Fixed Focus or Autofocus for Industrial Code Reading

How do fixed focus and autofocus work?

Fixed focus: A lens element is positioned so a defined working distance is in sharpest focus. Depth of field determines the range that remains acceptably sharp. With an adjustable iris, stopping down increases depth of field but reduces the light reaching the sensor.

Autofocus: Modern industrial code readers commonly use a liquid lens. Voltage changes the shape of a fluid-filled optical element, allowing the focal distance to change without mechanical lens movement. A processor selects the target distance and the lens adjusts through software.

Both approaches work. The application determines which one fits.

A code does not have to be perfectly sharp

Barcode decoding does not require a tack-sharp image. It requires enough contrast and image information for the algorithm to distinguish bars, spaces, or cells. Slightly soft focus can still provide reliable reads and may smooth print defects, glare, or surface texture.

The practical target is reliable decoding throughout the required working range, not maximum sharpness in every frame.

How stable is the working distance?

If products consistently pass the reader at the same distance, fixed focus is usually the simpler and less expensive choice. Once adjusted, nothing needs to refocus between reads.

Autofocus becomes valuable when working distance changes significantly, for example on multi-SKU lines with different product heights, in tote scanning, with pallets at different stack levels, or where the final mounting distance is difficult to predict.

Start with the application’s actual distance variation, not with the maximum flexibility the lens offers.

How much depth of field do you need?

An adjustable C-mount lens provides another option before autofocus becomes necessary: tune the depth of field. Stopping down the iris increases the range that remains acceptably sharp. Opening it admits more light but reduces that range.

A well-selected fixed-focus lens can therefore cover more variation in product height than its name might suggest. Liquid-lens systems address changing distances by refocusing, which introduces another consideration: time.

Is the part moving while you read it?

For moving products, autofocus settling time can become important. After a focus change, a liquid lens needs time to reach the requested focal state. If the image is captured too early, the part may not yet be sufficiently focused.

Fixed focus has no refocusing delay. At the tuned working distance, it is ready whenever the image is captured.

Worked example: At 100 m/min (330 ft/min), a product travels about 17 cm (6.6 in) in 100 ms. If refocusing occurs inside the read window, that movement must be included in the timing calculation.

This does not make autofocus unsuitable for fast lines It means focus time, conveyor speed, trigger position, and read-window length need to work together.

Fixed focus or autofocus: which should you choose?

The trade-off is straightforward: autofocus provides flexibility when the application changes; fixed focus provides simplicity when it does not.

Takeaway

Do not choose autofocus simply because it handles more variation. First determine the actual working-distance range, required depth of field, line speed, and available read window.

If a fixed-focus lens covers that operating range reliably, additional focusing capability adds little value. If the application genuinely requires multiple reading distances, autofocus can solve a problem fixed focus cannot.

Choose the lens that matches the job.