how to prepend /blog permalink for posts but not for custom post types

I was wondering, is there a way to have a certain base permalink like /blog for everything regarding posts, their categories and tags but have custom posts starting without this?
e.g.

• posts: /blog/%category%/%postname%/

and

• custom post types: /slug_name/item_name/

(now it’s /blog/slug_name/item_name/ for custom post types)