They always tell you not to believe what you clearly have seen.

That was not a full view of everything, police departments across the country say. You don’t know the danger that existed out there.

It would be a joke by now if it did not work.

Denver police Officer Devin Sparks clearly beat the stuffing out of a 23-year-old man on a downtown street corner in full view of the department’s own video camera. For lying about it, Sparks and Cpl. Randy Murr got as punishment a lousy three days without pay.

Mayor John Hickenlooper on Monday said he has asked the FBI to review what happened. I say good luck with that.

He should instead have had newly minted Manager of Safety Ron Perea flattened on his carpet as he repeatedly screamed “Three days?” while making Perea tell him why he should keep his job.

Michael DeHerrera on the videotape was so clearly victimized that it prompted independent monitor Richard Rosenthal, in a rarity of all rarities for him, to publicly announce Sparks and Murr should have been fired for trying to cover it all up.

Perea, not yet two months into the job of overseeing the police, fire and sheriff’s departments, in a statement explained his punishment decision this way:

“The video, however, does not tell the entire story . . .”

It is excuse-making as old as portable videotape itself, the self-same defense used by the Los Angeles police officers who stood trial for beating Rodney King.

It worked then, the defense attorneys thoroughly convincing jurors not to believe their own eyes. And the city burned. Fifty people died.

Old news, you say?

June 28, 2010, New York City. Police Officer David London, caught on videotape hitting an unarmed and handcuffed Iraq war veteran 20 times with a baton and lying to cover it up, is acquitted of assault and false-statement charges.

The video, his defense lawyer told the jury, didn’t provide a full view of the man’s provocation and the danger London faced. Of course.

This followed an April trial in New York of rookie Officer Patrick Pogan, charged with assault, harassment and lying for sending, unprovoked, a pro-bicycling activist, in a YouTube spectacular, hurtling from his bike.

The video, his attorney pleaded, did not tell the whole story. The jury, strangely, acquitted Pogan of all but the lying charge.

Last year a sheriff’s deputy in Kings County, Wash., is videotaped pummeling a 15-year-old girl, including flinging her to the floor by her hair, in a holding cell.

The deputy’s lawyer unsuccessfully pleaded to stop the public release of the videotape because, naturally, “it does not tell the whole story of the incident.”

I could go on.

There used to be a time when I believed in the videotape. Growing up in L.A., the most shocking aspect of the Rodney King beating was that it was playing on television. Such a thing played out in my L.A. neighborhood without the camera for decades.

They got ’em this time! you figured.

My wife saw the video of Michael DeHerrera getting licked on that street corner and was outraged. I simply said, poor kid.

And then Ron Perea, sure as anything, came forward with his “entire story” nonsense.

Will the FBI buy it? My hunch is probably.

Me, I’m just going to keep on believing my lying eyes.

Bill Johnson writes Mondays, Wednesdays and Fridays. Reach him at 303-954-2763 or wjohnson@denverpost.com.