Visual surveillance was unquestionably lawful because “the eye cannot by the laws of England be guilty of a trespass.”
Boyd v. United States, 116 U.S. 616,628 (1886)
Back-in-the-day pre-Internet and cell phone cameras, use of optical appliances and film cameras was a tricky business. Not only was the equipment somewhat bulky, it was obvious to any observer what you were doing, e.g. pointing a 35 mm telephoto-lensed camera at someone. Or even watching through binoculars. Movie cameras were even worse, and forget video – it didn’t exist for the average P.I.
Some of the P.I.s and P.I. hangers-on would assemble at the end of the day, or at a P.I. association meeting, and discuss custom vans with all manner of monitors, video equipment, and shotgun microphones. The group was usually divided into the haves, and the have-nots, the have-nots being 95% of the participants. Of course, the 5% got all the high-end work.
Most of my camera work was medium or close range, less than 75 feet. Get in, get the picture, get out. I did have a great camera, though.
A Contax 35 mm, with a 70-200 mm zoom telephoto lens, using a 2x teleconverter. It had an internal motor drive, as well. Burned lotsa film.
My binoculars were less impressive. A cheap plastic pair. The saving grace was they were 12 x 50. Could discern whether a spot was a mole or a zit @ 100 yards.
I still have the binoculars, but the camera and lens were on loan to my ex-wife when her apartment was burgled some years ago.
Oh, well, I’m not skulking about, taking photographs anymore. Film is too expensive, with processing and all. And old school. Everything’s digital, now…
And, after all, the government is doing that to all of us, anyway.