Often people assume that realistic or classical painting is merely copying subject matter. They do not realize that good painting is very much like a good game of chess. There are strategies according to which a painter will create his work. To emphasize his vision of the subject, the artist will create particular types of edges, make alterations in the composition, and alter values and colors. As he pays careful attention to these things, he must also pay parallel attention to the Unity of Effect. RH Ives Gammell, an excellent artist and legendary teacher in Boston, wrote on the Effect in his biography, Dennis Miller Bunker:
On Unity of Effect
"People untrained in the art of painting often believe that finish is attained by simply adding detail to detail and consequently they dismiss it as a mere by-product of industry and patience. Unfortunately this view does not correspond with the truth.
For an essential characteristic of all fine painting is unity of effect, and this unity is destroyed by any detail stated in a false relation to the other component parts of the picture. This is particularly true of the type of painting we are here discussing, the purpose of which is to recreate on canvas the impression made on the painter's eye by the landscape before him.
To achieve this end, each detail must be set down with the degree of definition and coloration which it holds for the eye when the focus of vision is adjusted so as to include the entire scene depicted. Piecemeal notation of individual detail immediately destroys the requisite unity of impression and turns the canvas into a compilation of separately observed visual facts. This invariably results in a hard, dry look, destroying all breadth of effect and offensive even to those who are quite unaware of its technical cause.
It is, in fact, one of the most serious defects which a painting can have and perhaps the most difficult defect for an earnest painter to avoid. The ability to carry a picture to a high degree of finish without losing its unity of impression is the mark of a master and requires artistry of the highest order. It is the central problem of the type of painting which takes for its main theme the interpretation of the beauty of the visible world."
from RH Ives Gammell's book, Dennis Miller Bunker, pp. 65, 66