Because it only records where it is pointing, and it will be pointing somewhere else when something happens.

A single PTZ covering a large area gives excellent detail of a small part of it and no record of the rest.

A patrol tour makes this worse in one sense: the camera is predictably elsewhere for most of every cycle.

Fixed cameras provide continuous coverage, and the PTZ provides the detail when somebody or something directs it.

Investigators frequently find that the PTZ was zoomed into a car park corner while the incident happened at the entrance.

Budget accordingly. A PTZ instead of three fixed cameras is usually a false economy.

Presets triggered by alarms are far more useful than a timed tour, especially on an unmanned site.

An alarm-driven preset points the camera at the event: a beam broken, a door opened, an analytic triggered.

That turns the PTZ into a camera that looks at what just happened rather than at a schedule.

Timed tours suit manned control rooms where an operator is watching, and are close to useless unattended.

A home position the camera returns to after a set idle time ensures it does not stay wherever it was last driven.

Record the preset list with what each covers. An undocumented preset numbering scheme becomes useless as soon as the operators change.

In a controlled scene with a single moving subject, well. In a busy one it follows whatever moved last and loses the subject.

Auto-tracking detects motion and drives the camera to keep the object centred, zooming as it goes.

It works best in areas that are normally empty, such as a yard at night, where any movement is the subject.

In a car park with vehicles and pedestrians, it hands off between subjects and often ends up tracking a tree.

Rain, snow and moving shadows all trigger it, so it needs the same environmental filtering as any analytic.

Use it with a return-to-home timeout so a lost track does not leave the camera pointing at nothing.

More than any other camera type - it has motors, gears and slip rings in constant use, and they wear.

Duty cycle is the key specification. A camera running a continuous tour is working every minute of every day.

Symptoms of wear are drift from preset positions, a camera that no longer returns accurately, and noisy or hesitant movement.

Seals on external units degrade with movement as well as with weather, and water ingress is the usual failure.

Check preset accuracy periodically against recorded reference images, because drift is gradual and easy to miss.

Specify the mechanism for the intended use. A unit rated for occasional operator control will not survive years of continuous touring.

Enough to make the subject the required size in the frame at the required distance - and optical zoom is the only kind that counts.

Identification needs the subject to occupy a substantial part of the frame, and that requirement, plus the distance, determines the zoom needed.

Digital zoom simply enlarges pixels and adds nothing usable for evidential purposes.

Very long zoom ranges are hard to use manually, because a small movement at full zoom sweeps a large area, and image stabilisation becomes necessary.

Atmospheric haze, heat shimmer and vibration all limit what a long lens can resolve outdoors regardless of its specification.

Work back from the identification requirement rather than buying the longest zoom available.