I am also interested in this, I have them in the stable block and the system would allow me to put more cameras on.....I would say that unless your field is really tiny you would struggle to get a whole view of the field....
I was wondering about those dome type cameras though that does a view all round it? I assume they only have a limited span of how far you can see with them though??#
"Stealth Camera's" are available over here, as k9h has said they are small, run off batteries and store images / video onto a memory card.
I was advised to get one with InfaRed flash else the normal flash would give away it's location. I haven't bought one yet but intend to before mine come in at nights again for the winter.