Working in a representational manner has always been to some degree about trying to trick the eye. The artist perceives the three dimensional world around them and has to represent this on a two dimensional plane. Most of us refer to this as "realistic" or "representational" art but it has also gone by another term. That term is "illusionism" or "illusionsistic" art. I think this term is appropriate for this topic in more ways than one especially when we consider what can happen in the way we perceive things.
When we begin drawing, most of us start by trying to get down the proportion and shape of what we are trying to represent. As anyone knows who's tried to do this, it can be quite difficult at times.Unfortunately, we can't seem to always be able to directly translate what our eyes are seeing onto the paper and instead things get turned around and muddled in our mind and the drawing starts to look completely wonky. The legs look too short, the head looks too big, basically the proportions are off. But in what way are they off is the real question. Sometimes things that we assume to be right or wrong turn out to be different than what we think they are.
Psychologists have been studying how we perceive visual information for some time now and their findings have turned up some interesting things. Some of these are optical illusions. There are quite a few of these that deal specifically with how we view scale or proportion. One of these is known as the Ebbinghaus Illusion.
In this illusion, two circles of the same size are perceived to be different sizes because of the size of the surrounding objects. This can happen to us while we are drawing. For example, we can think we have drawn a head that is too big compared to the subject when in fact we have drawn the neck, shoulders, or chest too small.
Another example is the Muller-Lyer Illusion where line segments of the same size appear to be of different lengths based on the shapes that are placed at the ends.
It can be extremely useful to remember that these sort of illusions occur and that just because we think we see something doesn't mean that it is so. It's something that should give us pause so that we really stop to try and consider what the actual nature of the problem is. We need to thoroughly check all parts of the drawing before we change or erase something to be sure we are not being fooled by the very illusion we are trying to create.