Industrial Camera Lens Selection

The Three Things That Have to Match

An industrial camera lens is chosen by three properties that must each match something else: the image circle must match the sensor format, the mount must match the camera interface, and the focal length must produce the required field at the working distance the machine allows.

These are checks rather than preferences. A lens that fails any of them produces an image that cannot be used for the intended measurement, no matter how good the lens is otherwise.

Image Circle Comes First

The image circle is the diameter over which the lens produces a usable image. If it is smaller than the sensor, the extra sensor area records vignetting and degraded edge accuracy, and no amount of field calculation recovers it.

Confirming the stated maximum format first removes incompatible models quickly and leaves a smaller set for the field comparison.

Using a lens designed for a larger format on a smaller sensor is safe: the sensor simply uses the central part of the image, where the lens is at its best.

Mount Is a Mechanical Standard, Not a Quality Grade

The C-mount, the F-mount and the M42 thread each fix a different flange distance. The mount determines what can be attached, not how good the lens is.

The consequence worth remembering is that an adapter changes the flange distance. On a lens with limited focus travel, an adapter can make the specified working distance unreachable, which is then reported as a defective lens.

The exact flange distances are fixed by the published standards rather than by any manufacturer, and they are worth looking up for the specific pair being joined rather than taken from a summary.

Focal Length Sets the Field

For a fixed working distance the field of view follows from the focal length and the sensor size. Longer focal lengths produce a smaller field and finer object-space resolution; shorter focal lengths produce a wider field and coarser resolution at the same standoff.

Across the POMEAS FA models that publish both figures, focal length spans 8 to 82 mm and field of view spans 8.4 to 89.1 mm at each model's specified working distance.

That the two spans coexist in one family is useful: it means the trade can be resolved by choosing a model rather than by changing the station.

Working Distance Is a Mechanical Constraint

Working distance is the free space between the front element and the part. Across the POMEAS FA models that publish it, working distance spans 6.6 to 690 mm.

The short end leaves little room for illumination and for any mechanism that approaches the part. The long end leaves room for both, at the cost of a larger lens and usually a smaller aperture.

Because the choice is mechanical, it is often decided before the optical comparison begins: a station with a conveyor and a ring light simply cannot use the shortest models.

What Accuracy Follows From This

The accuracy of a lens-based measurement follows from the object-space resolution that the field and the sensor imply, plus the geometric errors the lens introduces.

Across the POMEAS FA models that publish it, object-space resolution spans 12 to 1000 um and TV distortion spans 0.002 to 3.73 %.

The distortion span is wide because the family covers wide angles as well as moderate focal lengths, and the wide models carry most of it. For dimensional measurement the distortion figure for the specific model, at the specific field position used, is the one that enters the budget.

A Selection Order That Avoids Rework

Confirm the sensor format and therefore the required image circle; confirm the camera mount; compute the field required at the available working distance; then choose the focal length that produces it.

Only after those four are fixed is it worth comparing models on distortion, resolution and price. Comparing before the constraints are fixed produces a shortlist that cannot be used.

Where a Standard Lens Is the Wrong Tool

A standard FA lens cannot hold magnification constant when the part moves in height, and it carries more distortion toward the field edge. For an application that measures a dimension on parts presented at varying height, a telecentric design is the correct choice and the price difference is buying a property the application needs.

For presence detection, classification and guidance, neither property changes the result, and a standard lens is the right answer.

Frequently asked questions

How do I know which image circle a lens needs?

It must be at least the diagonal of the sensor. A lens whose stated maximum format is smaller than the sensor will vignette and lose edge accuracy regardless of the field calculation.

Does the mount affect image quality?

The mount itself does not. An adapter inserted to convert between mounts does, because it changes the flange distance and therefore the focus range. On a lens with limited travel that can make the specified working distance unreachable.

What focal length do I need for a given field of view?

The field is the sensor size multiplied by the working distance divided by the focal length. Solve for the focal length once the sensor, the field and the available working distance are fixed.

Is a longer focal length always sharper?

No. Resolution depends on the aperture and the design, not on the focal length alone. A longer focal length gives a smaller field at the same distance, which makes a given feature cover more pixels; whether more detail is actually resolved depends on the optics.

What distortion can I expect from a standard FA lens?

Across the POMEAS FA models that publish it, TV distortion spans 0.002 to 3.73 %. The wide-angle models account for the upper end, so the figure for a specific model matters more than the family span.

When should I use a telecentric lens instead?

When the part moves in height during measurement, or when the measurement reaches the field edge. Those are the two conditions under which a standard lens introduces errors that no calibration removes.