background subtraction& fast moving particle image analyzing

[derya k=?UTF-8?Q?=C3=83=C2=B6se?=]

Hi to all,
I work on a project requires to analyse videos of fast moving spherical particles. I need to obtain number of particle versus time data and perimeters of particles. On the other hand, i have some problems during image processing. Actualy i do not know how to analyse the video. Under process menu there is an option as "subtract background". Which image does image j subtract as background? How can i increase the frame rate? Thank you for any help.

Best Regards