Mobile: Apply text field focus with keyboard or not?

I'm interested in the drawbacks when applying auto focus with keyboard on text fields.

I'm redesigning an application where the user have to enter PIN etc. Normally the user have to enter maximum twice some type of content, depending on the functionality. I'm not sure if the automatic focus will enhance the user experience, or will only annoy the user because they can't decide what to do on their own?

On the other hand, the user is unable to continue or use the functionality at all if they don't enter any information. But, at the same time they are unable to explore the app, when they get the keyboard in their face all the time.

Any experiences?