Auto exposure vs iris control: which one the camera sets

Point a camera at the sky and the histogram swings from blown-out white to near-black every time a cloud crosses the sun. The question that follows is a common one: does auto exposure mean the camera can place the aperture anywhere between f/2 and f/16, or does it only set exposure time? On machine-vision and small-board cameras the answer is firm: auto exposure adjusts two dials only, exposure time and gain. The aperture is a mechanical iris inside the lens, set by hand. What follows is what that means in stops, which f-number to lock for changing sky light, and what hardware it takes when the iris truly must move from software.

The decision, first

Auto exposure is a brightness feedback loop. It meters the frame, compares the reading with a target, and turns the two actuators every industrial and embedded camera owns: integration time lengthens or shortens, gain rises or falls, until the average brightness lands on the target. How that loop is tuned, from target level to response speed and limits, is covered in automatic iris, exposure and gain control in machine vision.

What the loop can never do is reach the aperture. The iris is a ring of blades inside the lens; unless it carries a motor, and the camera carries a driver for that motor, no algorithm will move it. So the decision for a scene whose brightness keeps changing, like a sky, is this: buy a manual-iris lens, set the ring two stops down from full open, lock the screw, and let auto exposure trim time and gain around the fixed aperture. As the arithmetic below shows, one setting holds from noon sunlit cloud to dusk overcast.

Buy a motorized iris only when the aperture must move with no hands on the lens: a mast-mounted rig nobody can reach, or a station that must re-range between day and night faster than shutter and gain can follow. Then order lens and driver together, because an iris motor with nothing driving it stays wherever it was last left.

Three levers, measured in stops

Every exposure control trades light in factors of two, which makes stops the only currency worth counting:

The iris term carries the square because collected light falls with the square of the f-number. Set typical machine-vision ranges side by side:

Shutter plus gain span about 19.6 stops, close to three times the whole iris. The aperture was never the brightness workhorse; it is the image-quality dial, trading light against depth of field and diffraction. That trade is treated at length in aperture, f-number and depth of field in machine vision cameras.

Gain is the last dial you spend

A well-behaved loop lengthens exposure first and raises gain last. Gain amplifies signal and noise together: every 6 dB doubles brightness and doubles the noise riding on it, and on small pixels it lifts fixed-pattern structure into view. Cap auto-gain at 12 dB while shutter headroom remains; allow the full 24 dB only when the blur ceiling blocks a longer exposure.

If the camera is driven from code, query the exposure-time and gain ranges the device actually reports before writing limits into it: query the option range before setting exposure or gain shows the call pattern. Set the loop's brightness target deliberately too; for cloud work, aim it at the deck, not the blue gaps, the way set an auto exposure brightness target for inspection describes.

Which f-number to lock

Two stops from full open is the default answer, and it survives contact with the physics.

Lens aberrations are worst wide open. By two stops down the corners have sharpened and the centre has given up almost nothing. Stop down further and diffraction collects the bill: the Airy disc diameter is 2.44 × λ × N, which at 550 nm gives 5.4 µm at f/4, 10.7 µm at f/8 and 14.8 µm at f/11. On 2.4 µm pixels that is 2.2, 4.5 and 6.2 pixels of blur. Stay at or below f/8 on pixels that size; f/11 spends resolution buying depth of field you already have. The full diffraction arithmetic is in Airy disk size and the diffraction limit: match pixel to feature.

Depth of field at sky distances is not the constraint it is on a conveyor. Focus a 16 mm lens at its hyperfocal distance, H = f² / (N × c), and with a two-pixel circle of confusion (4.8 µm) at f/5.6 that distance is 16² / (5.6 × 0.0048) ≈ 9.5 m. Focused there, everything from about 4.8 m to the horizon is sharp. Run your own focal length and aperture through the calculator:

The blur ceiling on your shutter

Clouds move, so the shutter has a ceiling set by sharpness, not by the sensor. Image-side motion speed is angular rate times focal length:

Take a fair-weather cumulus deck 500 m away in a 15 m/s wind: ω = 15 / 500 = 0.03 rad/s. With a 16 mm lens, v = 0.03 × 16 mm = 0.48 mm/s, or 480 µm/s across the sensor. Allowing two pixels of 2.4 µm, 4.8 µm of blur, t_max = 4.8 µm / 480 µm/s = 10 ms. That is the design number: 10 ms for a 16 mm lens on near cumulus. High cirrus at 8 km drifting 30 m/s gives ω = 0.00375 rad/s, v = 60 µm/s and an 80 ms ceiling, never the binding constraint. The nearer and faster the deck, the tighter the ceiling; halve the distance and the ceiling halves with it.

Worked example: one setting from noon to dusk

The rig is a 1", 20 MP-class camera with a 16 mm manual-iris lens. The MV-CS200-10UM is a 20 MP USB3 camera with a 1" sensor, a C-mount thread and a rolling-shutter IMX183 sensor in mono. Pair it with the LS-FFL-0567, a 16 mm 1.1" C-mount lens rated for 20 MP, its iris closing from F2.8–22 with distortion under 0.3%. Set the ring two stops down from full open, to f/5.6, and tighten the lock screw. Call the pixels 2.4 µm and the sensor width 13.1 mm; both are example inputs for the arithmetic, not data-sheet lines.

Framing: 2 × atan(13.1 / (2 × 16)) = 44.5°, so the frame holds about 45° of sky, roughly 410 m of cloud deck across at 500 m (2 × 500 × tan(22.25°) = 409 m).

The exposure budget, in stops:

1. Noon, sunlit cumulus: sunny-day exposure arithmetic, the f/16 rule, puts a bright deck near 1/4000 s at f/5.6 and base gain: 0.25 ms.
2. Design dim point, heavy overcast half an hour after sunset: eight stops down, so the required time at 0 dB is 0.25 ms × 2^8 = 64 ms.
3. The blur ceiling is 10 ms, so the shutter alone falls 6.4× short.
4. Spend gain: 24 dB ≈ 4 stops ≈ ×16, which brings 64 ms down to 4.0 ms, inside the ceiling with 1.3 stops to spare.
5. Quieter variant: cap gain at 12 dB (2 stops, ×4) and the need is 16 ms, over the ceiling; open the ring one stop to f/4 and it halves to 8 ms, which fits.

Decision: lock f/5.6, a 24 dB gain allowance and a 10 ms shutter limit, and the rig holds exposure from noon to dark overcast. If the frames will be cropped hard and noise matters, lock f/4 with a 12 dB cap instead: the same day span, cleaner dusk frames.

Two other bodies suit this work:

When the iris must move from software

Lenses come with four kinds of iris. A fixed iris has no ring at all; the aperture was set where the optics were finished. A manual iris is the knurled ring with a lock screw that most machine vision lenses carry, and the setting lives in metal, not in software. A DC auto-iris, the type that dominates CCTV lenses, drives the blades with a coil: it needs a driver amplifier fed by a control signal, and the connection is a separate plug, never the mount thread. A P-iris replaces the coil with a stepper and position feedback over a serial line, so software can command an exact stop.

Software aperture control is therefore two purchases, not one: the motorized lens and the electronics that drive it, whether a camera body with a DC-iris output, a standalone iris controller, or a serial channel for P-iris. Most machine-vision and small-board cameras expose none of these; their control interfaces list exposure time, gain and a brightness target, and no iris node. On those platforms the honest answer is the one above: fix the ring and let the loop work around it.

For remote framing rather than remote exposure there is motorized zoom: the LS-CTV-0066 is a 1" C/F-mount motorized zoom lens. Wiring a motor in the barrel, from drive voltage to polarity and position feedback, has its own failure modes, mapped in motorized zoom lens control: voltage, polarity and position feedback.

When the usual answer is wrong

Fix-the-ring fails in four specific ways.

The sun in frame. A focused solar image can bloom the sensor in one frame and damage it in a few more.

Night skies. Past the end of twilight the sky background, not motion, limits the image. Open the iris wide, let the shutter run to seconds, and hold gain low: amplifying a background-limited frame only amplifies sky glow.

Series you will compare. Auto exposure re-solves every frame, so two frames of one cloud differ in settings and any brightness trend inherits those jumps. Lock time, gain and iris; the drift you get when you do not is the subject of camera image brightness drifts over time: turn every auto mode off.

Broken-cloud flicker. When the sun strobes through fast cloud, the loop hunts and lags behind. Meter the sunlit state, fix exposure there and accept dark frames in shadow, or write your own loop with 0.3–0.5 s of smoothing; the trade-offs are laid out in build a software auto exposure algorithm: the control law.

Checklist before you order

Run this against the lens and camera pages before either goes in the basket:

• Iris type and lock. A manual ring with a lock screw; an unlocked ring walks with vibration and temperature swings.
• Printed iris range. F2.8–22 is about six stops, (22/2.8)² ≈ 62; any span past five stops is plenty for a fixed setting.
• Coverage. The image circle must meet or exceed the sensor diagonal, with the mount matching: a 1.1" lens is comfortable over a 1" sensor.
• Controlled limits. Set the shutter ceiling at the blur limit and the gain ceiling at the noise budget, after querying the ranges the camera reports.
• Filter plan. If the sun can enter the frame, the neutral-density or solar filter belongs on the same order.
• Motor path. If the aperture must move remotely, order lens and driver in one pass and confirm the control appears in the camera's interface.

When the parts are chosen, the system configurator walks camera, lens and mounting through the same steps.