Shadowing concerns the position of an object's own shadow in relation to the object. Objects that cast a shadow have more of a three-dimensional appearance, while objects with no visible shadow appear flattened.
Shadows can be used to judge distance. Generally, an object casting a shadow onto another means the object casting the shadow is closer to the light (than the object the shadow is cast upon). A bright object may also appear to be closer to an observer than a darker one, but this is not necessarily so.
If an object's shadow is not attached to the object, the object is not in contact with the ground. The shadow's size may then give a clue to the height of the object.
Interposition concerns objects and their placement relative to one another. When an object partially masks another, the masking object is closer to the observer. For example, a tree between the observer and more distant natural formations (like a mountain) may screen a part of the formation from view.
Proximity, or distance between an object and observer, influences the amount of detail that can be seen on an object. Objects closer to the observer may appear to have more texture, more visible features.
In the field, haze tends to make an observer more aware of distance (proximity) than clear vision will. A clear day may make a mountain range appear closer to the observer because more detail can be discerned. Haze eliminates some of this visual feedback, affects large terrain features like mountains and helps to convince the observer's mind and eye that the perceived distance is greater and more true to the actual distance.
Relative size means the size of an object as it relates to the object's estimated distance. This helps the observer distinguish between near and far objects and can sometimes provide a more specific distance cue.
Observers use relative size as a guide based on their knowledge of general rules of perceived reality. We know that certain features in the environment do not rapidly change in size. When a fully grown tree appears greatly reduced in size, we can interpret that difference as a distance cue.
Motion parallax is the tendency of objects closer to an observer in motion to seem to move faster than those farther away. This effect varies depending on the object's position relative to the observer's fixation point.
The fixation point within the observer's field of vision changes the perception of the motion parallax effect. Put more simply, an extremely distant object that falls beyond this point will appear to move along with the observer (the moon).
Objects between the observer and the fixation point seem to move opposite to the direction of the observer's travel (roadside features). Objects close to the fixation point appear not to move or to move very little in relation to the observer (distant trees).
Perspective involves the tendency of the mind and eye to perceive parallel lines as seeming to converge as they grow more distant. Such lines seem to move toward a vanishing point on the horizon, when in actuality their relative distance does not change.
As our eyes make physical adjustments to interpret visual information about objects, the muscles of the eye make distinctive movements. We interpret this information without always realizing it is present (often subconsciously), as we alternate between looking at near and far objects.