Light and shade in portraiture is the second principle and just like anything else without light there is no shade. Wherever light comes from there has got to be shade and the best light and shade portrait is when the light is coming from above and from the front left or right side. The light is coming from the right and above in the example below so you get shadow in the eye sockets and down the left side of the forehead and cheek, underneath the nose, the top lip and under the bottom lip.
In this example there are dark areas in the portrait and if you were drawing a portrait say in charcoal you would put these in straight away because these show the structure of the face more than any detail you might put in. The light and shadow pattern here is one of the best light and shadow structures you can have in portraiture and there are others of course but this pattern is a well balanced pattern of light against shadow, the light side balances out the shadow side.
We then come onto the grey areas that emerge from the dark areas and into the light like in between the bridge of the nose, the crease of the right cheek and the dark shadow of the forehead where the middle tones emerge into the light areas. There are other tones but these are details like frown marks and indentations thus we have created tones which gives us an extra dimension to a portrait.
Look at the reflected light on the left side of the face in this example, always put this in because it gives a portrait the feeling of three dimensions. There needs to be a balance of light against dark throughout the picture whether it is a portrait, a landscape or a still life. If you half close your eyes and look at the light and darks in these examples you will see that there is a balance of light against dark, one doesn't outweigh the other. These are the basics of light and shade which you can carry on in portraits, landscapes or any other art you are interested in.
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