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One of the most recognisable characteristics of a Zeon mobile suit is its monoeye.

Typically illuminated magenta, this large circular camera swivels around from side to side, perhaps also up and down, giving the impression that the lumbering brute is looking around.

However, the pilot's view inside the cockpit is typically shown to be the same/similar to that of a Federation mobile suit or a Gundam - A full view from the front and sides of the machine's head, reminiscent of human peripheral vision. Now that makes sense with Federation suits, as those have cameras in the eyes/visor, forehead and the back of the head, but Zeon suits are only ever shown to have one, swivelling camera. You could say that said camera has some wacky fisheye stuff going on to get the all-encompassing view, but that still doesn't explain why it swivels around.

Am I missing something?

They have a few uses, such as:

• aiding in visual-range combat (a rangefinder)


• laser and infra-red sensors (arranged around an electro-optical camera), which make accurate firing possible despite Minovsky particle interference

Regarding the mono-eye movement:

Including the rotation of the neck, the main camera could scan 260 degrees to the left and right, and some models also had upward and downward swiveling functions...but in many models the head rotation mechanism is omitted or has a limited turning radius

The RX-78 is equipped with a dual sensor unit known as the twin eye for its main camera.

Source

I can't find anything specific about why the mono-eyes glow in-universe, so here's a handy TVTropes link to explain this phenomenon.

If you read this, know that I did not look any of this up and it may not be cannon.

my belief is that the mono-eye of the zaku is an extremely acute visual and infrared sensor with a limited field of view. this makes it very useful for long range engagements and for picking out the details of certain objects at closer range. these could also be linked to the scopes used by the pilots during long range engagement and reconnaissance.

Maybe these sensors were very expensive and difficult to produce, especially in the space colonies. thus, it made sense to have it swivel around and be able to select what it focused on. When finding objects, the eye swivels around acquiring large amounts of long range data to be processed into something the pilot can look at.
from a war perspective, it makes sense to have a minimal quantity of expensive and rare parts on a zaku. especially if it is not necessary.

The gundams having two eyes does not mean that they have two cameras. they may have more than two hiding behind the eyes in the head, thus allowing for a much larger amount of data acquisition and finding capabilities. maybe it is not that difficult to make the cameras on earth with the vast amounts of materials and large swaths of land at hand. or maybe it was that the gundam has a compound eye that processed many lower quality images into a higher quality image, just like the hubble telescope. It would still be vastly expensive to produce the compound eye, but it is an alternative to the single, scope eye that the Principality of Zeon has chosen for their mobile suits.

These two methods of getting high quality images both would work great, it is just that Zeon's methods produce a quicker image and is easier to salvage when a battle is over. that is, if the zaku didn't detonate.

that, and the zaku eye is sexy. and the zaku was not originally designed to be sold as gunpla. only the gundam and the federations mechs were designed to be both cool and modelable. the zaku was the brainchild of utility and looks, not sellability.