Visual weight is a measure of the force that an element exerts to attract the eye. Two-dimensional objects can attract attention.
Composing an image (or any type of artwork) balancing correctly the visual elements in order of weight is of utmost importance and the purpose of visual weight, in design, is to balance out how the user will view your page. By properly managing six contributing elements, you can achieve balanced visual weight, therefore a pleasing or impactful image to the human eye.
The first six classic design principles are shape, form, line, colour, texture and space. Patterns can also be included in photography. The photo below taken for illustration purposes of a Little owl (Athene noctua) shows these six visual elements.
As humans we're automatically attracted to eyes, therefore in this shot the owls' eye being the heaviest in visual weight, also due to the colour, shape and contrast with the yellow eye and round black iris. Secondly, would be space (negative and positive) of the owl and chimney silhouetting against the evening sky. The third (could be second) element would be the rusty pole, a red vertical line, so the line and the reddish colour having a 'heavy' visual weight, too much and is distracting. The tail of the owl is also distracting because of the small form size and negative space.
Finally, the colour yellow of the lichen textures on the chimney are reasonably 'light' in visual weight but could have been heavier if in focus and the lighting produced brighter yellow colours.
When it comes to shapes certain have more weight than others, for example, take two same sized shapes, a round black circle on white background will be heavier than a black square on the same background colour. Probably due to our human attraction to round shaped eyes. Then there are colours which have their different weights too, such as red being heavier than blue. And so on...
These design elements have been around, in one way or another, for centuries with artists relying on them to help them create beautiful images that make people stop and look and it's for this reason visual weight should be an important consideration when composing your photo.
However, there's a lot more to image composition when we begin including visual direction and Gestalt principles. Maybe that's for a later post.
SUMMARY
The visual weight of an element is a measure of how much the element attracts the eye of the viewer. A visually heavy element will attract the eye to it.
Visual direction is the perceived direction of forces acting on and exerted by elements. The direction is a cue to the viewer’s eye to move elsewhere.
Many characteristics can be modified to make an element visually heavier or lighter. A few can be used to establish an element’s visual direction, as can the page itself.
Visual weight and visual direction lead to principles like dominance and hierarchy, flow and rhythm and, ultimately, compositional balance.
A great book I read many years ago on the subject of visual perception is 'Concerning the Spiritual in Art' as seen through the eyes of famous modern art painter Wassily Kandinsky (1866–1944) where he discusses the psychology of colours and the language of form and colour.