The 49ers' recent exodus included the losses of defensive mainstays Justin Smith, Patrick Willis, Aldon Smith and up-and-comer Chris Borland, but veteran safety Antoine Bethea sees a new foundation rising.

"We'll look really different than we did last year," Bethea on Tuesday told KNBR.

Bethea likened the approach to the Jim Harbaugh era, when the 49ers' defense consistently ranked among the top in the league under coordinator Vic Fangio.

"We're going back to similar to when Vic Fangio was here, you know, our front seven would stop the run and get after the quarterback, and let the DBs sit back and make plays.

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"So we're kind of back to those days," he added. "You know, just trying to go out there and make plays, and really get back to the form where the 49ers defense used to be top five in all categories. That's what we're aiming to be."

Due to head coach Chip Kelly's up-tempo offense, Bethea also noted the 49ers defense "should be in great shape," and "at the top of our game."

And the three-time Pro Bowl defender is optimistic about all three levels of the defense, starting with the back end.

"At our corner position, it's going to be a battle," Bethea, who had four picks in his first year with the 49ers, said. "We have a lot of guys, you know, Dontae Johnson, Keith Reaser, Rashard (Robinson), we have a lot of good talent outside, man.

"A lot of people are talking down about our secondary, but I think — no, I'm not going to say I think, I know we're going to surprise a lot of people," Bethea stated. "On the back end, we have a lot of young talent, a lot of guys competing to get on the field."

But as it did under Fangio's guidance, the defensive impact starts near the line of scrimmage. And Bethea is confident the young players recently drafted by general manager Trent Baalke are ready to contribute.

"Even up front. Arik Armstead, coming into his second year, he's going to do a lot of good things for us, man. There's a lot of guys on our side of the ball, a lot of young guys. I really believe they're going to help us out a lot this year."

The 49ers in 2015 ranked 29th in total defense, and finished 18th in points allowed per game.

In Fangio's first three seasons in San Francisco, the 49ers never ranked outside the top five in total defense or points allowed per game.