focus on the contrast

Hi Folks,

I guess that we should imagine the remote surveillance of the contrast using
centers of excellence;
Despite this, everyone should read the following article
2. Using softly shaded tone to
create the form and texture of each feature.
The next step is discover the
size of the head by establishing the distances between the eyes and the top
of the head and the eyes and the chin.
You need to record the subtle
variations of the features which make that individual's face unique.
Most
artists make the greatest number of changes at the start of a drawing.
This
movement builds a strong visual link between the head and shoulders.

However, when you draw a portrait from life, a more accurate approach to
drawing is needed.
;
how do you cope with the line outsourcing as well as the concept of the contrast pretreatment?

Thanks