Accept focus across the required depth, not at one target point

Bottom line: camera focus should be accepted as a three-dimensional operational zone. A face, badge, vehicle, or package can be sharp at the commissioning marker while the places where incidents actually occur fall outside useful depth of field.

Source fact: lens and scene geometry determine the useful image

Axis’s Lenses in surveillance explains that field of view depends on focal length and image-sensor size. It describes depth of field as affected by factors including focal length, aperture, focus distance, and viewing conditions. It also warns that a lens must be compatible with the sensor; a mismatch can produce effects such as dark corners or an unintended telephoto-like crop.

The settings interact. Opening an iris to gather light can reduce depth of field. Increasing focal length can narrow the scene and change the in-focus range. Digital sharpness cannot restore detail that was never optically resolved.

Source boundary and applicability

The paper provides optical principles and Axis product context, not an acceptance threshold for a security task. Actual results depend on lens and sensor tolerances, focus method, aperture control, temperature, housing window, illumination, shutter time, motion, compression, and viewing scale. Requirements must be defined for the real scene.

Applicability questions

What must be observed or identified, and at which nearest and farthest points?

Does the iris change automatically between daylight and low light?

Can autofocus select a high-contrast background instead of the target zone?

Is the installed lens designed for the camera’s sensor size and resolution?

Do a dome, protective window, IR reflection, vibration, or temperature change the focus result?

DSE recommendation: commission near, middle, and far task planes

The following steps are DSE recommendations based on the cited source.

Mark at least the nearest, nominal, and farthest positions at which the defined task must succeed. Place appropriate test targets at each plane and capture them simultaneously where possible. Run the test in daylight, the lowest intended illumination, IR mode if used, and any other condition that changes aperture or focus. Retrieve the recorded stream rather than relying only on the setup preview.

If the full range cannot meet the task, adjust placement or lens, add illumination, narrow the operational zone, or add a targeted camera. Lock or monitor focus settings where the product permits, but preserve a controlled method for refocusing after maintenance.

Verification and evidence

Retain the task definition, scene drawing with distances, camera and lens models, sensor and mounting information, focus and exposure settings, retrieved clips or stills from every plane and lighting condition, and acceptance signatures. Repeat after lens, housing, camera, illumination, or scene changes.

Official references

Lenses in surveillance – Axis Communications