1. Their eyes are not yet taught to see the subtle variations.
2. The fear of messing up a drawing.
Starting artists can see big lights and big darks and halftones easily enough but it takes eye-training to see the finer variations in tone.
In this commentary we will focus on the interplay of strong unconventional lights and darks that are cast upon the model's skull, i.e., light-dappled patterns.
As usual, we first strike the arabesque and place the facial features (brow line and base of the nose).
When dealing with a complicated value pattern the principal task is striving to simplify the patterns. Sketching, in general, is invariably an exercise in simplification, i.e., making choices.
The next step is that of blocking-in the overall dark/light pattern with single tones. At this point do not yet refine the tones because doing so will surely lead to failure.
Using a paper or your fingers, blend the graphite. If you use a paper stump be careful not to dull the values. The lights can be added, improved, and refined with your putty eraser.
The facial features and the hair are now carefully plotted in.
Keep these structural lines very soft. Also, continue constructing the value shapes|forms keeping in mind that, except for the sun-dappled pattern, the tone must be kept down, i.e., nothing in the shadows must stand out.
This is a subtle balancing act. Tone must be added and then delicately modeled and/or removed. This requires you to constantly going back and forth.
Also, do not be tempted to finish the facial features but be sure that the likeness is already there and that the sizes are correct. That is, at this point, your drawing should look like an under-painting.
Like color, tone is affected by the neighboring tone. You can see, for example, that once the hair is hatched-in and somewhat refined that the facial area now looks lighter than before.
Now, you can return to the facial features and break down the large value masses into their various shapes and relationships by employing hatching, stumping and delicate eraser work.
The test with dappled light is to make the delicate patterns while still keeping an interconnected entire. As a general rule, cast shadows have hard edges while shape shadows have soft edges of varying degrees. When using a stump, you should barely tap the paper. This allows you to maintain the tiny lights that bounce off the paper thereby maintaining the image alive and vibrating with light.
To produce the most fragile values utilize the blackest, hardest pieces of putty eraser you can find.
As is the case with the facial features, the tones of the hair must be subordinated to the overall light and must match with the face both physically and emotionally.
Finally, at this point you can leave the sketch as is or you can push it further by adding more detail. That is an creative decision left to you.
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