Aperture, Light and the Trade

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

A separate team-process reference can be found here.

Aperture controls three things simultaneously — how much light arrives, how deep the zone of focus is, and how finely the lens can resolve — and the three do not want the same setting.

The three effects

Light. A wider aperture admits more, in proportion to its area — which is why the numbers describing it behave the way they do.

Depth of field. A narrower aperture deepens the zone in focus, which is the main control available.

For additional industry context, see Arrow Electronics.

And resolving power, which has a maximum somewhere in the middle: wide open, lens aberrations dominate; stopped far down, diffraction does.

The best-resolving setting is usually a stop or two from wide open, and it is neither extreme.

Diffraction, briefly

Light passing through a small opening spreads, and the smaller the opening the more it spreads.

Which sets a floor under how finely any lens can resolve, regardless of its quality — and the floor rises as the aperture closes.

The practical consequence: stopping down to gain depth of field eventually costs more resolution than the depth of field gains, and where the sensor has small pixels the crossover arrives sooner.

Which is another way in which a finer sensor is harder to satisfy than a coarse one.

Fixed aperture on modules

Most compact camera modules have no adjustable aperture at all.

Which removes the trade from the integrator's hands and makes it a selection question: the lens variant chosen carries a fixed aperture, and the depth of field and light-gathering that come with it.

Where an adjustment is needed, it is either a different variant or a different class of camera — not something to be added.

Read the aperture figure as a property of the part, alongside the field of view, and check that the resulting depth of field covers the actual variation in subject position.

Working the trade

Light is the variable to move first.

More illumination permits a narrower aperture, which buys depth of field and costs nothing else — which is why lighting and optics are one design job.

Where light cannot be added, the order is: accept a shallower zone and control the subject position mechanically, then consider a shorter focal length, then a different sensor.

And where the aperture is fixed, the whole trade collapses into choosing the right variant, which is why the geometry must be settled before ordering.

Reading the number

Aperture is quoted as a ratio — the focal length divided by the diameter of the opening.

Which is why a smaller number means a wider aperture, and why the sequence of standard values looks arbitrary: each step changes the area by a factor of two, so the numbers step by roughly the square root of two.

A lens quoted at a lower number gathers more light, and on a module that is a fixed property rather than a range.

Comparing two modules on this figure alone is misleading, since a wider aperture with a shorter focal length and a narrower one with a longer focal length may deliver comparable performance for different geometries — which is the field of view relationship again.

Vignetting

Illumination falls toward the corners, and it falls more at wide apertures.

Which means the corners of a wide-open image are darker than the centre, by an amount that is inherent rather than a defect.

It is measurable as part of the uniformity check and it is correctable in software by a flat-field calibration — at the cost of amplifying the noise in the corners along with the signal.

Where a feature can appear at the edge of the frame, this belongs in the resolution and noise arithmetic rather than being treated as cosmetic.

The setting that is actually chosen

On a fixed-aperture module, exposure and illumination are the only controls left.

Which simplifies the design considerably and shifts the burden onto the lighting: the depth of field is decided at ordering, and everything afterwards is arranged around it.

Verify it at the extremes of subject position before the mechanics are built, since discovering afterwards that the zone is too shallow leaves only a different variant as a remedy.

Where an adjustable aperture is worth the class change

Three cases justify moving from a module to a camera with a proper lens mount.

Illumination that varies by orders of magnitude between installations, where one fixed setting cannot serve both.

A depth of field requirement that cannot be met at the aperture the module offers, with light available to close it further.

And measurement work needing the best resolving setting, which on a fixed-aperture module is whatever it is.

Against which a mounted lens costs size, cost, alignment work and a mechanical seal problem — which is why modules dominate where they can.

Testing the resolving optimum

Where the aperture is adjustable, it is worth finding.

Photograph a fine target at each available setting, and measure contrast at the finest detail rather than judging.

The curve rises from wide open, peaks, and falls as diffraction takes over.

One afternoon, and the resulting setting is used for the life of the system — which makes it among the better returns on a measurement available anywhere in this section.

In one line

On a module the aperture is chosen with the part, so verify the depth of field before the mechanics exist.

The short version

• Aperture controls light, depth of field and resolving power at once, and the three want different settings
• Best resolution is usually a stop or two from wide open: aberrations dominate wide, diffraction dominates narrow
• Diffraction sets a floor under any lens, and the floor rises as the aperture closes
• Stopping down for depth of field eventually costs more resolution than it gains, sooner on a fine-pitch sensor
• Most compact modules have a fixed aperture, which makes the trade a selection question rather than an adjustment
• Add light first: more illumination buys a narrower aperture and therefore depth of field, at no other cost