Depth of Focus
The tolerance for image plane displacement behind the lens while maintaining acceptable sharpness.
Depth of Focus
noun | Camera & Optics
The range of positions along the optical axis at which the camera's image plane (the film or sensor surface) can be located while still producing an acceptably sharp image of a subject at a fixed focus distance. Depth of focus is the image-side counterpart to depth of field: where depth of field describes the range of subject distances that appear sharp, depth of focus describes the tolerance for image plane position at the camera. It is a lens design and camera engineering concept rather than a creative cinematography tool.
Quick Reference
| Also Known As | Lens-to-film tolerance, image-side focus tolerance |
| Domain | Camera & Optics |
| Also Used In | Post-Production (VFX departments use depth of focus data for virtual camera matching) |
| Opposite / Antonym | N/A (no direct opposite; shallow depth of focus is simply a tighter tolerance) |
| Measured At | The camera's image plane (film or sensor) |
| Distinct From | Depth of field (range of sharp subject distances in front of the lens) |
| Affected By | Aperture, focal length, circle of confusion standard, image magnification |
| Related Terms | Depth of Field, Deep Focus, Shallow Depth of Field, Focus, Aperture, Lens |
| See Also (Tools) | Depth of Field Calculator, Circle of Confusion Calculator |
| Difficulty | Advanced |
The Explanation: How & Why
Depth of focus and depth of field are often confused because they sound similar and both involve the word "depth." The distinction is which side of the lens you are measuring.
Depth of field is measured in object space: in front of the lens, in the world being filmed. It describes how much of the scene, at varying distances from the camera, appears acceptably sharp in the image.
Depth of focus is measured in image space: behind the lens, at the camera's film or sensor plane. It describes how far the image plane can move forward or backward from the ideal focus position while the image of a specific subject remains acceptably sharp.
The formula for depth of focus is: $t = 2 \times N \times c \times (v / f)$, where $t$ is total depth of focus, $N$ is the f-number, $c$ is the circle of confusion, $v$ is the image distance, and $f$ is the focal length. For non-macro distances, this simplifies to approximately $t = 2 \times N \times c$, because $v$ approaches $f$ at normal subject distances. At f/2.8 with a Full Frame CoC of 0.024mm, the depth of focus is approximately 0.134mm. At f/1.4, it shrinks to roughly 0.067mm - a tolerance thinner than a human hair.
The practical implications:
Film gate steadiness: In film cameras, the film must pass through the gate at a precise, consistent distance from the lens's focal plane. Any variation in film flatness or gate registration affects focus. The depth of focus determines how much tolerance exists - how much the film can deviate from the ideal plane before focus is visibly degraded.
Lens registration: When a lens is mounted to a camera, the distance from the lens mount to the image plane (the flange focal distance) must be precisely matched to the lens design. The ARRI PL mount has a flange focal distance of 52.00mm. A lens registered incorrectly by more than the depth of focus will produce soft images even when the focus ring is set correctly.
Digital sensor alignment: In digital cameras, sensor alignment and the consistency of the sensor's position relative to the lens mount are engineering concerns governed by depth of focus tolerances. Higher-resolution sensors (4K, 6K, 8K) effectively demand tighter tolerances because they resolve smaller details, making depth of focus violations more visible.
Focus pulling tolerance: At very large apertures (small f-numbers) where depth of field is minimal, depth of focus is also minimal. The focus puller has little tolerance for error in either the subject's position or the camera's calibration. At smaller apertures, both depth of field and depth of focus increase, providing more tolerance.
Historical Context & Origin
The distinction between depth of field and depth of focus has existed since the early development of optical theory. The American Cinematographer Manual has documented both concepts since its first edition, and the formulas trace back to 19th-century photographic optics. In the context of cinematography, depth of focus became particularly relevant with the introduction of precision optical systems in studio filmmaking. The film gate steadiness requirements of high-precision optical printing - compositing multiple film elements through contact printing or optical printers - required careful management of depth of focus tolerances. Even a few thousandths of an inch of gate weave could soften a composite element.
In the digital era, sensor alignment and lens registration tolerances have replaced film gate considerations, but the underlying optical concept governs the same engineering constraints. The Cooke Optics Camera and Lens Definitions for VFX document formalizes these tolerances for modern virtual production and VFX workflows, where depth of focus data feeds into computer-generated camera models for matchmoving and compositing.
How It's Used in Practice
Scenario 1 - Lens Registration (1st AC): A Cooke S7/i 50mm prime is mounted on an ARRI ALEXA 35 for the first time on a shoot. The 1st AC checks focus at 6 feet using a Siemens star chart and finds the focus mark on the lens ring does not correspond to actual sharp focus. The 1st AC suspects a lens registration problem: the flange focal distance is off, placing the sensor outside the depth of focus tolerance for this lens at T2. The lens is swapped to a backup, and the suspect lens is sent back to the rental house for collimation on their lens-testing bench.
Scenario 2 - Large-Aperture Focus Pulling (1st AC): Shooting at T1.4 on a 50mm Zeiss Supreme Prime on a Sony VENICE 2 in Full Frame mode, the 1st AC has approximately 0.067mm of depth of focus tolerance. Any camera movement - a slight flex of the tripod head, a vibration from a passing truck, even thermal expansion of the lens barrel in direct sun - can shift the image plane enough to soften the image. The 1st AC verifies that the camera is solid on a Mitchell mount, the lens is properly seated, and the focus marks have been confirmed with a chart before the first take at this aperture.
Scenario 3 - Behind-the-Lens Filters (DP / 1st AC): A DP wants to use a behind-the-lens gel filter on a 35mm lens at T2. The 1st AC warns that inserting glass behind the lens shifts the focal plane, and at T2 the depth of focus tolerance is too thin to absorb the shift. The DP either moves the filter in front of the lens or stops down to T5.6, where the depth of focus expands enough to absorb the optical shift.
Usage Examples in Sentences
"The focus marks are off because the lens registration is outside the depth of focus tolerance - send it back for collimation."
"At T1.4, we have no margin for error - not in depth of field and not in depth of focus either. The camera and lens must be perfectly calibrated."
"Depth of focus is an engineering concern. Depth of field is a creative one. Both matter, but in different ways."
"Don't put a behind-the-lens filter on anything wider than 25mm at T2 - the depth of focus is too thin to absorb the optical shift."
Common Confusions & Misuse
Depth of Focus vs. Depth of Field: The most common confusion in cinematography terminology. Depth of field is the subject-side range of acceptable sharpness - the creative parameter the DP controls through aperture, focal length, and focus distance. Depth of focus is the image-side tolerance for sensor or film plane position - primarily an engineering and calibration concern. In casual conversation, these terms are frequently used interchangeably; in precise technical discussion, the distinction matters. The Edmund Optics technical reference documents both concepts from an optical engineering perspective.
Depth of Focus vs. Deep Focus: Deep focus is an aesthetic technique in which all planes of the scene are kept in sharp focus simultaneously by maximizing depth of field. Depth of focus is an optical engineering measurement with no direct aesthetic dimension. Despite the similarity of the names, they describe entirely different things. A film shot with deep focus technique has large depth of field; the depth of focus of the camera system is unaffected by the creative choice.
Variations by Context
| Context | How Depth of Focus Varies |
|---|---|
| Film vs. Digital | Film cameras must manage depth of focus through gate steadiness (film flatness, registration pins). Digital cameras manage it through sensor alignment and mount precision. Digital sensors are rigid, eliminating gate weave, but introduce thermal expansion considerations |
| High-Resolution Sensors | 4K, 6K, and 8K sensors resolve smaller details, effectively demanding tighter depth of focus tolerances. A focus error invisible on a 2K sensor may be visible on an 8K sensor because the smaller pixels resolve the blur circle |
| Macro Photography | At macro distances (1:1 magnification), depth of focus increases while depth of field decreases. This is the inverse of normal shooting distances and confuses photographers transitioning to macro cinematography |
| Behind-the-Lens Filtration | Inserting gels or filters behind the lens alters the optical path, shifting the focal plane. At wide apertures where depth of focus is thin, this shift can push the image plane outside the tolerance zone. The ASC Manual advises against behind-the-lens filters on lenses wider than 25mm |
Key People & Films
The American Cinematographer Manual, first published by the ASC in 1935 and now in its 10th Edition, remains the definitive reference for depth of focus formulas and lens registration tolerances in cinema. Cooke Optics published their Camera and Lens Definitions for VFX document in 2023, formalizing depth of focus data for virtual production and matchmoving workflows. The ARRI PL mount (52.00mm flange focal distance) and the Leica M mount (27.80mm) are engineering standards defined by depth of focus requirements - the shorter the flange distance, the tighter the manufacturing tolerance required. Films shot at extreme apertures like The Revenant (2015, DP Emmanuel Lubezki, shot on the ARRI ALEXA 65 at T1.8) push depth of focus tolerances to their practical limits, requiring meticulous lens collimation and camera setup.
Equipment / Tools Reference
Lens collimation is verified using a Siemens star focus chart, a collimator bench (such as the Trioptic Collimator used by rental houses), or a DSC Labs camAlign chart. The ARRI ALEXA 35 and Sony VENICE 2 both publish sensor alignment tolerances in their technical manuals. The ARRI PL mount (52.00mm flange focal distance) and the Canon RF mount (20.00mm) represent different engineering approaches to depth of focus management - shorter mounts require tighter manufacturing tolerances. Lens testing tools like the DSC Labs ChromaDuMonde chart allow 1st ACs to verify focus accuracy on set. The Circle of Confusion Calculator on this site provides the CoC values needed for depth of focus calculations.
Standards & Specifications
The ASC Manual, 10th Edition defines the depth of focus formula as $t = 2 \times N \times c \times (1 + m)$, where $N$ is f-number, $c$ is circle of confusion, and $m$ is magnification. The ARRI PL mount standard specifies a flange focal distance of 52.00mm with a tolerance of +/-0.01mm. The ISO 10110 standard series covers optical element drawing requirements including tolerances relevant to depth of focus in lens manufacturing. The Cooke Optics VFX definitions document (2023) specifies that depth of focus data must be encoded in lens metadata for virtual production workflows, using the SMPTE lens metadata standard. For 4K delivery, depth of focus tolerances should be calculated with a CoC divisor of 2000 rather than the standard 1500.
Common Questions / FAQ
Q: What is the difference between depth of focus and depth of field?
A: Depth of field is measured in front of the lens (object space) and describes the range of subject distances that appear sharp. Depth of focus is measured behind the lens (image space) and describes the tolerance for sensor or film plane position. Depth of field is a creative parameter; depth of focus is an engineering parameter.
Q: Why does my lens show soft focus even when the focus ring is set correctly?
A: The most likely cause is a lens registration error: the flange focal distance is off, placing the sensor outside the depth of focus tolerance. The lens needs collimation on a test bench. This is common with adapted lenses (using a mount adapter that is slightly too thick or too thin) or with lenses that have been dropped or mishandled.
Q: Does depth of focus change with focal length?
A: For non-macro subject distances, depth of focus is relatively insensitive to focal length when the f-number is held constant. In the macro range, depth of focus increases with longer focal length or closer subject distance, while depth of field decreases. This inverse relationship at macro distances is a common source of confusion.
Q: Can I use behind-the-lens filters at any aperture?
A: Inserting glass behind the lens shifts the optical path. At wide apertures (T1.4-T2.8), the depth of focus tolerance may be too thin to absorb the shift, resulting in soft images. The ASC Manual advises against behind-the-lens filters on lenses wider than 25mm. Stop down to T5.6 or smaller to increase depth of focus tolerance, or move the filter in front of the lens.
Related Terms
- Depth of Field - The subject-side counterpart; the creative range of acceptable focus in front of the lens. The two are mathematically related through the same optical variables.
- Deep Focus - The aesthetic technique of maximizing depth of field; often confused with depth of focus despite being an entirely different concept.
- Shallow Depth of Field - The opposite aesthetic approach to deep focus; does not directly relate to depth of focus but both shrink at wide apertures.
- Focus - The fundamental optical concept that depth of focus bounds at the image plane; the point of maximum sharpness.
- Aperture - Controls both depth of field and depth of focus simultaneously; stopping down increases both.
- Lens - The optical system whose design and registration determine depth of focus tolerances; flange focal distance precision is the primary engineering variable.
See Also / Tools
The Depth of Field Calculator calculates the subject-side parameter mathematically related to depth of focus through the same optical variables. The Circle of Confusion Calculator provides the CoC values needed for depth of focus calculations on any sensor format. For the theory behind how sensor format affects both parameters, see Depth of Field in Cinema in the blog.
