Working Distance and Minimum Object Distance for Machine Vision and Robotics Lenses
The machine sets the working distance and the lens sets the minimum object distance. This guide covers both, and what happens to image quality at short range.
Working distance (WD) is the gap from the front of the lens housing to the object. Minimum object distance (MOD) is the closest that gap can get while the lens still focuses, and it is set by the lens. Early in a project there may be room to move WD. Once the mechanical design is committed it is fixed, and the lens must cover the required field of view at that distance.
C-mount and M12 lenses reach focus in two different ways. A C-mount lens uses an internal cam that moves lens groups relative to each other, compensating for aberrations across the focus range. An M12 lens moves as a rigid body, threading in or out of its holder. Both work well at the distances they are designed for and behave differently at the edges of their range. This guide covers working distance selection, the optics behind minimum object distance, and how to hold sharpness across the full field.
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 set by conveyor height, fixture geometry, or the inspection station layout; in robotics it follows from arm reach, end-effector clearance, or bin depth. Early on, WD is a design variable, since brackets and gantry heights can move. Once the mechanical design is committed it is fixed, and the lens has to perform at whatever distance the system requires.
How working distance drives lens selection
WD sets the required focal length. A longer distance needs a longer EFL to cover the same field of view; a shorter distance needs a shorter one. The relationship comes from rectilinear projection geometry (Hecht, Optics, 5th ed., §5.2):
For wide-angle or fisheye lenses with significant distortion, use the Commonlands distortion-corrected field of view calculator instead of the thin-lens estimate.
Working distance versus depth of field
A robot bin-picking at 150mm has a much shallower DOF than the same camera inspecting at 600mm.
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 focus in the center of the image, measured from the front of the lens to the object. Not every datasheet lists it, so you may need to verify it experimentally or request it from the manufacturer. How MOD behaves depends on whether the lens is C-mount or M12, because the two focus in different ways.
C-mount: the cam refocus system
A C-mount lens contains an internal cam that turns focus-ring rotation into linear motion of lens groups inside the barrel (Smith, Modern Optical Engineering, 4th ed., §15.5). That relative motion does more than shift the focal plane: it rebalances aberrations, including field curvature, across the focus range.
MOD is the hard limit set by the end of cam travel. When the ring hits its stop, the groups are at maximum relative displacement and the object cannot be focused any closer without an extension tube. Shorter-EFL C-mount lenses generally have a closer MOD, since less cam travel is needed to cover the defocus (Kingslake, Lens Design Fundamentals, 2nd ed., §18.3).
M12: rigid-body focus by threading
An M12 lens has no internal moving groups. It is a rigid assembly that threads into a holder, and focus is set by screwing it in or out to change the lens-to-sensor distance. So M12 has no traditional MOD: with a tall enough holder the center can always be brought into focus, and the practical limit is thread engagement.
The real limit at short range is off-axis aberration. Because the elements never move relative to each other, there is no rebalancing as focus changes, so the center can be sharp while the corners soften. How soon depends on the design, the pixel pitch, and how far the WD sits from the distance the lens was optimized for.
Commonlands can confirm the MOD for any of its C-mount and M12 lenses on request.
Working distance vs minimum object distance
These two numbers show up in many lens specs, though not every manufacturer publishes MOD. They are not the same thing, and confusing them is one of the most common reasons a vision system fails to focus after integration. Commonlands lists working distance guidance and, where defined, the MOD for its machine vision lenses.
| 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. |
When a datasheet lists MOD, check whether it refers to center focus or full-field performance. The physics behind it is the thin lens equation, 1/f = 1/do + 1/di (Hecht, Optics, 5th ed., §5.2). As the object moves closer, the image distance di grows. The C-mount cam or the M12 thread depth is what supplies that extra distance.
What happens at short working distance
Center focus is rarely the problem at short working distance; corner sharpness is. The reason is the design conjugate. Every lens is aberration-corrected for a target object distance, and many M12 lenses are corrected for longer distances, so field curvature and astigmatism are balanced there. As the object moves off that conjugate, the best-focus surface for the corners no longer sits on the flat sensor and they degrade. How much depends on the prescription, so two M12 lenses with similar headline specs can behave differently at the same short WD.
Smaller pixels make this more visible, since the same blur spot spans more pixels on a fine-pitch sensor, so re-verify lens performance whenever a sensor upgrade reduces pixel pitch.
At short range, bench-test 2-3 candidate M12 lenses at the actual distance on the actual sensor and check the corners, not just the center. If one passes and another does not at the same distance, that is the prescription difference at work. Commonlands can confirm the finite conjugate correction for any of its lenses on request.
Mount selection by working distance
These bands group working distances by packaging, not by optical thresholds. The point where corners degrade shifts with prescription, f-number, pixel pitch, and sensor size.
| Working distance | M12 standard lens | M12 finite conjugate (CIL064, CIL142, 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 | Corrected for the 50mm-500mm range; verify performance beyond it | 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 across the corrected 50mm-500mm range | Often more forgiving due to iris control and finite conjugate correction |
| Very close range (<100mm) | High risk. Not recommended without bench testing. | Supported from 50mm; bench-verify corners at the exact WD | Also strong; verify conjugate match. |
Lenses by working distance range
Representative Commonlands lenses for short and medium working distances in machine vision and robotics applications.
Best lens picks by working distance
The right pick depends on the focal length the 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 follow below.
| Working distance range | Recommended lens | Mount | Focus range / iris | Why this pick |
|---|---|---|---|---|
| Short (50mm-500mm) wide field, 2/3" sensors |
CIL064 6mm | M12 | Finite conjugate, F/2.9, 11.0mm image circle | Finite conjugate correction reduces field curvature at close range; widest field of the three. |
| Short (50mm-500mm) narrower field |
CIL142 14.4mm | M12 | Finite conjugate, F/2.6-F/5.2 variants, 9.3mm image circle | Mid-telephoto framing with the same reduced field curvature across the 50mm-500mm range. |
| Short (50mm-500mm) 500mm design point |
CIL121 21.8mm | M12 | Finite conjugate, corrected at 500mm | 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. |
Each range starts from the focal length the working distance requires (confirm it with the EFL calculator), then the mount follows from whether depth of field needs iris tuning. Focus and iris values are each Commonlands SKU's published figures, not numbers computed here. 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 lenses
Medium working distance (several hundred mm)
Frequently asked questions
Common questions on working distance and MOD for Commonlands machine vision and robotics lenses.
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 focus in the center of the 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 a softer limit: the center can be focused with enough holder depth, 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.
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 infinite conjugate designs, 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.
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.
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.