JSD8028 M12 Finite-Conjugate 850nm Lens – IP67, Lockable Focus for 76mm Working Distance
Compact M12 Lenses for 850 nm Short-Working-Distance Imaging: How to Choose a Lens That's Actually Designed for Your Conjugate
For fixed-geometry monochrome systems at 850 nm, a finite-conjugate M12 lens — corrected for your working distance rather than refocused on the bench — is what keeps contrast across the sensor.
Key takeaway: At 76 mm working distance, an infinity-corrected M12 lens loses field contrast the moment it is refocused for a close object. A finite-conjugate design — corrected for that object distance with an 850 nm-centered coating and lockable focus — holds edge sharpness across the IMX287. JSD's semi-custom route re-optimizes proven M12 designs for your exact conjugate, wavelength, and field of view, backed by measured validation data before volume.
The Silent Failure of Refocused Infinity Lenses
Most M12 board-level lenses are corrected for infinite object distance. Set one to a 70–80 mm working distance and the design behaves differently than intended: magnification rises, field curvature shifts, and edge contrast drops. Turning the focus ring until the center looks sharp masks the problem; it does not fix it. For a fixed-geometry monochrome system running at 850 nm, that gap shows up directly in measured image quality.
"Turning the focus ring until the center looks sharp masks the problem; it does not fix it."
— JSD Optics EngineeringWhy "Finite Conjugate" Is the Real Spec, Not a Buzzword
At 76 mm working distance with an 8 mm effective focal length, magnification sits near 0.09–0.10× — roughly ten times the focal length, well outside the infinite-conjugate regime where typical camera lenses are corrected. Three effects matter in that range:
The effective F/# changes as magnification grows, so a nominal f/2.8 no longer describes the real aperture behavior at close range.
The image plane for a close object curves differently than for an object at infinity — a real issue on a sensor with 6.9 µm pixels where contrast is at a premium.
Refocusing an infinity design at close range usually centers the best focus and leaves the field edges behind.
A lens specified as finite-conjugate is corrected for that object distance, so contrast holds across the sensor. This is the specification that separates a usable short-working-distance lens from a merely refocused one.
What to Look for in a Short-WD 850 nm M12 Lens
Four parameters decide the design; a fifth decides whether it stays useful on the line.
Effective focal length sets the field of view. On the 4.97 mm wide IMX287 at ~76 mm working distance, an 8.00 mm EFL yields roughly 50–51 mm of horizontal field; a 7.5 mm EFL reaches 52–53 mm. At short conjugates, small focal-length differences move the FOV more than they do at infinity.
Sensor coverage with margin. A lens covering Ø9.0 mm behind a 6.21 mm diagonal sensor uses only about 69% of its image circle — the difference between adequate edge sharpness and comfortable edge sharpness.
Wavelength and coatings. At 850 nm, an AR coating centered on that band cuts reflection and ghosting more effectively than a visible-range broadband coating. A single-wavelength design can also be corrected more aggressively than one that must stay sharp across the visible spectrum.
Aperture. An adjustable f/1.4–f/5 range suits prototype characterization. For production, a fixed stop at f/4–f/4.5 is usually where the design is sharpest, with enough depth of field for a fixed-geometry station and a lower mechanical cost.
Lockable focus. Focus that drifts after setup becomes a repeatability problem in a fixed-geometry system. A lock ring or set-screw removes that variable.
Two M12 Designs That Fit the Brief — and How to Choose Between Them
JSD Optics has two production M12 designs close to this requirement profile, both available as semi-custom finite-conjugate variants validated at the customer's working distance and wavelength:
| Specification | JSD8028 | JSD7524 |
|---|---|---|
| Effective focal length | 8.00 mm | 7.50 mm |
| Aperture | f/2.8 fixed | f/2.4 fixed |
| Mount | M12 | M12 |
| Construction | 2G4P (two glass elements) | 1G4P |
| Image circle | Ø9.0 (1/1.8") | Ø6.8 (1/2.7") |
| Total track length | 24.07 mm | 19.82 mm |
| IP67 | Yes | No |
| Est. horizontal FOV @ 76 mm WD | ~50–51 mm | ~52–53 mm |
JSD8028 — the 8 mm primary match
The 8.00 mm EFL aligns with the target spec. Two glass elements give more predictable NIR transmission than plastic aspheres across temperature and batch. The Ø9.0 mm image circle provides corner margin behind the IMX287, and IP67 sealing suits industrial washdown environments.
JSD7524 — the wider-field option
Where a 52–60 mm horizontal field is the binding constraint, the 7.50 mm design lands in that band with a shorter total track (19.82 mm) for tighter mechanical envelopes.
Both are M12, both carry mechanically lockable focus as standard, and both can be built without an IR-cut filter. Distortion is roughly ±0.5% on both designs — low enough that a geometrically calibrated system can ignore it, since software correction handles the residual.
The Semi-Custom Route: Proven Design, Validated for Your Conjugate
Instead of developing a new prescription from scratch, the semi-custom route starts from one of these production designs and runs three steps:
Re-optimize the prescription — at the target working distance (~76 mm), wavelength (850 nm), and sensor field.
Bench-validate on a physical sample — measured field of view, MTF across the field, and focus-lock repeatability.
Freeze a production variant — typically with a fixed internal stop, for repeatable low-cost volume.
The result is a lens with custom-like performance at the specified conjugate, without the development cost and schedule of a from-scratch program.
Where This Lens Class Earns Its Keep
Compact M12 optics in this class serve fixed-mount readers that image a small target from a short, repeatable distance:
Barcode and 2D data-matrix reading — a 52–60 × 39–45 mm field at 76 mm matches the geometry of a code or label on a passing package.
OCR and logistics sorting — conveyor-mounted readers need consistent field contrast across long shifts.
Small-part inspection — a short working distance keeps the whole station compact inside a machine.
NIR gauging and metrology — 850 nm monochrome illumination is a workhorse in industrial measurement, and a lens tuned to that wavelength carries the contrast those systems depend on.
Embedded NIR imaging — biometric and security readers at 850 nm benefit from the M12 form factor and the IP67 option on selected models.
Engineering Considerations Worth Knowing
Mechanical iris availability. M12 S-mount lenses normally do not carry a mechanical iris. Prototypes that need an adjustable aperture use an external stop on the bench or a custom iris barrel (moderate NRE); production favors a fixed internal stop.
Mount options. M12 is the default. If the camera holder is built around C/CS, adapter paths exist, though the M12 barrel keeps the assembly shorter.
What validation data to expect. A semi-custom sample ships with measured FOV and MTF-vs-field data at the specified conjugate, plus focus-lock repeatability results — so the customer can sign off on the design before committing to volume.
NRE and lead time. Modification of a proven design is bounded: re-optimization, bench validation, and a frozen production variant. NRE stays well below a from-scratch program, and the timeline is confirmed against the current production schedule at quotation.
Why JSD
JSD is an imaging lens developer and manufacturer, not a camera or vision-system supplier. The business is organized around the sensor: a catalog of standard M12 designs plus custom development, with an internal design department and in-house manufacturing under ISO 9001 and IATF 16949. That structure is what makes the semi-custom route practical — the design database, the re-optimization capability, and the production line sit under one roof, so validation results on a prototype batch carry through to volume.
❓ Frequently Asked Questions
Practical answers for evaluating a short-working-distance 850 nm M12 lens.
How do I choose an M12 lens for 850 nm short-working-distance imaging?
Apply the four parameters above — EFL for FOV, sensor coverage margin, 850 nm-centered coatings, and lockable focus — then request validation data at the working distance rather than accepting an infinity design refocused on the bench.
What counts as NRE in a semi-custom adaptation?
The re-optimization engineering, any iris-barrel tooling (only if a prototype iris is required), and the bench-validation setup. A fixed-stop production variant typically carries little or no NRE.
What drives pricing at 20, 50, and 100 units?
The mechanical content of the production variant — a fixed stop costs less than an iris barrel — plus the one-time NRE for re-optimization. Volume pricing for these quantities is quoted with the sample proposal.
What lead time and MOQ apply to a semi-custom variant?
First validated samples typically ship within weeks for a design already in the catalog; the MOQ and production timeline are confirmed once the variant is frozen.
Request Finite-Conjugate Validation Data
Send the sensor, working distance, field of view, and wavelength — the JSD engineering team will recommend a starting design and quote samples, NRE, and volume pricing.
Or email us at info@jsdoptical.com
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