When that happens gently bring your mind back to the chant and back to the task at hand. If you let your mind roam it will affect the PhotoReading. You may as well not be doing it. Bring your mind back and it's fine.
Bringing the mind back is reminding yourself that you want to do this and it is important to you. If you allow the mind to wonder off without calling it back you're telling yourself this isn't that important and you don't really want to bother with it.
Alex
