Windows into heaven
Icon painting has its traditional techniques. In an ordinary picture things seem to get narrower as they go into the distance. This gives the picture its feeling of depth. It is called "perspective". Icons are different. Almost all icons have inverted perspective, when the picture seems to get wider as it goes into the distance - the perspective is back to front. Natural perspective of a usual picture produces the illusion of remoteness of a subject, the inverted prospective in an icon indicates the grace emitted by it. It does not push a person who is telling a prayer into deep sensual experience, but allows him or her to feel this grace.
In an ordinary painting you can often see the sun, or else you can see light and shadow. You can tell the time of day, or you can see that it is night. You can not see these things in an Icon. There are no shadows, or ways of showing day and night. An Icon shows a view of heaven, so it is lighted by the unchanging light of God.
Icons are painted this way on purpose. An Icon is a window into Heaven. The veneration granted to the Icon is said to pass on to Heaven and the person depicted therein.
Icon is a visible representation of an invisible inner world that is why icons are painted in the language of symbols. For this reason, it can seem that it is impossible to paint in this way. The task of an icon painter is to express by means of painting that what is inexpressible in the material world. All icon painters without exception are ordered by the Stoglav Council to paint icons according to the sample. Why? Because ancient icon painters (mainly monks like St. Andrey Rublev) had a gift of spiritual vision of an inner world that they could express in colours. We don't have anything of that kind, therefore we should copy GENUINE ancient icons without unnecessarily complicating matters.
adapted from "Explaining Icons" Stylite Publishing Ltd.