C-Mount Lens Selection for Industrial Cameras: Sensor Size, Resolution and Focal Length

Three of the four numbers that decide a C-mount lens are already fixed before the lens is chosen: the camera's sensor size, its pixel pitch, and the working distance the mechanics allow. The fourth — focal length — is then calculated rather than guessed. Selecting a lens by focal length alone, without checking whether it covers the sensor or resolves the pixel pitch, is the most common reason a new camera delivers a softer image than the one it replaced.

Step 1: The lens must cover the sensor

A lens is built for a maximum image circle, quoted as a sensor format (2/3-inch, 1-inch, 4/3-inch, 1.1-inch). If the camera sensor is larger than the lens format, the image circle does not reach the corners and the frame vignettes.

The check is a single comparison: lens format ≥ camera sensor format. A 2/3-inch lens on a 1-inch sensor will darken the corners; a 4/3-inch lens on a 2/3-inch sensor works, and the unused image circle is simply not used.

Sensor format is not the same as the diagonal in millimetres, though the two are related. Approximate active areas:

Step 2: The lens must resolve the pixel pitch

A 20-megapixel camera paired with a lens designed for 5 megapixels produces a 20-megapixel image of a 5-megapixel optic. The pixels are there; the detail is not.

The practical check is the lens's stated resolution rating against the camera's pixel count. POMEAS FA lenses are grouped by the resolution class they are built for — standard, 10 MP, 12 MP, and 20 MP classes — and each group is matched to a sensor format:

Reading the matrix: a 1.1-inch 20 MP camera is served by the PMS-LFPHM series (12 / 25 / 50 mm); a 4/3-inch 12 MP camera by PMS-LFHHG (12 / 35 / 50 mm). Choosing a 2/3-inch lens for either camera fails both checks at once — coverage and resolution.

Line-scan cameras are a separate case. POMEAS 16K line-scan lenses (PMS-LLC026 / LLC05 / LLC1 / LLC2, all -14640M-M72) are built for line sensors and are not selected by the area-sensor method above.

Step 3: Calculate the focal length from field of view and working distance

With the format and resolution class fixed, focal length follows from the required field of view and the available working distance:

f = (sensor width × working distance) / field of view width

Worked example: a 2/3-inch camera (8.8 mm sensor width) needs to cover a 44 mm wide part from 300 mm away.

f = (8.8 × 300) / 44 = 60 mm

The nearest available standard focal length is 50 mm, which at 300 mm gives a field of 8.8 × 300 / 50 = 52.8 mm — wider than needed, which is usually preferable to a field that is too narrow, since the part still fits with margin. If the mounting allows 375 mm instead, the 50 mm lens gives exactly 8.8 × 375 / 50 = 66 mm; conversely, at 264 mm the same lens covers 46.5 mm.

The same formula rearranged gives the working distance for a chosen lens:

working distance = (f × field of view width) / sensor width

Step 4: Check distortion where the output is a measurement

If the station measures rather than merely inspects, the lens's distortion figure belongs in the uncertainty budget. POMEAS labels low-distortion variants explicitly in its FA line — the 12 mm, 16 mm and 35 mm PMS-LFEH models and the 12 mm and 35 mm PMS-LFGH and PMS-LFHHG models are designated low-distortion (低畸变) in their product titles.

Distortion should be read with its measurement condition. A single percentage without the field position or wavelength is a best-case figure, and for gauging work the value at the edge of the field is the one that matters.

Step 5: Confirm the mechanical interface

C-mount is the 1-inch-32 UN thread with a 17.526 mm flange focal distance, and it is the interface across the POMEAS FA range. Two mechanical details still need checking: whether the camera body and any housing leave clearance for the lens barrel diameter, and whether a filter thread or a spacer is inserted, since a spacer changes the flange distance and therefore the achievable focus range.

Worked selection: three real configurations

Configuration A — 2/3-inch, 5 MP, 200 mm working distance, 60 mm field. f = (8.8 × 200) / 60 ≈ 29 mm. Nearest option: PMS-LFEH-2514M(A) 25 mm 2/3-inch (covers 70.4 mm at 200 mm) or PMS-LFEH-3514(A) 35 mm (covers 50.3 mm). If the field has to be at least 60 mm, the 25 mm is the safe choice.

Configuration B — 1-inch, standard resolution, 150 mm working distance, 100 mm field. f = (12.8 × 150) / 100 ≈ 19 mm. Available: PMS-LFGH-1616(A) 16 mm 1-inch, giving 12.8 × 150 / 16 = 120 mm.

Configuration C — 1.1-inch, 20 MP, 400 mm working distance, 150 mm field. f = (14.1 × 400) / 150 ≈ 37.6 mm. The 20 MP 1.1-inch series offers 12 / 25 / 50 mm; PMS-LFPHM-5028M(B) 50 mm gives 14.1 × 400 / 50 = 112.8 mm, and the 25 mm gives 225.6 mm. With only these three focal lengths, the working distance has to move to about 530 mm for the 50 mm lens to reach 150 mm of field, or the station accepts a narrower field.

Configuration C illustrates a real constraint: the FA line offers discrete focal lengths, not a continuum. When the calculated focal length falls between two models, the working distance is adjusted, not the lens.

What this method does not cover

This is a selection method, not a compatibility list. No claim is made that any POMEAS lens is a drop-in replacement for a lens currently fitted to a specific third-party camera, and no third-party specification is quoted here. Selection also does not end at the lens: illumination, filter choice, and the algorithm's edge definition all contribute to what the station can actually measure, and none of them appears in the focal length formula.

Camera brand and model names in this article appear only where publicly documented in customer-supplied intelligence (integrators describing their own camera equipment). They are evidence of what the market uses, not a source of third-party optical specifications.

FAQ

Does a 1-inch lens work on a 2/3-inch camera?

Yes. The image circle is larger than the sensor, so the corners are covered; part of the image circle is unused. The reverse — a 2/3-inch lens on a 1-inch sensor — vignettes.

How do I know what resolution class of lens I need?

Match the lens's stated megapixel class to the camera's pixel count. A 20 MP camera needs a 20 MP-class lens; pairing it with a standard-class lens wastes the sensor's resolution.

What is the formula for focal length?

f = (sensor width × working distance) / field of view width. All in millimetres.

Why is my calculated focal length not available?

FA lenses come in discrete focal lengths. Adjust the working distance to land on an available focal length, or accept a slightly different field of view.

Is low distortion necessary for every station?

Only where the output is a dimension. For pass/fail presence inspection, distortion matters far less than coverage and resolution.

Do I need a different type of lens for a line-scan camera?

Yes. Line-scan lenses are specified for line sensors; the area-sensor selection method above does not apply.

Next Step

Contact POMEAS with your requirements.