VISION AND THE TECHNIQUE OF ART. 11 



From the point of view of the technique of art the question arises is 

 it necessary that these characteristics be reproduced in the depicting 

 of an object upon which the eye is focused? 



The care used by the better artists to paint broadly, i.e., not to get, 

 even in the most detailed parts of their pictures, any more detail than 

 is apparent to them at the distance at which they stand from their 

 model or scene and their quite common practice of softening and treat- 

 ing their edges, is evidence that to be technically satisfactory from an 

 artistic point of view the detail of an object on which the eye is focused 

 should be depicted with the same characteristics with which it is 

 imaged upon the retina. This evidence is further supported by the 

 fact, as will be shown later, that the characteristics of the images of 

 objects upon which the eye is not focused, must be reproduced to 

 bring out the appearance of depth and to make the pictures pleasing. 



Chapter II. 

 DEPTH OF FIELD (AXIAL). 



In this chapter we will deal with the nature of images of objects that 

 are in the line of vision nearer and farther away from the observer than 

 the object upon which the eye is focused. 



The images of such objects will have markedly different charac- 

 teristics from those of objects in focus. 



These characteristic differences are due primarily to two factors. 

 First, Depth of Field of the lens system; second, Chromatic Aberra- 

 tion. 



Figure 9. Diagram showing diffusion of images of point source objects 

 not in focus. 



DEPTH OF FIELD. 



For sake of brevity and simplicity a full explanation of Depth of 

 Field will not be gone into. It is evident however that an object which 

 lies nearer or farther from the observer than the object upon which he 

 is focused will be imaged not on the retina but behind it or in front of 

 it. This is shown in Figure 9. 



