Parallax is defined as the difference in alignment between two objects, when looking at them from a different viewpoint or angle. You can experience it simply, by looking at close objects with your left and right eyes separately, and observe the difference of alignment of the objects in front of you.
Stitching blends the different video streams into a full 360 video. Parallax error is introduced by the slightly different points of view of each camera.
To have a perfect stitch, each camera and lenses would have to be at the same position, which is physically impossible.
Can I avoid parallax?
It's impossible to remove it for all the case (and don't trust people who'll tell you they don't have any parallax in any case, it can't be true!), but there are different ways to reduce it and make less invisible:
- The rig should be as compact as possible, and the camera should be the closest the one from the others.
- The closest the objects are from the camera, the more likely parallax effect will be visible. Usually, parallax effect is more or less invisible after 2 meters, and starts to be very noticeable to less than 1 meter.
- If you need to have a very close object from the camera, the parallax effect will not be visible if it's in front of just one lens. If it's on the overlap between several lenses, then parallax will be more visible.