Almost all Renaissance and later portraiture uses light coming from
the subject's right (viewer's left).


This is simply not true. A broad survey of western painting shows only
a slight bias toward portraits lit from the left (about a 55/45 split),
with some painters favoring left, some right and some exhibiting no
preference.


In the early days before people read as much as we do these days, heck,
before it was common for people to read, it didn't matter which side you
placed the light. Today, since most all readers have a natural tendency to
enter an image from the left side (this has been shown in studies were they
use a laser to map where the eye is directed as it moves across a page.

painters and early photographers used northlight studios and I would imagine
the side it was placed depended on the shape of the room.



Even more interesting is photographic portraiture, which shows a
virtually even 50/50 distribution of left-right preference in a survey
of fine art and commercial portraiture over the last 100 years. The
only exceptions are certain amateurs and camera-club members who are
unduly influenced by dogmatic rules telling them that the light must
come from the left.


use of right light in a commercial image is often a deliberate rule break to
cause the eye to stop, commercial imagery often uses deliberately bad
composition to cause the viewer to stop and mentally try and balance the
composition, usually implanting the item in your brain.

the use of a strong or contrasty right light can slow the eye down like a
speed bump.

at least for me