External Video Monitor
For even the best eyed director of photography the LCD and viewfinder can be a little small to really gauge and frame images. On real film productions they will usually include an outside monitor that is attached so that they can get a larger image to operate on. There are a number of different uses for an external monitor on your motion film or digital video camera, as well as different monitor types.
Monitor and Viewfinder
A monitor is going to be essential for the artistic choices in image framing and planning, but not for perfect calibration. A large monitor can be used when blocking a shot and arranging what it will look like essentially. With this you can get a pretty clear idea of how the image is going to end up looking in the final print. The monitor is not a clear way of seeing exactly how sharp the image is. For actual focus the best way to calibrate is to do it through the viewfinder. It is hard to get into the focus object, or focus card, on the monitor. Instead the monitor is simply to be used to get a general idea of the image itself instead of the finer technical areas.
You will have to go through the same checks on the external monitor that you do on the display on the camera itself, like the LCD display and the viewfinder. The main thing that you will have to adjust on the monitor is the color settings. To do this you are going to initiate the color bars on the camera once the monitor is attached. Make sure that the camera image is showing up on the monitor ahead of time. Once the color bars are coming up you can adjust the color balance, brightness, and other specifics until it looks adequate. Once you return to the regular image you can check it against the image you are receiving on the LCD display.
External Monitor Types
Usually the larger and highest resolution monitor you can find will be the best. With high quality HD cameras you may want to try to use an HD monitor along with it. HD monitors are made specifically for this purpose, but for a low budget digital video filmmaker this may not be the best choice. Instead you can use a plasma or LCD television for this purpose and can attach one to a movable stand. This can then be plugged in to your camera during production. A great option is to use a laptop, most specifically a Macbook Pro. These will have great image clarity and give you a host of options. One great option is that you can do a live capture right onto the laptop while you are filming. This can take the part of actually recording to a tape or digital storage medium, or just giving you a double set of exposure files.