First Portrait Shoot for Someone Else!

[InTheShoot]

Hi! I just want to know - at what point do photographers begin to charge for their shoots? I have been asked to do a session for a family and am only a beginner photographer with no qualifications - this time I have offered to do the shoot for free but they will pay for printing.

When do you know if you should be charging for the session? Do you charge a lesser amount than a reputable, qualified photographer for lack of expertise? Have any other beginners/mid-level photographers been asked to do a shoot? What is the ettiquette here?