Zac Efron -- My Butt Is Not for Your Viewing Pleasure (PHOTO)

Zac Efron is showing plenty of skin for his new movie -- we've seen the photos -- but there is one body part he's not willing to put on camera ... his butt.

Zac continued filming "Dirty Grandpa" down in Georgia, and it looks like there's a naked motorcycle scene -- but for some reason Zac's a little ass-shy. A butt-double was photographed stepping in for Efron.

Based on pics ... you'd think Zac would be happy to flaunt his cheeks, but instead it was the double's butt in the spotlight.