[ 231 ] 



XXXI. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from p. 166.] 



May 2, 1867. — Lieut.-General Sabine, President, in the Chair. 

 H^HE [following communication was read : — 

 * " Optics of Photography. — On a Self-acting Focus-Equalizer." 

 By A. Claudet, F.R.S. 



When a solid figure is brought too near the object-glass of a 

 camera obscura, the difference of focus for its various planes is com- 

 paratively so great, that it is impossible that all the images should 

 be equally well defined. Hence, in the case of photographic por- 

 traiture, there is a want of harmony in the representation of the 

 various parts ; some are too sharply delineated, and some others are 

 confused in proportion as they are more and more distant from the 

 plane in focus. But there is another defect which- is the consequence 

 of the difference of distance of the various planes bearing too great a 

 proportion to the distance of the whole, which is that the nearest 

 parts of the figure are too much enlarged, and the farthest too much 

 reduced. 



In a paper I read at the British Association at Nottingham in 

 1866, I proposed a plan to obviate these defects, which consisted in 

 bringing all the planes consecutively into focus, by moving, during 

 the exposure, the tube of the lens or the back frame of the camera ; 

 the consequence of which was, of course, that the planes were also 

 during that movement brought out of focus, so that a sharp image 

 of every plane was impressed upon a confused image ; but they 

 were all in the same degree in that mixed state, and the result was 

 an equality of effect producing harmony in the whole, and that 

 kind of softness in the picture so much approved by artists, as 

 resembling, more than the sharpest photographs, the effect that they 

 aim at producing. 



The original simple idea of equalizing the focus of the various planes 

 by moving either the frame holding the plate, or the tube of the 

 lens, during the exposure had, it appears, occurred to several persons 

 engaged in photographic pursuits (of which I was not aware before 

 reading my paper) ; but it is certain that the plan had never been 

 practically and generally adopted, and that, at all events, no speci- 

 mens of the process had at any time been exhibited in public, pro- 

 bably because it presented several difficulties which could not be easily 

 overcome. The greatest of these difficulties I soon found during 

 my investigations, which was that, in changing the focal distances 

 merely by moving the frame or the tube, the size of the various 

 superposed images was unavoidably reduced or increased according to 

 the alteration of focus during the movement applied. 



Therefore I turned my attention to the means which might be 

 found capable of avoiding this defect ; and a fortunate idea presented 

 itself, by which I found that it was possible to preserve the size 

 of the various images during the adaptation of the focus to the dif- 

 ferent planes of the figure. 



