"I See You" (1976) by Damon Knight
It was originally published in The Magazine of Fantasy and Science Fiction, November 1976. The synopsis on isfdb for the short story matches your description, albeit in the very broadest of terms.
The invention of a machine to view events distant in time and space alters human society.
"Assigned to enhance image quality for a (let's say) viewfinder, a developer is puzzled when the display seems to show movement when no one else is in the room."
In the spring of 1990 he was working on an image intensification device that was puzzling because it was too good. He had it on his bench now, aimed at a deep shadow box across the room; at the back of the box was a card ruled with black, green, red and blue lines. The only source of illumination was a single ten-watt bulb hung behind the shadow box; the light reflected from the card did not even register on his meter, and yet the image in the screen of his device was sharp and bright. When he varied the inputs to the components in a certain way, the bright image vanished and was replaced by shadows, like the ghost of another image. He had monitored every television channel, had shielded the device against radio frequencies, and the ghosts remained. Increasing the illumination did not make them clearer. They were vaguely rectilinear shapes without any coherent pattern. Occasionally a moving blur traveled slowly across them.
"I See You"
"Figures out that the device is seeing through walls. Refines device and still gets unexpected imagery. Discovers that the device can display images from any distance and time in the past."
It took Smith six weeks to increase the efficiency of the image intensifier enough to bring up the ghost pictures clearly. When he succeeded, the image on the screen was instantly recognizable. It was a view of Jack McCranie's office; the picture was still dim, but sharp enough that Smith could see the expression on Jack's face. He was leaning back in his chair, hands behind his head. Beside him stood Peg Spatola in a purple dress, with her hand on an open folder. She was talking, and McCranie was listening. That was wrong, because Peg was not supposed to be back from Cleveland until next week.
After a moment he turned to the breadboard controls of the device and changed one setting slightly. In the screen, Peg turned and walked backward out of the office. When he turned the knob the other way, she repeated these actions in reverse. Smith tinkered with the other controls until he got a view of the calendar on Jack's desk. It was Friday, June 15th—last week.
"I See You"
"Realizing the consequences of the technology, he produces a few and goes to elaborate lengths to hide his identity, selling them in consignment shops."
Late in February he bought a house and an electronics dealership in a small town in the Adirondacks. In March he signed over his interest in the company to his partner, cleaned out his lab and left. He sold his co-op apartment in Manhattan and his summer house in Connecticut, moved to his new home and became anonymous.
"I See You"
"Instructions said to point in the desired direction and adjust DISTANCE knob appropriately, but always leave TIME knob set on zero."
The production model was ready for shipping in September. It was a simplified version of the prototype,with only two controls, one for space, one for time. The range of the device was limited to one thousand miles. Nowhere on the casing of the device or in the instruction booklet was a patent number or a pending patent mentioned. Smith had called the device Ozo, perhaps because he thought it sounded vaguely Japanese. The booklet described the device as a distant viewer and gave clear, simple instructions for its use. One sentence read cryptically: “Keep Time Control set at zero.” It was like “Wet Paint—Do Not Touch."
"I See You"