While reading a question here whose answer was Stephen Baxter's "Light of Other Days" book, it reminded me of a short story I read with a similar idea.
Probably read at least 30-40 years ago, in English.
The fact that the Government has invented a device that can view the past is made public (by a whistleblower, I believe). Historians go nuts besieging the government to look at their favorite time periods to reveal the truth about various historical "facts" and or "mysteries" (Who shot JFK? Who was Jack the Ripper? Was Helen of Troy really that beautiful? Did Jesus exist? etc, etc.)
And the government is stonewalls most of them and claims it's expensive to operate, hard to control to get exact time/location, etc.
So someone (maybe the whistleblower?), knowing it's possible, does math/construction and builds his own version, and realizes it's easy to make, cheap to operate, and easy to use. So he makes plans to stick it to the government by releasing the info.
Maybe he tries to force the government to release the info, or they realize he has made it, so they grab him and try to convince him to stop. But he is not convinced until they tell him, it can't see back very far (maybe just a few days/weeks, because it needs exponentially more power the further you look back.)
Finally the agents tell him: "Don't you see, you can use it to see what's happening just a few minutes ago, or even now. Your company can't make widgets? Zoom in on your competitors and see how they make them. Think your wife is fooling around on you? Zoom in on her and track her to see if she is. Your pretty neighbor is taking a bath now? Zoom in and watch her... These devices mean no one will have any privacy now."
At which point he tells them it's already too late. They kept him there so long that the letters he mailed will already be delivered to various newspapers, scientists, historians, etc.
