Fixed Focus, Autofocus and the Choice

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

For the non-technical side of coordinating engineering work, see mouse jiggler detection software.

Focus arrangement is decided at ordering on most module families, which makes it a selection question rather than a setting — and one that constrains the mechanical design around it.

The four questions

One. Does the working distance vary? Between installations, between parts, or not at all — three different answers with three different arrangements.

Two. How deep is the acceptable zone? Depth of field at the chosen aperture and focal length, against the actual variation in subject position.

A useful external technical reference is Microchip Technology.

Three. Is there time to focus? A triggered inspection with a part passing has no settling time available; a system imaging a static scene does.

And four. What environment will the mechanism live in? Vibration and shock are hostile to moving parts in a way a bench is not.

Answer those and the arrangement follows without any discussion of image quality.

Where fixed focus wins

A known, stable working distance — which describes most fixed industrial installations.

And it wins on more than reliability. No settling time, no variation between captures, no mechanism drawing power, nothing to fail — and one fewer thing to lose in service.

The constraint it imposes: the distance is decided when the part is ordered, so the mechanical design must hold to it and a later change of geometry means a different part.

Which is a reason to settle the geometry early rather than an argument against fixed focus.

Where a mechanism earns its cost

Field-installed equipment where the distance differs by site and is fixed once there — for which a focus-once-at-commissioning arrangement is frequently sufficient and avoids continuous refocusing entirely.

Applications imaging objects at genuinely varying distance, where no single setting covers the range.

And systems needing close-up and distant capability from the same camera.

Outside those, a mechanism is solving a problem that does not exist, at the cost of everything listed above.

The middle option

Adjustable but locked.

A threaded barrel set during assembly or commissioning, then bonded or locked.

Which gives flexibility at build time and the reliability of a fixed lens afterwards — and is the arrangement most often appropriate where a family does not offer a fixed variant at exactly your distance.

Its failure mode is the adjustment nobody locked, which is how a set focus is lost and is prevented by a documented step rather than by good intentions.

What varying distance usually means

Worth interrogating, because the answer is frequently narrower than the question suggests.

"The distance varies" sometimes means it varies between two known values — two product sizes, two fixture positions — which is a case for two fixed cameras or one repositionable mount rather than for a focus mechanism.

Sometimes it means it varies within a range, which the depth of field may already cover once the arithmetic is done.

And sometimes it varies genuinely and continuously, which is the case a mechanism exists for.

Measure the variation before concluding which, since the three have different answers and only the third is expensive.

Ordering the right variant

A fixed-focus module is set to a distance, and that distance is part of the part number on families that offer the choice.

Which means the geometry has to be settled first — the calculation from installation to part number — and it is the same calculation that fixes the lens.

Where a family offers focus set at a standard distance only, check that the depth of field at that setting covers your subject: a module focused at infinity and used at 200 mm may or may not work, and the answer is arithmetic rather than optimism.

The setting-once question

Where a mechanism exists, when does it run?

Continuously — responsive, unpredictable, and consuming time per frame.

On command — from the machine controller, at a known safe moment.

Or once at commissioning, held thereafter.

The third is frequently what a "we need autofocus" requirement actually means, and it is much cheaper in every respect than the first.

Testing a fixed-focus choice before committing

An evaluation kit at the intended distance, with the real part.

Photograph at the nominal position, then at the near and far extremes of the measured variation, and measure the contrast at each rather than judging.

Three images and three numbers. If all three meet the requirement, the fixed variant works and the mechanism was unnecessary.

If the extremes fall short, the choices are a smaller aperture, a tighter mechanical constraint on the part, or a mechanism — in that order of cost.

An afternoon, and it settles a decision that cannot be revised after the module is designed in.

Documenting the decision

The distance, the depth of field, and the measured variation it must cover.

Three numbers with the part number, because the next person to ask "why is this fixed focus" needs an answer, and because a change to the mechanics makes the consequence visible immediately.

Where the answer is that a mechanism was rejected, say why — the four questions and how each was answered, in three lines. A decision without a recorded reason gets reopened annually, and reopening it costs more each time the design matures.

In one line

Measure how much the distance actually varies, and two of the three cases turn out not to need a mechanism.

The short version

• Focus arrangement is decided at ordering on most module families, which makes it a selection question
• Four questions settle it: does the distance vary, how deep is the zone, is there time to focus, and how hostile is the environment
• Fixed focus wins on more than reliability: no settling, no variation, no power, nothing to fail
• Its constraint is that the geometry must be settled early, since a later change means a different part
• A mechanism earns its cost for field-installed equipment, genuinely varying distance, or close and far from one camera
• The middle option is adjustable-then-locked, whose failure mode is the adjustment nobody locked