how can i zoom the backdrop image interactively using mouse/pen ?

in compositing , how can i zoom backrop interactively ? i can only zoom step in and step out. Is it any way i can interactively zoom in/out like zooming in uv image editor ?

You can only zoom in/out in steps. You can change how big those steps are (make them smaller) in the User Preferences / Input panel. Go to the shortcuts, expand the shortcut settings and change the factor value