The retinal image of object will be larger if the object is close to
the eye, and smaller if it is further away. For this reason, it makes
sense to think of an object's visual size in terms of the angle that
it subtends on the retina. That way, objects with different real-world
sizes or which are viewed from various distances can be compared in
terms of their effective size on the retina.
The
visual angle approach also provides a useful metric for measuring acuity.
If an observer can resolve a minimum gap or feature in an acuity target
(e.g., a letter, grating or a gap in a Landholt C) that is 1 minute
across (1/60 of a degree), he or she would have 20/20 acuity. Similarly,
someone with 20/10 acuity could detect a minimum gap of 0.5 minutes
(i.e., 30 seconds of arc) regardless of target size and distance.