Working Distance and Minimum Object Distance for Machine Vision and Robotics Lenses
Working distance is usually the first number in a vision system design. The machine frame, conveyor height, robot arm reach, or end-effector geometry typically determines it, but it is a design consideration, not an immovable constant. Early in a project, there may be room to adjust mounting height or reposition a camera bracket. Later, it becomes fixed. Either way, the lens must cover the required field of view at the target distance.
C-mount and M12 lenses handle focus at different working distances in two different ways. C-mount lenses use an internal cam system that moves lens groups relative to each other, compensating for aberrations across the focus range. M12 lenses move as a rigid body: the entire lens threads in or out of its holder. Both approaches work well at the distances they are designed for, but each has different behavior at the edges of its range. This guide walks through working distance selection, the optics behind minimum object distance, and how to get sharp images across the full field at any working distance.
What working distance means
Working distance (WD) is the distance from the front of the lens housing to the target surface. In machine vision, it is typically driven by the conveyor height, fixture geometry, or inspection station layout. In robotics, it follows from the arm reach, end-effector clearance, or bin depth.
Early in a project, WD is a design variable: mounting brackets can be repositioned and gantry heights adjusted. Once the mechanical design is committed, WD becomes effectively fixed. Either way, the lens needs to perform at whatever distance the system requires.
How working distance drives lens selection
WD directly determines the required focal length. Longer distance needs a longer EFL to cover the same field of view; shorter distance needs a shorter EFL. The relationship comes from the rectilinear projection geometry (Hecht, Optics, 5th ed., §5.2):
This is equivalent to EFL = sensor_dim / (2 × tan(AOV/2)), which is the formula used in the Commonlands angle of view calculator. For lenses with significant distortion (wide-angle or fisheye), use the distortion-corrected FOV calculator instead.
Working distance versus depth of field
Shorter WD compresses depth of field. A robot bin-picking at 150mm has a much thinner DOF than the same camera inspecting at 600mm. If targets have height variation (stacked parts, irregular surfaces, PCB components), model DOF at the actual WD before committing.
The depth of field calculator reports DOF and focus sensitivity for any lens/sensor combination. Here, focus sensitivity means how strongly image focus changes as object distance changes. At shorter WD, this sensitivity rises quickly, so small part-position or robot-position errors produce more blur.
What minimum object distance means and why it differs by mount type
Minimum object distance (MOD) is the closest distance at which a lens can produce a sharp image, measured from the front of the lens to the object. Not every lens datasheet lists MOD. Some manufacturers omit it entirely, so you may need to verify it experimentally or request it from the manufacturer.
How MOD works depends on whether the lens is C-mount or M12, because the two systems focus in different ways.
C-mount: the cam refocus system
C-mount lenses contain an internal cam mechanism that converts rotation of the focus ring into linear motion of lens groups inside the barrel (Smith, Modern Optical Engineering, 4th ed., §15.5). As you turn the focus ring, two or more lens groups move relative to each other along precisely machined cam tracks. This relative motion does more than shift the focal plane. It also rebalances aberrations (including field curvature) across the focus range. It is a compensating system.
MOD for a C-mount lens is a hard limit set by the end of the cam travel. When the focus ring reaches its mechanical stop, the lens groups are at their maximum relative displacement. The object cannot be focused any closer without adding an extension tube. Shorter EFL C-mount lenses generally have shorter MOD because less cam travel is needed to compensate for defocus as working distance decreases (Kingslake, Lens Design Fundamentals, 2nd ed., §18.3).
M12: rigid-body focus by threading
M12 lenses have no internal moving groups. The lens is a rigid optical assembly that threads into a holder. Focusing is done by screwing the lens in or out, which changes the distance between the entire lens and the sensor. The optical elements do not move relative to each other.
This means M12 lenses do not have a traditional MOD in the same sense as C-mount. If the holder is tall enough, an M12 lens can bring the center of the image into focus at any working distance meaningfully beyond its focal length, because there is no cam travel limit. The practical limit is thread engagement: at some point the lens runs out of threads in the holder. But with a taller holder or a different mounting approach, the center can still be brought into focus.
The real limit for M12 lenses at short working distances is not center focus: it is field curvature. Because the lens groups do not move relative to each other, there is no aberration rebalancing as focus changes. At short working distances, the center may be sharp but the corners may soften. The severity depends on the specific lens design, pixel pitch, and how far the WD is from the distance the lens was optimized for.
Why MOD matters for packaging
Since MOD is measured from the front of the lens, it directly tells you the minimum barrel-to-object gap. A C-mount lens with 100mm MOD means the object cannot be closer than 100mm from the front element and still be in focus. That number determines whether a ring light, gripper, or conveyor clearance is sufficient.
Working distance vs minimum object distance
These two numbers appear in many lens specs, though not all manufacturers publish MOD. They are not the same thing. Confusing them is one of the most common reasons a vision system fails to focus after integration.
| Term | Measured from | Measured to | Who defines it | Notes |
|---|---|---|---|---|
| Working distance (WD) | Front of lens housing | Object surface | Machine designer | Fixed by mechanical layout. Not a lens property. |
| Minimum object distance (MOD) | Front of lens | Object surface | Lens manufacturer | C-mount: hard limit (end of cam travel). M12: practical limit (thread engagement or field quality). Not always published. |
For C-mount lenses, MOD is a hard mechanical limit: the cam system cannot travel further. For M12 lenses, MOD is less clear-cut: the center can be focused with sufficient thread depth at any distance meaningfully beyond the focal length, but field curvature may prevent full-field sharpness at very short distances. When a datasheet lists MOD, verify whether it refers to center focus or full-field performance.
Physical clearance at closest focus
Since MOD is measured from the front of the lens, the published MOD directly gives you the minimum barrel-to-object gap. Plan your lighting, gripper geometry, and conveyor clearances around this number. For final integration, verify against the manufacturer's mechanical drawing, especially for C-mount lenses where the barrel protrudes differently depending on focus position.
The optics behind MOD
The thin lens equation 1/f = 1/do + 1/di (Hecht, Optics, 5th ed., §5.2) relates object distance do to image distance di for a given focal length f. As the object moves closer, di must increase: the image forms further behind the lens. In a C-mount lens, the cam system accommodates this by moving internal groups until it runs out of travel. In an M12 lens, you thread the lens further from the sensor to increase di. The physics is the same; the mechanical implementation is different.
What happens at very short working distances
The behavior depends on whether you are using a C-mount or M12 lens, because the two focusing systems hit different limits.
C-mount: the cam system runs out of travel
When a C-mount lens is focused to its closest distance, the cam mechanism is at its mechanical stop. Move the object closer and the focus ring has nowhere left to go. The image is uniformly soft across the entire field, center and corners alike. This is a hard limit. The only fix is an extension tube between the lens and camera body, which increases the lens-to-sensor distance and shifts the focus range closer.
M12: center focuses, but corners may not
M12 lenses do not have a cam system, so there is no mechanical stop in the same sense. If the holder has enough thread depth, an M12 lens can bring the center of the image into focus at any working distance meaningfully beyond its focal length. The practical limit is thread engagement: at some point the lens physically runs out of threads.
But even with center focus achieved, the corners may be soft. Because all optical elements in an M12 lens are fixed relative to each other, there is no aberration rebalancing as the lens moves closer to the sensor. Field curvature and other off-axis aberrations can increase, producing an image that is sharp in the center but degraded at the edges. This is the real working distance limit for M12 lenses: not whether the center can focus, but whether the full field meets the application requirements.
How to recognize you are at the limit
- C-mount: The focus ring reaches its stop and the image is uniformly blurred. No position produces a sharp image anywhere in the field.
- M12: The center is sharp but corners are soft, or the lens has run out of thread engagement in the holder. If corners are the issue, a taller holder will not help. It is an optical limit, not a mechanical one.
- Contrast drops: edges that should be crisp become low-contrast gradients
- Vision algorithms fail on feature detection or edge finding even after parameter adjustment
C-mount (uniformly blurred): Add an extension tube, or select a shorter-EFL lens with a closer MOD. M12 (sharp center, soft corners): A taller holder will not fix field curvature. Try a different M12 lens design optimized for shorter working distances, or switch to a C-mount lens whose cam system can compensate.
Fixes, in order of preference
Select a lens matched to the target WD. For M12, try a design optimized for shorter working distances. The CIL121 is corrected for a 500mm finite conjugate. For C-mount, a shorter-EFL lens will have a closer MOD.
Verify the field of view still covers the inspection area with the FOV calculator.
C-mount: add an extension tube. This increases the lens-to-sensor distance and shifts the focus range closer. Tradeoffs: the far focus limit moves in, FOV changes, and you lose some of the cam system's aberration compensation at the new conjugate. Fine for prototyping; for production, a lens designed for the distance is more reliable.
M12: try a different lens design first. Two M12 lenses with similar headline specs can have different field curvature behavior at the same short WD. Before switching to C-mount, test 2-3 M12 candidates at the actual distance on the actual sensor.
Depth of field tightens at close range
As WD decreases, depth of field compresses. A system at short WD faces both a tighter DOF window and (for M12 lenses) increasing field curvature. Stacked parts, irregular surfaces, or robot positioning variance may not fit within the available DOF.
Use the depth of field calculator to check DOF and focus sensitivity at your target WD. Focus sensitivity measures how strongly image sharpness changes with object displacement. At close range this rises quickly, so small positioning errors from a robot arm or conveyor vibration produce disproportionate blur.
Corner blur and field curvature at short working distance
The most common image quality issue at short working distances is a sharp center with soft corners. Understanding why requires knowing the difference between how C-mount and M12 lenses handle the transition to close range.
Design conjugate and aberration balance
Every lens is aberration-corrected for a target object distance, the design conjugate (Smith, Modern Optical Engineering, 4th ed., §3.5). Many M12 lenses are optimized for longer working distances. Field curvature, astigmatism, and other off-axis aberrations are balanced at the design distance.
As the object moves away from the design conjugate, this balance shifts. The best-focus location for the field center and the best-focus locations for off-axis points no longer coincide on a flat sensor. Corner softness may be dominated by field curvature, astigmatism, or a combination. Which one dominates depends on the specific lens prescription. Two M12 lenses with similar headline specs from different manufacturers can behave noticeably differently at the same short WD. This is a bench-testing problem, not a spec-sheet problem.
Why C-mount lenses handle this differently
When you refocus a C-mount lens, the cam system moves internal lens groups relative to each other. This relative motion does not just shift the focal plane. It rebalances the aberration correction for the new conjugate (Kingslake, Lens Design Fundamentals, 2nd ed., §18.3). A well-designed C-mount lens can maintain reasonable field quality across a wide focus range because the compensating groups were designed to do exactly that.
An adjustable iris adds a second tool: stopping down increases depth of field and reduces the visible blur from any residual focus mismatch across the field. In machine vision and robotics, controlled lighting (LED ring lights, backlights, structured light) makes stopping down practical without losing exposure.
C-mount lenses are not immune to off-axis degradation: a lens used far from its corrected conjugate will still degrade. But the cam system gives them a much larger operating envelope than a rigid-body M12.
Why M12 refocusing is different
Threading an M12 lens deeper into its holder moves the entire lens as a rigid body. This shifts the focal plane, and at short working distances you can almost always get the center sharp. But because no optical elements move relative to each other, the aberration balance does not change. Field curvature that was negligible at the design distance can become significant at short range, and no amount of threading adjustment can correct it: threading moves the whole focal surface, curved and all.
Smaller pixels make residual corner softness more visible: the same blur spot spans more pixels on a fine-pitch sensor, so a sensor with smaller pixels can reveal degradation that a coarser-pitch sensor renders less obvious. Whenever a sensor upgrade reduces pixel pitch, plan to re-verify lens performance at the new pitch.
Test before committing at short working distances
At short working distances, off-axis behavior varies enough between individual M12 lens designs that datasheet specs alone are not reliable. The point where corners degrade is not a fixed threshold; it shifts with the lens prescription, f/#, pixel pitch, and sensor size.
The practical approach: order 2-3 candidate M12 lenses and bench-test each one at the actual WD on the actual sensor. Check corners, not just center. If one lens produces acceptable full-field results and another does not at the same distance, that is the prescription difference at work. Bench-testing several candidates at the actual working distance is a sound way to de-risk short-WD system design.
Standard M12 lenses typically do not provide an adjustable iris, so stopping down is not available as a mitigation. If corners are soft at short WD with one M12 lens, try a different lens design before switching mount types. A different M12 prescription optimized closer to your WD may resolve the issue. If none pass, consider a finite-conjugate M12 design (CIL121) or move to a C-mount lens with cam-based aberration compensation and iris control.
Finite-conjugate M12 lenses
Some M12 lenses are designed specifically for shorter working distances, corrected at a finite conjugate rather than optimized for long range. The Commonlands CIL121 is optimized for 500mm working distance. Its optical prescription is different from standard M12 designs, so it maintains better field quality at its target distance.
C-mount at short WD
C-mount lenses with cam-based refocus and an adjustable iris have two mechanisms working in their favor at short WD: aberration rebalancing through the cam groups, and depth-of-field extension through the iris. This combination often makes them more forgiving at close range. That said, every C-mount lens has its own design limits: a lens used far outside its intended conjugate range will still degrade. The mount provides the mechanisms; the optical prescription determines how well they work.
Mount selection by working distance
| Arbeitsabstand | M12 standard lens | M12 finite-conjugate (CIL121) | C-Mount |
|---|---|---|---|
| Longer working distances | A wider range of standard lenses may be viable; verify full-field performance for the actual sensor and resolution target | Works well | A wider range of lenses may be viable; verify full-field performance |
| Short range (below a few hundred mm) | Risk zone: verify corner sharpness at the actual WD, sensor, and pixel pitch | Good at or near design WD | Often more forgiving due to iris control and finite-conjugate correction |
| Very close range (<100mm) | High risk. Not recommended without bench testing. | Possible with correct design | Preferred. Verify conjugate match. |
At longer working distances, a wider range of lenses may be viable, and packaging, weight, and cost become more flexible trade variables, but full-field performance still depends on the specific lens, sensor, and resolution target. At short working distances, off-axis behavior diverges between lens designs. Plan to test 2-3 candidates on your actual sensor at the actual WD. If all fail corners, move to finite-conjugate M12 or C-mount with iris.
Lenses by working distance range
Representative Commonlands lenses at three WD ranges for machine vision and robotics applications.
Best lens picks by working distance
The lens that holds full-field sharpness at a set working distance depends on the focal length that distance requires and on whether the job needs iris control for depth of field. The table pairs each working distance range with a Commonlands lens whose published focus range covers it, split by mount. Detailed cards for every pick follow below.
| Working distance range | Recommended lens | Halterung | Focus range / iris | Why this pick |
|---|---|---|---|---|
| Short, finite-conjugate embedded, medium-short range |
CIL121 21.8mm | M12 | Optimized for a 500mm finite conjugate | Aberration balance set for a shorter conjugate, so corners stay sharp where a long-conjugate M12 softens. |
| Medium, several hundred mm iris and DOF control |
CIL532 12mm | C-Mount | Finite conjugate from 100mm, F/2.0–F/16 | Cam refocus rebalances aberrations across the range and the iris tunes depth of field. |
| Medium, several hundred mm wide FOV, low distortion |
CIL062 6mm | M12 | 50mm to ∞, uncorrected | Low-distortion wide field for measurement at standard inspection distances in a small M12 package. |
| Long, over ~800mm high-resolution, iris |
CIL544 25mm | C-Mount | Working distance 130mm to ∞, F/1.8 iris | 20MP 1.1-inch coverage with iris control for distant, high-resolution inspection. |
| Long, over ~800mm M12 form factor required |
CIL160 16mm | M12 | 50mm to ∞, uncorrected | Narrow field for distant targets when the embedded M12 form factor rules out C-mount. |
Each range starts from the focal length the working distance requires (EFL = WD × sensor width / FOV width; confirm it with the EFL calculator), then the mount follows from whether depth of field needs iris tuning. Focus and iris values are each SKU's published figures, not numbers computed here, and M12 working distance runs 50mm to infinity uncorrected. Bench-test full-field sharpness on your own sensor before ordering; the focal length selection guide covers the full method.
Short working distance: finite-conjugate M12
Medium working distance (several hundred mm)
Long working distance (>800mm)
Choosing a machine vision lens for a long working distance
A long working distance calls for a focal length calculation, not a "long-range" product category. When guarding, heat, or robot clearance forces the camera back, calculate the focal length that produces the required field of view at that distance, then pick the mount family that provides it on your sensor.
Worked example: a camera fixed 600mm from an 80mm-wide label, with 7.4mm of active sensor width, needs EFL = (7.4 × 600) / 80 = 55.5mm. The nearest standard focal lengths, 50mm and 75mm, cover roughly 89mm and 59mm at that distance; 75mm is narrower than the label, so 50mm is the right choice. Use the active width from the sensor datasheet, not the nominal format number. The focal length guide covers the full selection method and telephoto tradeoffs.
Ground sampling distance (scene width divided by horizontal pixel count) must put at least 2 pixels on the smallest feature, a practical minimum derived from Nyquist sampling; reliable detection typically needs 3-4 pixels. Longer focal length is not free: at the same working distance, a 75mm lens at F/5.6 has far shallower depth of field than a 6mm lens; if you lengthen both WD and focal length to keep the same field of view, DOF is set by magnification and f-number and stays roughly constant. And a 0.5° mount tilt at 1000mm WD shifts the aim point roughly 8.7mm.
Telephoto M12 lenses cover much of this range. The CIL250 25mm M12 covers a 20° field of view on 8-10MP sensors up to 1/1.7" (fixed F/2.0). When depth of field must be tuned with an iris, C-mount takes over: the CIL579 75mm (sensors up to 1/1.7") is finite-conjugate optimized with a 5.8° field of view, F/3–F/20 adjustable iris, and 500mm minimum object distance. Sensors larger than 1/1.7" step up to the CIL544 (1.1", 20MP+).
Long working distance does not itself require telecentric optics. Telecentricity matters when object height variation causes unacceptable perspective error; most long-distance tasks are detection at a fixed plane, where an entocentric lens is sufficient (see what is a telecentric lens). Commonlands does not currently stock telecentric lenses.
Extending the focus range with extension tubes and holder height
When a lens cannot focus at the required working distance, there are mechanical approaches depending on the mount type. Both increase the lens-to-sensor distance and shift the achievable focus range toward closer object distances.
C-mount: extension tubes
Extension tubes are hollow spacer rings inserted between the C-mount lens and camera body. A lens that cannot focus at 80mm may reach that distance after a 5mm or 10mm tube is added. They thread onto the flange, so adding or removing one takes seconds.
One important caveat: extension tubes increase the lens-to-sensor distance but bypass the cam system's designed operating range. The compensating lens groups were optimized for a specific range of cam travel. With an extension tube, the cam may not rebalance aberrations as effectively as it does within its designed focus range.
M12: holder height
M12 focus is set by how deep the lens sits in the holder: threading it further from the sensor shifts focus closer. This is the standard M12 focus mechanism, not a workaround. A taller holder extends the range further.
Since M12 lenses are rigid assemblies, moving the lens further from the sensor changes the center focus position but does not change the aberration balance. At short WD, a taller holder can bring the center into focus but will not improve corners if field curvature is the limiting factor.
Tradeoffs for both approaches
- Far focus limit moves in. The lens may not reach distances needed for setup or calibration.
- FOV changes: effective magnification increases. Recalculate coverage with the FOV calculator.
- C-mount: extension tubes add mechanical length and misalignment risk, especially on robot end-effectors under vibration.
- M12: taller holders bring center into focus but do not correct field curvature at close range.
When to select a different lens
For production systems (both fixed inspection stations and robot-mounted cameras), a lens designed for the target WD is more reliable than mechanical workarounds. For M12, a finite-conjugate design (CIL121) corrected at the target distance will maintain better field quality than a long-conjugate lens at any holder height. For C-mount, a purpose-built close-range lens provides better alignment stability than spacers stacked behind a standard lens, especially under vibration or thermal cycling.
If a sensor upgrade to smaller pixels is planned, verify that spacer-based or tall-holder systems still meet your MTF requirements at the new pixel pitch. What passed on 5.86µm pixels may not pass on 2.74µm.
Working distance selection checklist
Each item maps to a specific failure mode. Skip one and you find out during mechanical integration or first-article inspection, when changes are expensive.
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Define WD from the mechanical layout. Measure from camera mount position to target surface in the actual fixture, robot cell, or station. Early in the project, evaluate whether WD can be adjusted for better optical performance.
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Calculate required EFL. Use EFL = (WD × sensor_width) / FOV_width to get a starting focal length. Verify with the EFL calculator.
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C-mount: verify MOD ≤ required WD. M12: verify field quality at the target WD. For C-mount, check the datasheet for MOD (not always published, so request it from the manufacturer if missing). For M12, MOD is less meaningful; test full-field sharpness at the actual WD.
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Check physical clearance (barrel-to-object gap). MOD is measured from the front of the lens, so the MOD value gives the minimum barrel-to-object gap. Verify against the manufacturer's mechanical drawing and confirm it fits your lighting, conveyor, gripper, and part handling geometry.
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At short WD: bench-test 2-3 candidate lenses. Mount each lens at the actual WD on the actual sensor. Inspect the full field including corners. Off-axis behavior varies between lens designs, so datasheet specs are not sufficient. Do not rely on center-point focus tests.
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If corners degrade at short WD: first test alternate prescriptions. Try a different lens design at the same WD. If none pass, move to a finite-conjugate M12 (CIL121) or a better-matched C-mount lens. This is a lens selection issue, not an adjustment issue.
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Verify DOF covers the part height variation, and check focus sensitivity. Use the depth of field calculator at the actual WD and f-number. Focus sensitivity indicates how strongly image sharpness changes with object displacement at that working distance.
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Confirm lighting and end-effector clearance. Ring light size, illuminator angle, and robot gripper geometry must fit within the barrel-to-object gap. Design lighting and mechanics in parallel with lens selection.
Häufig gestellte Fragen
What is working distance in machine vision and robotics?
Working distance is the distance from the front of the lens housing to the target surface. In machine vision, it is typically driven by the machine frame or conveyor geometry. In robotics, it follows from the arm reach, end-effector design, or bin depth. WD is a system-level design consideration. Early in a project there may be room to adjust it, but once the mechanical design is committed, the lens must perform at that distance.
What is minimum object distance and how is it different from working distance?
Minimum object distance (MOD) is the closest distance at which a lens can produce a sharp image, measured from the front of the lens to the object. WD is also measured from the front of the lens. The key difference: WD is a system-level design choice, while MOD is a lens characteristic. For C-mount lenses, MOD is a hard limit: the cam refocus system has reached its mechanical stop. For M12 lenses, MOD is more nuanced: the center can be focused with enough holder depth at any distance meaningfully beyond the focal length, but field curvature may prevent full-field sharpness at very short distances. Not all lens datasheets publish MOD, so you may need to verify it experimentally.
What happens if my object is closer than the lens can focus?
It depends on the mount type. For C-mount: the cam system has reached its stop and the image is uniformly soft across the entire field. An extension tube is needed to shift the focus range closer. For M12: with enough holder depth, the center can usually still be focused, but at very short distances field curvature may make corners soft even while the center is sharp. Try a different M12 lens design optimized for shorter working distances, use a taller holder (which helps center focus but not corners), or consider a C-mount lens whose cam system provides aberration compensation.
Why are my image corners blurry at short working distance?
This is usually an off-axis aberration issue, most commonly field curvature and/or astigmatism when the lens is used far from its design conjugate. Many standard M12 lenses are optimized for longer working distances, and because they focus as a rigid body (no internal cam mechanism), there is no aberration rebalancing as you thread them closer. How badly corners degrade depends on the individual lens design. Two lenses with similar specs can perform differently at the same distance. At short working distances, bench-test multiple candidates. If none pass, try a finite-conjugate M12 (CIL121) or a C-mount with cam-based aberration compensation.
What is the difference between finite conjugate and infinite conjugate lenses?
A long-conjugate (infinite-conjugate) lens is aberration-corrected for objects at longer distances. Many M12 lenses are optimized for longer working distances. A finite-conjugate lens is corrected for a specific shorter distance, such as 500mm. At that distance, field curvature and other off-axis aberrations are minimized. C-mount lenses have a structural advantage here: the cam refocus system rebalances aberrations as you focus through the range. M12 lenses focus by moving as a rigid body, so the aberration balance is fixed by the design. For systems with a known working distance, a lens corrected near that conjugate will deliver better full-field performance.
Should I use M12 or C-mount for short working distances?
Both can work; the question is which one best fits your mechanical and optical constraints. C-mount lenses have two advantages at short WD: the cam refocus system rebalances aberrations across the focus range, and the adjustable iris provides depth-of-field extension. M12 lenses are simpler, lighter, and smaller, which matters for embedded robotics and space-constrained applications. If you need M12 at short WD, test 2-3 candidates because field quality varies between designs. The CIL121 is an M12 lens specifically optimized for a 500mm working distance.
Can extension tubes or holder adjustments extend the focus range?
Yes. C-mount extension tubes increase the lens-to-sensor distance and shift the focus range closer. For M12, threading the lens farther out of the holder does the same; a taller holder just maintains thread engagement. Tradeoffs: the far focus limit moves in, FOV changes, and for C-mount, extension tubes operate outside the cam system's designed range so aberration compensation may be less effective. For M12, extra extension brings the center into focus but does not correct field curvature. If corners are soft, a different lens design is needed.
How do I calculate the focal length I need from my working distance?
Use the rectilinear projection relationship: EFL = (WD × sensor_width) / FOV_width. Example: 800mm WD, sensor with 7.2mm active width, 400mm target width → EFL = (800 × 7.2) / 400 = 14.4mm. The formula is a close approximation for rectilinear (low-distortion) lenses when the working distance is much longer than the focal length. At close range, work from the thin lens conjugate equation instead, and note that WD is measured from the lens housing, not the principal plane. Verify with the EFL calculator, then confirm the selected lens can actually focus at that working distance before ordering.
What lens should I use for long working distance in machine vision?
Does stopping down the aperture fix corner blur at close range?
On a C-mount lens, the cam refocus system already rebalances aberrations across the focus range. Stopping down adds depth-of-field extension that further reduces visible blur from any residual focus mismatch. Both mechanisms work together, and in controlled-lighting environments stopping down is practical. Standard M12 lenses typically do not have an adjustable iris. For M12 at short WD, the best approach is a lens design optimized for the target distance: a finite-conjugate M12 where the aberration balance is set for shorter range from the start.
Related resources
- EFL calculator Find the required focal length for a given working distance, sensor, and field of view.
- Field of view calculator Verify coverage at the actual working distance before selecting a lens.
- Depth of field calculator Calculate DOF and focus sensitivity as a function of working distance for any sensor, focal length, and f-number.
- How to choose focal length for machine vision Full focal length selection guide with formula, examples, and sensor size reference.
- Depth of field in machine vision How DOF tightens at close range and how aperture and sensor size affect it.
- Lens aberrations in machine vision Field curvature, distortion, astigmatism, and how they appear in real images.
- What is an M12 lens M12 (S-mount) lens format, applications, and selection guide.
- What is a C-mount lens C-mount format, flange distance, and when to choose C-mount over M12.
- F-number and aperture in machine vision How f-number affects exposure, depth of field, and diffraction.
- Field of view in machine vision FOV calculation, sensor format, and the relationship to working distance.
- What is a telecentric lens When telecentric optics are necessary and how they differ from entocentric designs.
- CMOS sensor size guide Sensor format reference for matching lens image circle to sensor dimensions.
Find the right lens for your working distance
Commonlands publishes working distance and conjugate information for its machine vision lenses, and can confirm the MOD or finite-conjugate correction for any product on request. If nothing in the standard range fits your inspection station or robot cell, our San Diego engineering team can review application-specific configurations. Same-day shipping on orders placed before 12 PM PST.