· VisionSelect Team · Guides · 4 min read
Industrial Lenses from China: Fixed Focal, Telecentric and Line Scan Explained
The lens decides your image; the camera gets the credit. A buyer’s guide to industrial lens families, resolution matching, telecentric optics — and what the Chinese supply base now offers.
A 12 MP camera behind a lens that cannot resolve 12 MP produces a 2 MP image with extra noise. The lens is the cheapest place to lose resolution and the easiest component to blame the camera for. This guide covers the lens families, how to match them to your sensor, and why China is now a first-choice source.
The lens families
| Family | What it is | Use when |
|---|---|---|
| Fixed focal (C-mount) | Standard lens, fixed focal length, common mounts (C, CS, F) | Most area scan applications; the default starting point |
| Large-format / F-mount | Bigger image circle for line scan and large sensors | Line scan cameras, 29 MP+ area scan, full-frame coverage |
| Telecentric | Chief rays parallel to the optical axis; magnification independent of distance | Dimensional metrology where perspective error corrupts measurements |
| Line scan | Optimised for 1D sensors, high MTF across a line | Web, panel and wafer inspection |
| Zoom / varifocal | Adjustable focal length | Bench setups, flexible stations, prototyping |
| Macro | Close working distance, high magnification | Small parts, electronics, microscope-style imaging |
Resolution matching: the rule that saves projects
A lens must resolve what the sensor samples. The practical rule:
Lens resolution (lp/mm at your working aperture) ≥ sensor sampling (line pairs per mm at the sensor).
Sensor sampling is simply pixels per mm on the sensor. If your 5 MP sensor with a 2/3” format samples about 90 lp/mm, a lens that only resolves 60 lp/mm is the bottleneck — no camera upgrade will fix it.
For metrology, the usual practice is to oversample: aim for lens resolution comfortably above sensor sampling, so the optics — not the pixels — limit the system.
Telecentric lenses: when and why
Standard lenses suffer perspective error: a closer point appears larger. For presence checks this is irrelevant. For measurement it is fatal.
- Image-space telecentric lenses keep magnification constant within a depth range — the standard for in-line gauging.
- Bilateral telecentric lenses also parallelise the object side, eliminating parallax entirely — the choice for thick or tall parts where the object cannot be perfectly flat.
- The price is real: telecentric lenses cost several times a conventional lens and constrain working distance and field size. Use them where the measurement demands it, not as a default.
What the Chinese supply base offers
Industrial lenses are another category where domestic supply has gone deep:
- Full range. Leading domestic suppliers (such as OPT) offer fixed focal families up to 29 MP class and beyond, dedicated line scan lens series (Coloretto, Hawk, Grampus families), zooms, macros and high-precision telecentric lenses — one catalogue, one vendor, matched to the same camera line.
- Class coverage. From 2 MP budget fixed focals to 151 MP-class line scan optics, the domestic range now covers what used to require European or Japanese sourcing.
- Cost structure. Comparable MTF-class domestic lenses typically land well below imported equivalents, and custom mounts (F-mount, M42, proprietary flanges) are routine.
- Integration advantage. Buying lens, camera and lighting from one supplier means one spec chain — the lens is chosen against the sensor and the lighting geometry, not in isolation.
The premium tier (German and Japanese optical houses) still leads on the highest-end telecentric and line scan optics — but for the vast majority of stations, the domestic range resolves what the sensor can see.
Verification points before you buy
Lens defects are invisible on a datasheet and obvious in the image:
- MTF of the combo. Measure the lens-plus-camera modulation transfer at your working aperture and working distance — not the lens alone on a test bench at f/8.
- Distortion. For measurement applications, quantify distortion across the field (typ. <0.1% for measurement-grade, <1% acceptable for presence).
- Vignetting and field uniformity. Corner illumination drop-off interacts with your lighting; check the combination.
- Flange distance / back focus. Especially for C-mount adapters and spacers — the wrong stack kills resolution at close working distances.
- Line scan coverage. For line scan lenses, verify MTF across the full line length and at the scan direction, with your sensor’s pixel size.
The three most common mistakes
- Buying the lens after the camera. The lens must be specified against the sensor’s sampling and your working distance — order of operations matters.
- Trusting “5 MP compatible”. Compatible means the mount fits. Resolution is a measured property of the combination.
- Defaulting to the cheapest lens in the right focal length. On measurement stations, the lens is where your accuracy lives.
The short version
Specify lens, camera and lighting as one imaging chain; match lens resolution to sensor sampling with margin; verify the combination with measured MTF, distortion and uniformity — then choose the supplier.
That chain is exactly what our Selection Sprint builds and our Sample Verification measures: the combo on the bench, with your optics, your light and your part.
