for safe solar observations, further validating the model's geometric accuracy.^[5] In mathematical terms, the model projects a 3D point \mathbf{P} = [X, Y, Z]^T in camera coordinates onto the image plane at \mathbf{p} = [x, y]^T, where x = f \frac{X}{Z} and y = f \frac{Y}{Z}, with f denoting the focal length (distance from pinhole to image plane); this perspective projection preserves straight lines but introduces radial foreshortening for distant objects.^[1] For broader applications, the model incorporates intrinsic parameters (e.g., focal length and principal point) in a camera matrix and extrinsic parameters (rotation and translation) to relate world coordinates to camera coordinates, yielding the full projection equation s \begin{bmatrix} u \\ v \\ 1 \end{bmatrix} = \mathbf{K} [\mathbf{R} | \mathbf{t}] \begin{bmatrix} X \\ Y \\ Z \\ 1 \end{bmatrix}, where s is a scale factor, \mathbf{K} is the intrinsic matrix, and [\mathbf{R} | \mathbf{t}] handles pose.^[6] Real-world extensions account for lens distortions (e.g., radial and tangential) absent in the ideal pinhole, often corrected using models like Brown-Conrady.^[6] In computer vision, the pinhole model serves as the cornerstone for tasks such as camera calibration, 3D reconstruction from multiple images, stereo vision, and augmented reality, enabling algorithms to estimate scene geometry from 2D observations by inverting the projection process.^[1] Its simplicity facilitates homogeneous coordinates for efficient linear algebra computations, though limitations like infinite depth of field and perfect sharpness necessitate hybrid models for lens-based systems in practical imaging.^[2] Despite these abstractions, the model's enduring relevance stems from its alignment with projective geometry, influencing fields from robotics to photogrammetry.^[6]

Introduction

Definition and Overview

The pinhole camera model is a fundamental mathematical abstraction in computer vision and optics that describes the projection of three-dimensional (3D) world points onto a two-dimensional (2D) image plane through an infinitesimal aperture, known as the pinhole, without the use of any lenses.^[1] This model simulates the imaging process by assuming light rays emanate from scene points in straight lines and converge at the pinhole before intersecting the image plane, thereby establishing a perspective mapping from 3D space to 2D coordinates.^[7] At its core, the model embodies the principle of central projection, where rays from each 3D point in the scene pass through the single pinhole to form an inverted and reversed image on the opposite side of the aperture.^[1] This setup ensures a one-to-one correspondence between visible scene points and their projections, capturing the geometric essence of how human vision and basic cameras perceive depth through foreshortening and convergence of parallel lines.^[8] The pinhole model incorporates several idealizations to simplify analysis: it assumes an infinitely small pinhole for perfect ray convergence, resulting in infinite depth of field where all points remain in focus regardless of distance; it neglects optical aberrations such as distortion or chromatic effects; and it operates in continuous Euclidean coordinates without discretization or noise.^[1]^[7] Conceptually, the model is often visualized with the pinhole positioned at the origin of a 3D coordinate system, and the image plane placed parallel to the xy-plane at a distance f (the focal length) along the optical axis (z-axis), allowing rays from world points to project onto this plane.^[1] As a baseline for real imaging systems, it approximates the behavior of lens-based cameras by ignoring focusing mechanisms and aberrations, providing a clean foundation for understanding more complex models.^[9]

Historical Context

The earliest recognition of the pinhole effect dates back to the 4th century BCE, when the Greek philosopher Aristotle observed that during a solar eclipse, the shadows cast by light filtering through small gaps between tree leaves formed crescent-shaped images on the ground, demonstrating an early understanding of natural projection phenomena.^[10]^[11] In the 11th century, the Arab scholar Ibn al-Haytham (also known as Alhazen) advanced these observations significantly in his seminal work Book of Optics (circa 1021 CE), where he described the camera obscura as a darkened chamber with a small aperture that projects an inverted image of external objects onto an opposite surface, laying foundational principles for pinhole projection and refuting earlier emission theories of vision in favor of intromission.^[12]^[3] During the Renaissance in the late 15th century, Leonardo da Vinci further explored and illustrated the camera obscura in his notebooks, such as the Codex Atlanticus (compiled 1478–1519), sketching devices that used the pinhole principle to aid in achieving accurate linear perspective for artistic and anatomical drawings, thereby bridging optical theory with practical application.^[13]^[14] In the 17th century, astronomers like Johannes Kepler employed pinhole setups for safe solar observations, further validating the model's geometric accuracy.^[12] The mathematical formalization of perspective projections, essential to the pinhole model, advanced in the 18th and 19th centuries through projective geometry, with Girard Desargues introducing key principles in 1639 and Jean-Victor Poncelet formalizing it in his 1822 treatise Traité des propriétés projectives des figures, providing a rigorous framework for modeling image formation in engineering, architecture, and optics. The model's adoption in the 20th century marked its transition into computational fields, with early computer graphics efforts like Ivan Sutherland's Sketchpad system (1963) incorporating perspective projection techniques akin to the pinhole model for interactive 3D visualization.^[15] This foundation culminated in formal computer vision treatments, such as Berthold K. P. Horn's Robot Vision (1986), which rigorously defined the pinhole camera as a central projection model for image formation in robotic and machine perception systems.^[16]^[17]

Physical and Geometric Principles

Optical Basis

The pinhole camera model is grounded in the principle of rectilinear propagation, whereby light rays travel in straight lines from points on an object, pass through the infinitesimal aperture, and converge to corresponding points on the image plane.^[1] This geometric optics approximation assumes that light behaves as rays without deviation, enabling the formation of a sharp image solely through the aperture's restrictive geometry.^[18] The size of the pinhole plays a critical role in image quality; an ideal pinhole is a point aperture with zero diameter, which eliminates geometric blurring by allowing only a single ray per object point to reach the image plane.^[19] In practice, finite pinhole sizes introduce trade-offs: larger apertures increase light gathering for brighter images but cause overlap of light cones from off-axis points, resulting in blurred disks of confusion, while smaller apertures enhance sharpness up to the point where diffraction effects—arising from light's wave nature—dominate and further degrade resolution.^[18]^[19] Due to the straight-line paths of light rays crossing at the central pinhole, the resulting image on the plane is inverted both vertically and horizontally, with rays from the object's top projecting to the image bottom and vice versa.^[1] This inversion is a direct consequence of the rays crossing at the aperture between the object and image plane, ensuring that all rays from a given point intersect at the pinhole before diverging to the opposite side.^[18] The model relies on several key assumptions to maintain its ideal behavior, including the absence of scattering or absorption of light within the system, which would otherwise diffuse rays and reduce contrast.^[20] It further presumes wavelength independence in ray propagation, treating light as monochromatic or applying geometric optics where diffraction is negligible, thus ignoring chromatic variations that could arise from polychromatic sources.^[19] These simplifications hold in a vacuum or uniform medium where wave effects like diffraction are negligible compared to geometric propagation. The pinhole camera model finds physical embodiment in the camera obscura, a darkened enclosure with a small aperture that demonstrates these optical principles by projecting real-time, inverted images of external scenes onto an internal surface without lenses or mechanical aids.^[21] This device serves as an intuitive tool for illustrating ray propagation and aperture effects in educational settings.^[22]

Geometric Setup and Assumptions

The pinhole camera model establishes a foundational geometric framework for understanding image formation in computer vision, rooted in the idealization of light propagation through a tiny aperture. The three-dimensional world coordinate system is centered at the pinhole, denoted as point O, with the X_3-axis aligned along the optical axis and directed toward the scene being imaged. The X_1-X_2 plane is perpendicular to this axis, providing a reference for transverse directions in the scene. This setup positions the camera's viewpoint at the origin, facilitating the analysis of spatial relationships between objects and their projections.^[23] The image plane is positioned parallel to the X_1-X_2 plane, at a fixed distance f from the pinhole, known as the focal length. In the physical (real) configuration, this plane lies behind the pinhole along the negative X_3-direction, where light rays converge after passing through the aperture. Alternatively, a virtual image plane can be considered at positive X_3 = f, simplifying mathematical treatments by placing the plane in front of the pinhole while maintaining the same projection geometry. The principal point, or image center, is defined as the intersection of the optical axis with the image plane and is conventionally located at coordinates (0, 0) in the image coordinate system. A world point P = (x_1, x_2, x_3) in this setup projects onto an image point Q = (y_1, y_2) on the image plane, capturing the perspective distortion inherent to central projection.^[23]^[1] Several key assumptions underpin this geometric model to ensure idealized behavior. The pinhole is treated as infinitesimally small and point-like, eliminating effects such as diffraction or lens aberrations that would occur in real optical systems. The image plane maintains orthographic alignment with no tilt or skew relative to the X_1-X_2 plane, implying perfect perpendicularity to the optical axis. The model accommodates scenes at finite distances for perspective projection but can approximate orthographic projection when objects are sufficiently far away, such as at infinity along the X_3-axis. Additionally, it presumes a static environment with no motion blur, assuming instantaneous exposure and rigid camera positioning during imaging. These conditions, while simplifying reality, enable precise geometric derivations and form the basis for more complex camera calibrations. The underlying intuition draws from similar triangles, where rays from scene points through the pinhole scale proportionally to form the image.^[23]^[1]

Projection Formulation

Basic Projection Equations

The basic projection equations in the pinhole camera model describe how a three-dimensional point in space is mapped onto a two-dimensional image plane through the pinhole, assuming a coordinate system where the pinhole is at the origin, the optical axis aligns with the positive z-axis (with points in front of the camera having z > 0), and the real image plane is located at z = -f behind the pinhole, where f > 0 is the focal length.^[24]^[25] To derive these equations, consider a 3D point \mathbf{X} = (x_1, x_2, x_3)^T with x_3 > 0. The line from the pinhole to this point intersects the image plane at z = -f. Using similar triangles in the xz-plane (and analogously in the yz-plane), the ratio of the image distance to the object distance along the ray yields the horizontal projection coordinate y_1 = -f \cdot \frac{x_1}{x_3}. Similarly, the vertical projection coordinate is y_2 = -f \cdot \frac{x_2}{x_3}.^[24]^[25] In vector notation, the projected point on the image plane is given by

\begin{pmatrix} y_1 \\ y_2 \end{pmatrix} = -\frac{f}{x_3} \begin{pmatrix} x_1 \\ x_2 \end{pmatrix}.

^[24] The negative sign arises because the real image plane lies behind the pinhole, resulting in an inverted image relative to the object coordinates (upside-down and left-right reversed).^[25]^[24] These equations require x_3 > 0 to ensure the point lies in front of the camera; otherwise, the projection is not defined in the standard forward-facing setup. Division by zero occurs if x_3 = 0, corresponding to points at the pinhole itself, where no unique projection exists.^[24] The projection is defined up to a similarity transformation, as the equations involve homogeneous scaling by the factor $1/x_3, which normalizes the depth-dependent ray intersection.^[24]

Image Plane Configurations

In the pinhole camera model, the physical setup places the image plane behind the pinhole, resulting in an upside-down and left-right reversed projection due to the inversion.^[26] To address this, a common configuration employs a virtual image plane, conceptually located in front of the pinhole at distance +f.^[1] This configuration maintains the projection equations y_1 = f \cdot \frac{x_1}{x_3} and y_2 = f \cdot \frac{x_2}{x_3}, but interprets the rays as intersecting the plane before reaching the pinhole, avoiding the crossing of light rays behind the aperture that occurs in the physical model.^[24] The virtual plane is particularly useful in computational contexts, as it models the perspective projection geometrically without enforcing the inverted orientation of real optics, producing upright images from positive-depth scenes and simplifying ray tracing by placing the intersection forward, preventing artifacts from negative depths in simulations.^[1] In practice, image coordinates are normalized relative to the principal point—the intersection of the optical axis with the image plane—to center the projection and account for offsets in real sensors. This normalization shifts coordinates as y_1' = y_1 - c_1 and y_2' = y_2 - c_2, where (c_1, c_2) denotes the principal point, facilitating conversion to pixel coordinates via scaling by pixel size without delving into full intrinsic parameters.^[27] The virtual image plane configuration offers advantages in software rendering and computer vision algorithms, as it streamlines computations by aligning the projection with positive coordinate systems and enabling efficient visibility testing against plane boundaries, such as near-clipping planes in graphics pipelines.^[26]

Mathematical Extensions

Homogeneous Coordinates

In the pinhole camera model, homogeneous coordinates provide a framework from projective geometry to represent points and transformations linearly, facilitating the mathematical description of perspective projection. A three-dimensional point \mathbf{X} = [X, Y, Z]^\top in Euclidean space is represented in homogeneous coordinates as the four-dimensional vector \tilde{\mathbf{X}} = [X, Y, Z, 1]^\top. Similarly, a two-dimensional image point \mathbf{u} = [u, v]^\top is represented as \tilde{\mathbf{u}} = [u, v, 1]^\top. These representations are defined up to a nonzero scalar multiple, meaning \tilde{\mathbf{X}} and \lambda \tilde{\mathbf{X}} for \lambda \neq 0 denote the same projective point.^[1]^[28] Projective equivalence in homogeneous coordinates allows for the incorporation of points at infinity, which occur when the last coordinate is zero, representing directions rather than finite locations. To recover Euclidean coordinates—a process called dehomogenization—one divides the first three components by the fourth: for \tilde{\mathbf{u}} = [u', v', w']^\top, the image point is [u'/w', v'/w']^\top. This setup transforms the nonlinear perspective projection of the pinhole model into a linear mapping in projective space. For a basic pinhole configuration without extrinsic parameters (assuming the world coordinate system aligns with the camera), the projection is given by \tilde{\mathbf{u}} \sim \mathbf{K} [\mathbf{I} \mid \mathbf{0}] \tilde{\mathbf{X}}, where \mathbf{K} is the intrinsic matrix incorporating focal length and principal point, \mathbf{I} is the 3×3 identity, and \mathbf{0} is the zero vector; the symbol \sim denotes equality up to scale.^[1]^[28] The use of homogeneous coordinates simplifies key operations in the pinhole model, such as applying rotations and translations, which become linear transformations via matrix multiplication without separate handling of the perspective divide. The perspective division inherent to the pinhole projection—dividing image coordinates by the depth Z—is recovered through dehomogenization of the homogeneous output, where the third component of \tilde{\mathbf{u}} corresponds to Z. This linear formulation not only handles degenerate cases like parallel lines converging at infinity but also enables efficient computation in computer vision algorithms.^[1]^[28]

Camera Parameters

The pinhole camera model is parameterized by a 3×4 camera matrix C, which relates homogeneous 3D world coordinates \mathbf{x} = [X, Y, Z, 1]^\top to homogeneous 2D image coordinates \mathbf{y} = [y_1, y_2, y_3]^\top through the projection \mathbf{y} \sim C \mathbf{x}, followed by a perspective division to obtain pixel coordinates (u, v) = (y_1 / y_3, y_2 / y_3).^[1] This matrix decomposes into intrinsic and extrinsic components as C = K [R \mid \mathbf{t}], where K captures the camera's internal geometry and [R \mid \mathbf{t}] describes its external pose relative to the world coordinate system.^[29] The formulation assumes a perspective projection without lens distortions, aligning with the ideal pinhole geometry.^[30] The intrinsic parameters are encapsulated in the 3×3 upper-triangular calibration matrix K:

K = \begin{pmatrix} f_x & s & u_0 \\ 0 & f_y & v_0 \\ 0 & 0 & 1 \end{pmatrix},

where f_x and f_y represent the focal lengths along the image axes (in pixels), (u_0, v_0) denotes the principal point offsets from the image origin, and s is the skew coefficient measuring non-orthogonality of the image axes.^[30] For the ideal pinhole model, the skew is zero (s = 0), the focal lengths are equal (f_x = f_y = f), and the principal point is at the image center (u_0 = v_0 = 0), simplifying K to reflect symmetric projection without offsets or asymmetry.^[1] These five parameters (f_x, f_y, u_0, v_0, s) fully specify the intrinsics, encoding how 3D rays are mapped to the 2D sensor plane.^[30] The extrinsic parameters consist of a 3×3 orthogonal rotation matrix R and a 3×1 translation vector \mathbf{t}, forming the 3×4 block [R \mid \mathbf{t}]. The rotation R aligns the world coordinate frame with the camera's frame, while \mathbf{t} positions the camera center in world coordinates (often expressed as \mathbf{t} = -R \mathbf{c}, where \mathbf{c} is the camera center).^[29] Together, these six degrees of freedom (three for rotation, three for translation) define the camera's rigid pose in the scene.^[1] The complete projection equation is thus

\lambda \begin{pmatrix} u \\ v \\ 1 \end{pmatrix} = K [R \mid \mathbf{t}] \begin{pmatrix} X \\ Y \\ Z \\ 1 \end{pmatrix},

with the depth factor \lambda handled by the perspective divide to yield normalized image coordinates.^[30] Due to the homogeneous representation, the matrix C is defined only up to an arbitrary scale factor, resulting in 11 degrees of freedom overall (five intrinsic plus six extrinsic, minus one for scale).^[1] This scale ambiguity requires normalization, such as setting a specific element of C to 1, to ensure uniqueness in practical computations.^[30]

Applications and Limitations

Uses in Computer Vision

The pinhole camera model serves as a foundational abstraction in computer vision, enabling the mathematical inversion of image projections to recover three-dimensional scene structure and camera parameters from two-dimensional observations. By assuming ideal perspective projection without lens distortions, it underpins algorithms that process real-world imagery captured by digital cameras, facilitating tasks from 3D reconstruction to virtual overlay. This model's simplicity allows for efficient computation while providing a baseline for more complex extensions like radial distortion correction.^[24] In structure from motion (SfM), the pinhole model is central to estimating sparse 3D point clouds and camera poses from a sequence of 2D images, by solving the inverse projection problem through feature matching and bundle adjustment optimization. Seminal approaches, such as those in incremental SfM pipelines, initialize reconstructions using two-view geometry and iteratively refine them with multi-view constraints, achieving sub-millimeter accuracy in controlled environments like cultural heritage scanning. For instance, the COLMAP system leverages the pinhole intrinsics to bundle-adjust thousands of images, enabling large-scale 3D modeling with reported mean reprojection errors of 0.6-0.8 pixels on benchmark datasets such as the 1DSfM dataset.^[24]^[31]^[32] Camera calibration employs the pinhole model to determine intrinsic parameters (focal length, principal point) and extrinsic parameters (rotation, translation) by observing known calibration patterns, such as checkerboards, and minimizing reprojection errors via least-squares fitting. Zhengyou Zhang's flexible technique, using planar patterns viewed from multiple poses, solves a linear system for the projection matrix and decomposes it into intrinsics and extrinsics, requiring at least three images for robustness and achieving calibration accuracies of 0.1-0.5% in focal length estimation for standard lenses. This method is widely implemented in libraries like OpenCV, supporting applications from robotics to photogrammetry.^[33] The pinhole model also informs stereo vision, where epipolar geometry constrains correspondence searches between two images from rigidly separated pinhole cameras, reducing the 2D matching problem to 1D lines via the fundamental matrix \mathbf{F} = \mathbf{K}^{-\top} [\mathbf{t}]_\times \mathbf{R} \mathbf{K}^{-1}, with \mathbf{K} as the intrinsic matrix, \mathbf{R} and \mathbf{t} as relative rotation and translation, and [\mathbf{t}]_\times the skew-symmetric matrix. This relation, derived from the coplanarity of rays, enables disparity computation for depth estimation, as in semi-global matching algorithms that yield dense disparity maps with sub-pixel precision on stereo benchmarks like the Middlebury dataset.^[34] In image formation for computer graphics within vision pipelines, the pinhole model drives ray tracing by generating primary rays from the camera center through image pixels, simulating perspective projection in rendering engines; for example, OpenGL's gluPerspective function constructs a projection matrix that maps the view frustum to normalized device coordinates, ensuring correct depth buffering and anti-aliased perspective views in hybrid vision-graphics systems.^[35]^[36] For augmented reality, the pinhole model facilitates real-time pose estimation and virtual object overlay by transforming 3D world coordinates into image planes via calibrated projection matrices, allowing seamless integration of graphics with live video feeds; early marker-based systems, such as ARToolKit, use fiducial detection to compute extrinsics and render content accurately aligned in indoor tracking scenarios.^[37]

Model Limitations

The pinhole camera model idealizes the aperture as a point, enabling perfect geometric projection of scene points onto the image plane without overlap. In real systems, however, a finite pinhole diameter introduces geometric blur, as multiple rays from a single 3D point pass through the aperture and form a blurred disk on the sensor with radius equal to half the pinhole size projected at the image distance.^[1] This blur increases linearly with pinhole diameter, necessitating a trade-off with diffraction effects, where light waves bend around the aperture edges, producing an Airy disk pattern that limits resolution for smaller apertures.^[38] The optimal pinhole size balances these factors, often following Lord Rayleigh's criterion for minimizing total blur, yielding a diameter of approximately d \approx 1.9 \sqrt{f \lambda}, where f is the focal length and \lambda is the average wavelength of light (around 550 nm for visible spectrum).^[39] Larger apertures improve exposure by allowing more light but exacerbate geometric blur, while smaller ones enhance sharpness at the cost of diffraction and longer exposure times.^[1] Unlike lens-based cameras, the pinhole model assumes infinite depth of field, with all scene depths projected sharply since rays from any distance converge through the point aperture without focusing elements. In practice, this ideal is constrained by the finite pinhole and sensor characteristics; the uniform blur circle from the aperture size acts equivalently to a high f-number (e.g., f/200 or higher), but sensor pixel size and noise introduce depth-independent resolution limits rather than selective defocus.^[24] The model thus overlooks how real sensor arrays, with finite pixel dimensions (typically 1-10 μm), quantize the continuous projected image, leading to aliasing and sampling errors not captured in the ideal formulation.^[1] Additionally, sensor noise from thermal or read-out processes further degrades the projected signal, particularly in low-light conditions common to pinhole imaging due to limited light throughput.^[6] The pinhole model excludes optical aberrations inherent to real lenses, such as radial and tangential distortions that warp straight lines into curves, vignette that darkens image peripheries, and chromatic aberration that shifts colors across the field.^[24] These effects are absent in a true pinhole but must be modeled separately when approximating the ideal with lens systems, often using polynomial corrections like x_d = x_u (1 + k_1 r^2 + k_2 r^4), where r is the radial distance and k_i are distortion coefficients.^[40] The model also presumes a static setup, ignoring motion blur from object or camera movement during exposure, as well as dynamic sensor artifacts like rolling shutter distortion in CMOS arrays, where rows are exposed sequentially, skewing fast-moving features.^[6] For applications beyond narrow fields of view, the pinhole model's perspective projection fails, particularly in wide-angle or fisheye scenarios exceeding 120-180 degrees, where light rays no longer follow simple central projection and require specialized models like equidistant or stereographic mappings to handle the non-linear distortions.^[24] These extensions incorporate additional parameters to approximate real omnidirectional imaging, highlighting the pinhole's limitation as a first-order approximation valid primarily for moderate fields and controlled conditions.^[6]

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[PDF] An Introduction to Ray Tracing
3. The modified pinhole camera model as commonly used in computer graphics. modeling, the classic computer graphics version of the pinhole camera moves.
[36]
The Perspective and Orthographic Projection Matrix - Scratchapixel
The perspective projection matrix as used in OpenGL and its transformation upon transposition. Principle In summary, we understand that the matrix is correctly ...
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[PDF] A Survey of Augmented Reality - UNC Computer Science
In typical graphics software, everything is rendered with a pinhole model, so all the graphic objects, regardless of distance, are in focus. To overcome ...Missing: seminal | Show results with:seminal
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https://www.edmundoptics.com/knowledge-center/application-notes/imaging/limitations-on-resolution-and-contrast-the-airy-disk/
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How to calculate the optimal pinhole size?
Jan 3, 2014 · Optimal diameter for pinhole size is calculated by formula d = c × √(f × λ) where d - the optimal diameter for pinhole c - constant f - focal length.
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https://www.sciencedirect.com/science/article/pii/B9780128118894000014