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Depth of focus
Depth of focus
Depth of focus
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Depth of focus is a lens optics concept that measures the tolerance of placement of the image-capturing plane (the plane of an image sensor or a film in a camera) in relation to the lens. In a camera, depth of focus indicates the tolerance of the film's displacement within the camera and is therefore sometimes referred to as "lens-to-film tolerance".

Depth of focus versus depth of field

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The phrase depth of focus is sometimes erroneously used to refer to depth of field (DOF), which is the object position range over which objects are acceptably focused on an image, whereas the depth of focus refers to the zone behind the lens wherein the film plane or image sensor is placed to produce an in-focus image. Depth of field depends on the focus distance, while depth of focus does not.

Depth of focus can have two slightly different meanings. The first is the distance over which the image-capturing plane can be displaced while a single object plane remains on it with acceptably sharp focus;[1][2][clarify] the second is the image-side conjugate of depth of field.[2][clarify] With the first meaning, the depth of focus is symmetrical about the image plane; with the second, the depth of focus is slightly greater on the far side of the image plane.

While depth of field is generally measured in macroscopic units such as meters and feet, depth of focus is typically measured in microscopic units such as fractions of a millimeter or thousandths of an inch. In optometry depth of focus is usually measured in dioptres.

The same factors that determine depth of field also determine depth of focus, but these factors can have different effects than they have in depth of field. Both depth of field and depth of focus increase with smaller apertures. For distant subjects (beyond macro range), depth of focus is relatively insensitive to focal length and subject distance, for a fixed f-number. In the macro region, depth of focus increases with longer focal length or closer subject distance, while depth of field decreases.

Determining factors

[edit]

In small-format cameras, the smaller circle of confusion limit yields a proportionately smaller depth of focus. In motion-picture cameras, different lens mount and camera gate combinations have exact flange focal distance measurements to which lenses are calibrated.

The choice to place gels or other filters behind the lens becomes a much more critical decision when dealing with smaller formats. Placement of items behind the lens will alter the optics pathway, shifting the focal plane. Therefore, often this insertion must be done in concert with stopping down the lens in order to compensate enough to make any shift negligible given a greater depth of focus. It is often advised in 35 mm motion-picture filmmaking not to use filters behind the lens if the lens is wider than 25 mm.

Calculation

[edit]

If the depth of focus relates to a single plane in object space, it can be calculated from[1]

where t is the total depth of focus, N is the lens f-number, c is the circle of confusion, v is the image distance, and f is the lens focal length. In most cases, the image distance (not to be confused with subject distance) is not easily determined; the depth of focus can also be given in terms of magnification m:

The magnification depends on the focal length and the subject distance, and sometimes it can be difficult to estimate. When the magnification is small, the formula simplifies to

The simple formula is often used as a guideline, as it is much easier to calculate, and in many cases, the difference from the exact formula is insignificant. Moreover, the simple formula will always err on the conservative side (i.e., depth of focus will always be greater than calculated).

Following historical convention, the circle of confusion is sometimes taken as the lens focal length divided by 1000 (with the result in same units as the focal length);[2][3] this formula makes most sense in the case of normal lens (as opposed to wide-angle or telephoto), where the focal length is a representation of the format size. This practice is now deprecated; it is more common to base the circle of confusion on the format size (for example, the diagonal divided by 1000 or 1500).[3]

In astronomy, the depth of focus is the amount of defocus that introduces a wavefront error. It can be calculated as[4][5]

.

References

[edit]
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Depth of focus

View on Grokipedia
from Grokipedia
Depth of focus refers to the range of distances along the in image space within which the image of an object remains acceptably sharp, representing the tolerance for displacing the (such as a or ) from its nominal position before the resulting defocus blur exceeds an acceptable limit, typically defined by the circle of confusion. This concept is fundamental in , particularly in systems where precise alignment of the is critical, such as in , , and imaging devices. Unlike , which describes the corresponding range of object distances in object space that appear acceptably sharp when imaged onto a fixed plane, depth of focus operates in the conjugate image space and is scaled by the square of the system's . For instance, in a high-magnification system, the depth of focus is larger relative to the depth of field, while in low-magnification setups like , the image-space tolerance is smaller despite a deeper object-space depth. This distinction arises from the of optical conjugates, where defocus in object space is magnified in image space, affecting the allowable blur. The magnitude of depth of focus is influenced by several key parameters, including the wavelength of light (λ), the (or F/#) of the optical system, and the acceptable blur size. A common approximation for the depth of focus is Δz ≈ 2λ (F/#)^2, indicating that it increases quadratically with larger f-numbers (smaller apertures) and longer wavelengths, allowing greater positional tolerance in low-resolution or systems. In , where (NA) plays a prominent role, depth of focus also varies with objective magnification; for example, a 4× objective with NA 0.10 may yield an image depth of 0.13 mm, while a 100× objective with NA 0.95 can extend to 80 mm, facilitating precise sensor alignment despite shallow . Applications of depth of focus are widespread in precision . In and , it determines the mechanical tolerance for lens-sensor alignment, ensuring consistent image quality across production variations. In , particularly with intraocular lenses, it quantifies the eye's tolerance to defocus, influencing designs for extended depth of focus implants that enhance over a range of distances without accommodation. Overall, understanding depth of focus enables optimization of optical systems for sharpness, resolution, and manufacturing feasibility.

Definitions and Concepts

Basic Definition

Depth of focus refers to the axial range in the image space over which an image remains acceptably sharp when the or film plane is displaced from the precise focal plane. This tolerance allows for minor variations in the positioning of the image detector without significantly degrading image quality, as the blur caused by such displacement stays within an acceptable limit defined by the circle of confusion. Unlike concepts measured in object space, depth of focus specifically quantifies the permissible shift in the lens-to-sensor distance, which is generally small—on the order of millimeters—and contrasts with the much larger distances involved in object positioning. This image-space metric is crucial for practical systems where exact alignment of components may be challenging, providing a buffer against mechanical imperfections or vibrations. Visually, depth of focus can be understood through the of rays: a lens focuses rays from an object point into a converging that meets at the focal plane before diverging; the depth of focus represents the longitudinal segment along the where the cross-section of this cone remains smaller than the allowable blur diameter, often depicted as a diamond-shaped tolerance zone bounded by rays from the lens edges to the edges of the circle of confusion. This concept in image space corresponds to depth of field in object space, where the latter describes the range of object distances yielding sharp images for a fixed sensor position.

Comparison with Depth of Field

Depth of field (DOF) is defined as the range of distances in object space—typically in front of the lens—over which objects appear acceptably sharp when projected onto a fixed image plane, such as a camera sensor. In contrast, depth of focus (DOFoc) describes the range of positions for the image plane itself, where a stationary object in the scene maintains acceptable sharpness, allowing for variations in sensor placement or orientation. This distinction positions DOF as a property of the subject-to-lens relationship and DOFoc as a characteristic of the lens-to-image tolerance. The two concepts share a reciprocal relationship: a shallow DOF in object space, which limits the sharpness range for subjects at varying distances, corresponds to a deeper DOFoc in image space, providing greater leeway for adjustments, and conversely for deeper DOF scenarios; this interplay arises from the inherent in the optical system, scaling distances between object and image spaces. Beginners often conflate DOF and DOFoc due to their similar , mistakenly believing that DOFoc influences the direct sharpness of scene elements like backgrounds or subjects, when it instead governs the precision required in aligning the components behind the lens. Qualitative examples illustrate these differences clearly. In , a shallow DOF enables the subject's face to remain sharply rendered while the background blurs into a soft, non-distracting , emphasizing the foreground element within object space. By comparison, DOFoc comes into play during camera manufacturing or setup, where it determines the allowable misalignment of the —such as slight tilts or shifts—without compromising overall image clarity for a fixed subject.

Influencing Factors

Optical Parameters

The size of a lens, quantified by its , is a key determinant of depth of focus. A smaller , which corresponds to a larger diameter, reduces the depth of focus by creating a narrower bundle of light rays converging on the ; this limits the allowable displacement of the before the resulting defocus blur exceeds the resolution tolerance. Conversely, increasing the (stopping down the ) widens the depth of focus, as the broader ray bundle permits greater positional tolerance without significant degradation in sharpness. Wavelength of light plays a critical role in defining depth of focus through diffraction effects. Shorter wavelengths, such as those in blue light (around 470 nm), yield a smaller depth of focus because they result in tighter diffraction-limited spot sizes, imposing stricter limits on blur circle growth from defocus. In contrast, longer wavelengths allow for a more extended depth of focus by relaxing these diffraction constraints. Lens aberrations further modulate depth of focus by introducing deviations from ideal ray convergence. Chromatic aberration, which varies with wavelength, causes different colors to focus at slightly offset planes, asymmetrically narrowing the effective depth of focus and potentially shifting the best focus position. Spherical aberration, meanwhile, affects marginal rays more severely, leading to a curved focal surface that reduces symmetry and tolerance around the nominal focus; simple single-element lenses exhibit pronounced spherical aberration, resulting in a more restricted depth of focus compared to compound lenses designed to minimize such errors through multiple elements.

System and Environmental Factors

In imaging systems, the resolution of the or plays a critical role in determining the effective depth of focus by influencing the acceptable size of the , which defines the threshold for perceptible blur. Higher resolution sensors, featuring smaller sizes (typically around 8 μm in high-end digital single-lens reflex cameras), necessitate a correspondingly smaller —often on the order of the pitch—to preserve sharpness across the , thereby reducing the tolerance for axial shifts and compressing the overall depth of focus. System magnification further modulates depth of focus, with higher magnification levels—common in telephoto or macro configurations—extending the axial range over which the image remains acceptably sharp due to the quadratic relationship between depth of focus and magnification. This effect arises from the longitudinal magnification of the optical system, where increased amplifies the object-space depth into a larger image-space tolerance, with depth of focus scaled by the square of the magnification factor. For instance, in evaluations, this scaling accounts for the extended tolerance in image space. Temperature variations and mechanical stability introduce additional constraints on usable depth of focus by inducing shifts in the through and vibrations. in lens mounts and housings can displace the focal plane by up to ±80 μm over ranges from -40°C to +85°C, effectively narrowing the operational depth of focus unless compensated by athermalization techniques such as or mechanical adjustments. Similarly, environmental vibrations, prevalent in settings, can cause axial drifts exceeding 5 μm over hours, pushing the image beyond the depth of focus and degrading resolution; active stabilization systems, achieving ~21 nm precision, are often required to maintain the light sheet or within this tolerance during extended acquisitions. Illumination conditions indirectly affect depth of focus assessment by altering the perception of blur through contrast reduction and . Non-uniform lighting or introduces veiling , which spreads across the and exacerbates the of defocus-induced blur, making marginally out-of-focus regions appear softer or hazier even within the nominal depth of focus. This effect is quantified in response metrics, where reduces modulation transfer and perceived sharpness, particularly in systems with shallow depth of focus.

Mathematical Formulation

Core Equations

The depth of focus, denoted as DOFoc, quantifies the axial range in image space over which the can be positioned while maintaining acceptable sharpness, typically defined by the blur circle not exceeding the circle of confusion diameter cc. In the geometric approximation for distant objects (infinite conjugates), the primary is given by DOFoc≈2Nc,\text{DOFoc} \approx 2 N c, where NN is the of the lens (also known as the relative , N=f/DN = f / D with ff the and DD the diameter) and cc represents the maximum allowable blur diameter in the , often set to the size or a thereof based on resolution requirements. This expression arises from paraxial ray tracing considerations of defocus blur formation. Consider an ideal thin lens focusing parallel rays from a distant point source onto the image plane at distance v≈fv \approx f. A marginal ray parallel to the optical axis passes through the edge of the aperture and intersects the focal plane at height D/2D/2. If the image plane is displaced axially by δz\delta z toward the lens, the intersection of this ray with the displaced plane forms a blur circle. The diameter bb of this blur circle is b=δz⋅(D/v)b = \delta z \cdot (D / v). Since N≈v/DN \approx v / D for infinite conjugates, b=δz/Nb = \delta z / N. Setting the acceptable blur b=cb = c yields the one-sided tolerance δz=cN\delta z = c N; accounting for symmetric defocus on either side of the nominal focus gives the total depth of focus DOFoc=2cN\text{DOFoc} = 2 c N. This derivation assumes paraxial rays (small angles relative to the optical axis) and neglects higher-order aberrations. For finite conjugate systems involving magnification mm (where m=v/um = v / u with uu the object distance), the formula extends to account for the increased image distance v=f(1+m)v = f (1 + m) and the corresponding working f-number Nw=N(1+m)N_w = N (1 + m), yielding DOFoc=2Nc(1+m).\text{DOFoc} = 2 N c (1 + m). Here, the blur circle scaling incorporates the longitudinal effect in image space, where the ray cone angle adjusts with magnification, increasing the depth of focus relative to the infinite case for m>0m > 0. This adjustment follows from the paraxial equation and ray heights traced through the aperture stop. These formulations rely on key assumptions: an ideal model without aberrations, monochromatic illumination to ignore chromatic effects, paraxial for ray propagation, and diffraction-limited conditions where geometric blur dominates over wave optics effects (valid for c≫λNc \gg \lambda N, with λ\lambda the ). The NN and mm directly influence the tolerance as discussed in optical parameters.

Calculation Methods

To compute the depth of focus (DOFoc) in practical systems, the process begins by referencing the core DOFoc ≈ 2 N c, where N is the and c is the allowable blur circle in the . First, select c based on the 's resolution; for digital sensors, c is typically set to half the pixel pitch to ensure the blur does not exceed the Nyquist limit, such as c = 1.725 μm for a sensor with 3.45 μm pixels. Next, determine N from the aperture diameter D and f via N = f / D; for example, an f/4 lens has N = 4. Then, apply the core formula to obtain the nominal DOFoc. Finally, adjust for m if at close distances, using the extended form DOFoc ≈ 2 N c (1 + m) to account for the increased tolerance in the due to extension or macro setups. For complex systems involving varying object distances, approximations integrate the H = f² / (N c) to estimate effective DOFoc across a range; this scales the computation by treating distant objects as contributing minimal defocus while adjusting c dynamically for near-field variations. In tilted configurations, such as those using the to align the with the object plane's tilt, the DOFoc is briefly accounted for by rotating the or lens, which extends the effective range but requires iterative adjustment of the tilt angle to maintain focus across the field. Beyond analytical methods, ray-tracing software like enables precise simulation of DOFoc by modeling ray bundles through the optical system, incorporating aberrations and defocus to compute blur spots iteratively for non-ideal lenses. Assuming perfect alignment in calculations overestimates DOFoc, as manufacturing tolerances in camera assembly—such as sensor tilt of ±0.1 mm or back shifts—can reduce the effective range by up to 20-30% in high-resolution systems, necessitating active alignment during production to meet performance specs.

Practical Applications

In Imaging Systems

In imaging systems such as digital single-lens reflex (DSLR) and mirrorless cameras, depth of focus (DOFoc) determines the allowable range for positioning relative to the , ensuring acceptable image sharpness across manufacturing tolerances. The —the fixed mechanical distance from the mount to the plane—must be maintained with precision on the order of hundredths of a millimeter, as even minor deviations can introduce blur exceeding the system's criterion. DOFoc, approximated as ±B' × f/# (where B' is the maximum tolerable blur diameter and f/# the lens f-number), guides these tolerances by quantifying how much the can shift longitudinally from the nominal without degrading focus. For instance, in wide-aperture lenses (low f/#), DOFoc is shallower, demanding tighter alignment in camera assembly to prevent consistent defocus across the frame. DOFoc also imposes fundamental limits on autofocus (AF) precision, particularly distinguishing phase-detection AF (PDAF) in DSLRs from contrast-detection AF (CDAF) in many mirrorless systems. In PDAF, which uses a dedicated to split incoming light and measure phase differences, focus accuracy is typically calibrated to within one full DOFoc at the lens's maximum for standard points, or one-third DOFoc for high-precision central points (e.g., requiring f/2.8 or faster lenses on professional bodies like the series). This tolerance is essential for reliability, as PDAF's effective high (e.g., f/22–f/32 equivalent) yields a deeper DOFoc on the AF , reducing sensitivity to minor errors but still bounding overall precision. In contrast, CDAF relies on the main to analyze contrast gradients, achieving potentially higher accuracy within a narrower DOFoc but at slower speeds due to iterative hunting. For high-speed applications like , PDAF's faster acquisition—often tracking subjects at rates exceeding 10 fps—leverages these DOFoc limits to maintain focus on erratic motion, such as a soccer player sprinting at 30 km/h, where CDAF might lag and miss peak action. Lens mounting standards further highlight DOFoc's role in system compatibility, with variations in flange focal distance influencing adapter feasibility and focus tolerances. The Canon EF mount, with a 44 mm flange distance, supports broader interoperability than the Nikon F mount's 46.5 mm, as the shorter distance allows simple, optics-free adapters for lenses from longer-flange systems (e.g., adapting or medium-format optics to Canon bodies) while preserving within DOFoc bounds. Conversely, adapting shorter-flange lenses like to requires corrective optics or impossible negative-thickness spacers, as the 2.5 mm excess would shift the beyond typical DOFoc tolerances (e.g., 0.01 mm errors preventing sharp ). These differences affect professional workflows, where adapters must maintain sub-DOFoc precision to avoid chronic back- or front-focus issues in video rigs or hybrid setups. Post-processing software, such as , can enhance perceived sharpness through sharpening, effectively extending the usable DOFoc by amplifying captured edge contrast and mitigating minor defocus artifacts. However, this digital adjustment does not modify the underlying optical tolerance, as it cannot reconstruct details entirely absent from the due to shifts exceeding the true DOFoc—severe blur from misalignment remains irrecoverable, limited to enhancing what the lens and initially captured. Unlike , which governs object-space sharpness for creative blur control, DOFoc in these systems prioritizes mechanical and algorithmic reliability for consistent output.

In Scientific and Industrial Contexts

In , particularly at high magnifications, the depth of focus (DOFoc) becomes extremely limited, often less than 1 μm for 100× oil-immersion objectives with numerical apertures around 1.25–1.30, necessitating precise control to maintain sharp of fine cellular structures. This shallow DOFoc arises from the high required for resolving details as small as 0.25–0.27 μm, making even minor thermal drifts—such as a 1°C change causing 0.5–1.0 μm focal shifts—a significant challenge in live-cell . To address this limitation for three-dimensional samples, z-stack techniques capture multiple focal planes and computationally combine them into an extended-focus image, enabling comprehensive volumetric analysis without mechanical refocusing during acquisition. In for automated , DOFoc tolerances are critical for maintaining consistent focus across varying object positions, such as on moving conveyor belts in lines. For instance, in wafer scanning, wafer warpage can exceed 100 μm across a single die, far surpassing the typical DOFoc of conventional optical systems, which leads to defocus and unreliable defect detection at nanometer scales. Systems mitigate this by employing , where multiple images at different focal depths are merged to extend the effective DOFoc, ensuring all surface features remain sharp during high-speed scanning of wafers for voids or alignment errors. This approach enhances precision in dynamic environments, reducing false positives in processes. Recent advancements in ophthalmic and medical imaging, particularly as of 2024, highlight distinctions in DOFoc for retinal imaging, where the eye's accommodation dynamically influences the perceived focus range on the retina. In adaptive optics (AO) ophthalmoscopy, the inherently small DOFoc—often limited to specific retinal layers like the inner plexiform layer—requires ultrafast corrections to counteract accommodation-induced fluctuations, which can shift focus between layers such as the inner nuclear and nerve fiber layers during steady-state viewing. Ocular accommodation adjusts the lens to refocus the retinal image, affecting axial intensity distributions and necessitating non-cycloplegic stabilization techniques for accurate layer-specific imaging in non-paralyzed eyes. These considerations enable finer control, with focus steps as precise as 0.02 diopters (approximately 7.4 μm), improving diagnostic resolution for conditions involving retinal depth variations. Industrial applications leverage wavefront coding to artificially extend DOFoc through computational , minimizing the need for high-precision mechanical adjustments in systems. This technique employs a phase mask, such as a cubic element, at the pupil plane to render the point-spread function insensitive to defocus, achieving near-diffraction-limited performance over depths up to 30 times greater than conventional limits. By combining this optical preprocessing with digital , systems reduce sensitivity to misalignment in tools, such as those used for precision inspection in , thereby lowering costs associated with hardware. Pioneered in seminal work on incoherent , wavefront coding has been integrated into infinity-corrected microscopes and for robust performance across varied working distances.

References

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  2. https://micro.magnet.fsu.edu/primer/anatomy/focusdepth.html
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  17. https://www.cis.rit.edu/class/simg733/ray_optics_notes_01.pdf
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  19. https://www.largeformatphotography.info/forum/archive/index.php/t-11709.html
  20. https://www.sciencedirect.com/science/article/pii/S1888429610700078
  21. https://thinklucid.com/tech-briefs/active-sensor-alignment/
  22. https://www.the-digital-picture.com/Photography-Tips/Canon-EOS-DSLR-Autofocus-Explained.aspx
  23. https://www.photonstophotos.net/MarianneOelund/AutoFocus_System_Design/AutoFocus_System_Design.htm
  24. https://fotodioxpro.com/blogs/news/flange-focal-distance
  25. https://photographylife.com/advanced-post-processing-tips-three-step-sharpening
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  27. https://www.microscopyu.com/applications/live-cell-imaging/correcting-focus-drift-in-live-cell-microscopy
  28. https://www.microscopeworld.com/blog/how-to-use-z-stacking-microscopy-software/
  29. https://semiengineering.com/how-advanced-packaging-is-reshaping-inspection/
  30. https://www.baslerweb.com/en-us/use-cases/focus-stacking/
  31. https://www.nature.com/articles/s41467-024-54687-z
  32. https://graphics.stanford.edu/courses/cs448a-06-winter/dowski-wavefront-coding-optics95.pdf
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