There is an Earthlike planet inhabited by Earthlike lifeforms including beings that are human for all practical purposes. The primary visible human civilisation is roughly equivalent in technology to Ancient Greece in the period 200 BC - 100 BC. However, the god-emperor-like semi-secret rulers of the world possess technology equivalent or greater than what we have today (2022 CE).
The secret rulers are aware that there are pesky human offworlders with comparable technology visiting their world and they wish to detect these intruders and restrict their intelligence gathering activities. While there are many measures being undertaken to achieve this detection / restriction strategy, this question is only concerned with detecting camera usage within population centres by offworlder spies. (Detecting the offworlders as they arrive, fumble their way through the language or exchange diseases with the locals are all out of scope.)
- Camera detection equipment is built into statues placed such that any person within a town will always be within 100 m of a statue.
- There is no cultural restriction on the materials that the statue can be made of - the populace know that their rulers have powers available, they just consider them magic rather than technology.
- Detection station technology is limited to that available in 2022.
- Power supply and maintenance for the detection stations are non issues, provided maintenance is not required more frequently than daily.
- Active scanning, if any, must not have adverse effects on the citizens or be detectable by the citizens (eg a solution that scans the area constantly with a multi-kilowatt laser/reflection detector combination that will send everyone blind within a few years is unacceptable, as is even a visible spectrum low-powered laser.
- Offworlder cameras may be concealable digital cameras or film cameras. However, they are equivalent to off-the-shelf consumer items that have been available in the period 1992-2022 on Earth, not custom creations of a spy agency.
- Offworlder cameras' electronics (if any) are shielded so RF detection is not an option. The offworlders are also smart enough to physically disable the flash (if any), to avoid any possibility of accidental activation.
- Offworlders will always deploy a camera from within their robes / under the fold of a cloak etc. They will not hold it up in the open to take pictures nor will they leave it unattended.
- Budget: Assume that each detection station may contain up to USD 100,000 worth of hardware with a separate and effectively unlimited software budget. Offworlder cameras are price limited to USD 1,000 each.
- Detection stations are only expected to be able to detect cameras that are pointing at them.
- Detection stations must recognise that eyes (human or other animals) are not cameras for this purpose and must not return a false positive every time a human, sparrow, moth or goat (or equivalent) looks in that direction.
Question: Given the above conditions, how long would the lens of a covertly employed camera 100 m away from a detection station need to be exposed in order for there to be a 50% (or greater) chance of the station detecting it?
Note 1: Every search along the lines of "detecting cameras" provided "how to find a hidden camera in my hotel room", which is how to slowly scan a single room for a camera rather than constantly monitoring a large area for the presence of every camera.
Note 2: While the rulers and the offworlders both have access to particular technologies that are more advanced than current real-world Earth technology, these do not impact this surveillance / counter-surveillance issue.