Mr. A. Claudet on the Optics of Photography. 213 



To obviate this, or rather to equalize the effect to a certain 

 degree, it is customary to reduce as much as possible, by means 

 of diaphragms, the aperture of the lens, The object of such 

 diaphragms is to cut off all the oblique rays, and to employ only 

 the rays which emerge from the lens at the least angle possible. 

 It is evident that when the rays are emerging from the centre of 

 the lens, they follow a course so nearly parallel that any equal 

 points of the object on various planes are included between spaces 

 not varying much in size; so that although these points are 

 distant from the plane which is represented at the mathematical 

 focus with the greatest definition, they form their image within 

 a circle of confusion so near the circle of definition that the eye 

 cannot easily detect the difference, and the image of the solid 

 figure appears well defined in its various planes. 



But this result cannot be obtained without sacrificing a great 

 amount of the light which falls on the lens and is stopped by the 

 diaphragm ; consequently the time of exposure for the formation 

 of the image, is to be .increased as much as the surface of the 

 lens has been reduced. It is then obvious that, in the case of 

 portrait-taking, the advantage which would be gained, in point 

 of definition is lost by the unavoidable unsteadiness of the sitter, 

 and at all events is more than counterbalanced by the. constrained 

 expression resulting from a long sitting. 



Even supposing that the person could sit sufficiently long 

 without moving, and preserve all the while the same expression, 

 it is a question not difficult to decide in an ar.tistical point of 

 view, whether a photographic portrait showing all the pores and 

 asperities of the skin, with the smallest of its wrinkles, would 

 ever be an agreeable or artistic production. 



Excessive minuteness is the greatest reproach which has-been 

 made by, artists to the best photographic portraiture; and in 

 order to obviate it, some have gone so far as to suggest that it 

 would be desirable that photographers should take their portraits 

 a little out of focus. But these artists, forgetting certain]laws of 

 optics, failed to observe that it was impossible to represent the 

 whole of the figure in the same degree out of focus. If,, for ex- 

 ample, the nose was a little out of focus,, the eyes jvould be con- 

 siderably more so, aud the ear still more; in fact some parts of 

 the figure would be quite indistinct and confused, whilst one part 

 only would be a little softened down by a slight deviation from 

 the plane of sharp focus. 



Although such a method is therefore unavailable, this sugges- 

 tion, being made in a„true spirit of progress, was worthy of consi- 

 deration ; and a very useful lesson was to be learned from the well- 

 meantrecommendation that photographic portraits.tobe agreeable 

 and artistic in effect, should not partake too much of the mathe- 



