Field of view
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Draw a line from each horizontal edge of your monitor to your eye, and then measure the angle between those lines. If you set your flightgear field of view to that same angle (--fov=xx) then all the objects in your display will be the correct "life size".
Here is what I mean by "life size". In real life, an object of some size, located at some distance from you, will cover a certain angle of your field of view. If you setup your view parameters as described above, that same object in the simulator, at that same distance, will cover the same field of view relative to your eye. Thus I call it "life size".
When setting up a "serious" simulator with multiple displays to form a wrap-around view, (i.e. multiple monitors, or multiple projectors tiled together) you will get the best results if you carefully measure your real world screen/monitor dimensions, and setup your individual flightgear views to match those precisely.
If you think about, the angles you measure will be different when your eye point changes. This means you need to decide where the pilot is going to sit, do your measurements relative to that point, and your display system will look best when viewed from that precise spot. If you move your head (or sit in the copilot seat or view the screens from outside the cockpit, you will start to see heavy distortion in some screens (like looking at a picture from an extreme angle.) But it will all look perfect (or close to perfect) from the pilot's perspective.
One interesting thing that some people have done is to attach a head tracker to the pilot. Then as the pilot's head moves, you can adjust the individual display parameters for each monitor/projector and this gives an even better sense of 3d.
I've seen a system which used 2 projectors per display. Each projector had a polarizing lens mounted 90 degrees to each other. The projector pairs showed slightly different views from each other, slightly adjusted for each eye. Then the viewer would wear special polarized glasses with each eye having a filter mounted 90 degrees from the other. The result is that one eye would see the view for one projector, the other eye would see the other projector. Then combine this with a head tracker and you can create an extremely convincing 3d world. It would be an awesome thing for a virtual cockpit (except there wouldn't be anything out there if you reached out to touch the controls.)
These systems can get really expensive and are hard to setup at home, but they are pretty neat when they are all calibrated and working very well.
Finally, it's worth mentioning that our eyes are very adaptable, so you don't *have* to get it all perfect. When you look at a picture on the wall from an odd angle, your eye usually sorts out what you are seeing quite easily. So if you don't have your monitors configured perfectly, your eye can often adapt and get used to the layout quickly. You might want to show a larger field of view than your monitor actually covers, just so you can see enough of the world to operate your airplane. Going back to your original question ... a default of 55 degree field of view isn't one size fits all, but we determined that this was an acceptable compromise between seeing enough of the instrument panel and the outside world, but not so much that the view gets all distorted and noticably out of whack with reality.
One last, last point. Field of view is something that should be determined by the end user's physical monitor layout. Aircraft designers should never override this with a custom aircraft setting. This is just plain wrong to state it bluntly. Please fight the urge to do this!!!! If someone has carefully setup their field of view to match their own display hardware (like in the 7 monitor sim I show in the movie link above), then this shouldn't get all screwed up when they load some particular aircraft!