Direct concentration on photoreading is not necessary, ie thinking nothing but photoreading. However, it is best if you direct your train of through to the book you are photoreading, ie thoughts about the subject pertaining to the book.
Concentrating at the white spaces between text as well as employing some sort of chanting is also a good way of keeping your mind from going off on a tangent.
When photoreading, you may want to direct your attension inward, ie try to notice how your unconcious mind is communicating the things you are photoreading to your concious mind. Things such as feelings is a big sign. Since your unconscious mind acts mostly in the domain of feelings and creativity, it is logical to think that the primary messagner between the conscious and the unconsious mind is feelings and insight.
try it out for yourself 
