Simultaneous Contrast

The percepted brightness of an area depends not only on the real brightness but also on the brightness of the surrounding surfaces.
A grey area seems to be lighter, when the surrounding area is dark and it appears darker, when the surrounding faces are bright. When drawing this means, that if you want to present a bright area you have to place it “in front” of a dark background.

As illustration the following pictures:

 

In both pictures are the inside lying squares equal – both are coloured with the same grey. Nevertheless they seam to have a different brightness, the light and dark backgrounds result a different perception of the squares. Areas in front of a dark background seem to be brighter, in front of a bright background they seem to be darker. This effect is called “Simultaneous Contrast”

When drawing you have to handle this effect continuously. In monochromic drawings you always will have dark lines and areas beside brighter parts.

You can use the effect consciously to let surfaces appear brighter and/or darker. Since brightness can only be “drawn” on white paper by “less drawing”, means keeping an area more or less clean, the highest brightness is defined by the colour of the paper being used. But you can “light up” a bright area by surrounding it with darker surfaces.

There is another effect with the perception of brightness and shadings (and colours too). It is an effect of adaptation. When standing in front of a white wall with small contrasts our sensitivity for differences in brightness and shading is increasing. We notice nuances, which we wouldn’t see on a wall with high contrasts of black and white.

This increasing sensitivity for small differences is the reason that we notice at once repaired places on a wall. For the same reason we recon, that black marker or felt pen lines in a pencil drawing are unpleasant. The pencil drawing has small contrasts, the great contrast between the marker lines and the rest of the drawing is so big, that it disturbs the perception of the pencil drawing. The same contrast is percepted as completely naturally in a woodcut or an india ink painting. By the way, this effect is no reason, not to draw with a black marker into a pencil drawing, in the contrary - remember §1!

This adjustment of the "attention" eaable us to see strong light-dark contrasts in a silver-pen drawing, although these drawings are realy “pale” with week lines and " very low-contrasts.
On the other hand, if you see a silver-pen drawing between India-ink sketchings this will appear almost contrastless to you- because of the extreme high contrasts between the black India-ink and paper.

Again the warning: don’t take what I wrote as „rule“ or “law”, not even as advice. I for myself often sketch with marker into pencil drawing, BECAUSE the contrast between pencil and felt pen is so strong, that the drawing seams to be cut and BECAUSE it is so beautiful unpleasant.