Focus, Fixing It and Losing It

Generic engineering material, applicable to any module from any supplier.

A separate team-process reference can be found this page.

A fixed-focus module is set at manufacture and never moves, which is a reliability advantage and a constraint — and the choice between it and the alternatives belongs early, because it interacts with the mechanics.

The three arrangements

Fixed focus. Set at manufacture to a distance, then locked or bonded. No mechanism, no drift, no adjustment to lose — and the working distance is decided when the part is ordered.

Adjustable focus. A threaded lens barrel set during assembly or commissioning. Flexible, and it can be set wrong, and it can move.

A useful external technical reference is Embedded.

Autofocus. A mechanism that focuses continuously or on demand. Handles varying distance and introduces a moving part, a settling time, and a decision about when it refocuses.

Each is right somewhere. Where the distance is known and stable, fixed focus is the better engineering choice rather than a limitation.

What autofocus costs

Time. A frame captured while the mechanism is settling is a blurred frame, and in a triggered inspection that is a defect rather than an inconvenience.

Repeatability. Two focus operations on the same scene may not land identically, which for measurement introduces a variation nothing else in the system has.

Reliability. A moving part in an industrial environment, subject to vibration and to the same temperature range as everything else.

And unpredictability. A mechanism deciding what to focus on can decide wrongly — focusing on a reflection, on the window, or on whatever crossed the frame.

Where the distance varies genuinely, it earns all of that. Where it does not, it is a mechanism solving a problem that does not exist.

How a set focus is lost

Adjustment ring vibrating loose, which is the commonest and is prevented by locking it — with a thread-locking compound, a grub screw, or bonding.

Thermal movement, since materials expand and the sensor-to-lens distance changes — small, and significant where the depth of field is narrow.

Mechanical shock moving a lens in its mount.

And somebody adjusting it. A ring that turns is a ring somebody will turn, particularly when an image looks wrong for an unrelated reason.

Which is an argument for bonding after commissioning on anything that will be serviced by people who did not set it.

Setting it properly

At the working distance, on the real target, measured rather than judged.

Photograph a target with fine detail and measure the contrast at several ring positions — the maximum is the correct setting, and it is frequently not where the image looks best to an eye.

Then check the extremes of the part's actual position variation, since a focus set at nominal may not cover the range.

And record the setting so that a replacement can be set to the same place rather than adjusted afresh.

Focus and the infrared question

A lens does not necessarily bring visible and near-infrared light to the same plane.

Which means a system focused under visible light may be slightly out of focus under an infrared illuminator, and the shift is largest on simple lens designs.

Where a dual-band system switches between the two as described elsewhere, the focus that suits one may not suit the other.

Two responses: a lens corrected across both bands, or a compromise focus set between the two and verified at both — which works where the depth of field is generous and fails where it is not.

Test at both wavelengths rather than assuming, since the effect is easy to measure and easy to overlook.

Checking focus in service

A reference target permanently in the field of view, or a known feature of the scene.

Contrast measured on it at intervals makes a focus shift visible before it causes failures — and distinguishes it from contamination, which produces a similar-looking loss from a different cause.

The two are told apart by the shape of the loss: defocus blurs fine detail preferentially, contamination reduces contrast across all scales.

Liquid lenses

A focus mechanism with no moving mechanical part, changing shape electrically.

Fast, and with no wear surface — which addresses the reliability objection to autofocus directly.

It costs a driver, a supply, and a component with its own temperature behaviour, and availability in module families is limited.

Mentioned because it is the answer where varying distance and vibration coexist, which is otherwise the hardest combination on this page.

The adjustment that should exist and often does not

A way to verify focus without disassembly.

Either a live view accessible during service, or a stored reference image the current one can be compared against.

Because a technician at a machine cannot judge focus from a still frame alone — the contrast comparison needs a baseline, and without one the assessment is guesswork.

Providing it costs a menu item, and it converts a service visit from an investigation into a check.

Autofocus that runs once

A middle position worth knowing about.

A mechanism that focuses at start-up or on command, then holds, rather than continuously.

Which removes the settling-time and unpredictability objections while keeping the ability to accommodate a distance that changes between installations rather than between frames.

It suits field-installed equipment where the working distance differs by site and is fixed once there — and it is a software decision as much as a hardware one, since the mechanism is the same.

In one line

Fix it, lock it, and record the setting, because a ring that turns is a ring that will be turned.

The short version

• Fixed, adjustable and autofocus: the first has no mechanism to drift, the last handles varying distance and adds one
• Autofocus costs settling time, repeatability, reliability and the risk of focusing on the wrong thing
• Where the distance is known and stable, fixed focus is the better engineering choice rather than a limitation
• A set focus is lost by vibration, thermal movement, shock, and by somebody turning a ring that turns
• Bond or lock the adjustment after commissioning on anything serviced by people who did not set it
• Set focus by measuring contrast at several positions rather than by eye, then check the extremes of part position