A field mixer will be generally RUGGED design. Meant for taking a bit of punishment. Usually metal cased. Has pro metering, tone generators, means of providing audio to external devices by a number of output routes and varying levels (such as mic level and line level at the same time by different outputs. it will have facility for monitoring control, allow listening in on any input, have adjustemnt for mic types on every input separately, sich as dynamic mic, phantom 48v, t12, etc etc
The mic pre amps are a key consideration when working at a high spec and this is where the money goes. Pro users will spot the difference as will the post production chain.
They are expensive (good ones)
Peter
Post Production Dubbing Mixer