When we think about eyesight, we usually think about visual acuity: how clearly we can read, recognize a face or see fine details.
But that's only one part of our vision. The visual field is everything we can perceive around ourselves, while looking at a fixed point.
Imagine for example right now, reading here this article. While your eyes are going word by word, you know that your hands are on the desk; if you are using a laptop you somehow see its keyboard. Indeed you also might see a person approaching you from a side: you are not looking at the side but you perceive the context around you anyway.
That's the Visual Field. It's an area horizontally large about 200-210 degrees and vertically 100 to 135 degrees.
Changes in the visual field can occur for different reasons and can sometimes be surprisingly difficult to notice. When they develop gradually, our brain compensates and may adapt to missing information, making a loss of vision much less obvious to perceive in everyday life.
This is one reason visual-field testing is used in eye care: it can identify areas where visual sensitivity is reduced or absent, even when the person may not consciously perceive the change.
Visual Field Tracker was built to let people record where they perceive a moving stimulus and compare those self-recorded maps over time. It cannot determine whether vision or a medical condition has changed.