1867,] Mr. Claadet on a Focus-Equalize?' for Photography. 



457 



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 photo- 

 graphic 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 specimens of the process had at any time been 

 exhibited in public, probably 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 different planes of the 

 figure. 



The desideratum was, when changing the focus, to increase the power of 

 the double lens for the planes the most distant and to reduce it for the 

 nearest planes. At first this seemed to be an impossibility. But in con- 

 sidering the subject attentively, I was suddenly struck with the fact that 

 the power of any double combination of lenses being proportionate to the 

 distance which separates the two lenses greater when they are more sepa- 

 rated, and smaller when they are less separated, it was possible to alter the 

 power of the combination by changing the distance between the two lenses. 



Therefore, if, instead of moving the whole tube containing the two lenses, 

 we move only the back lens nearer the plate, when we want to focus for 

 more distant planes, we increase at the same time the power of the double 

 combination, and consequently the size of the image ; and if we move the 

 lens further from the plate, when we want to focus for the nearest planes, 

 in doing so, by reducing the separation of the two lenses, we reduce the 

 power of the combination, and consequently also the size of the image. 

 This is a most fortunate property ; for by this means it is possible not only 

 to equalize the definition of the various planes, but at the same time to 

 equalize the size of their images, and consequently to avoid the exaggeration 

 of perspective by which the nearest planes are increased, and the furthest 

 disproportionately reduced, a defect which is so detrimental to the ap- 

 pearance of large photographs. 



I submitted my plan to M. Voigtlander, the celebrated optician, and I 

 had the satisfaction to meet with his entire approbation. He found that 

 I had solved the problem in a way w^hich was perfectly correct and suf- 

 ficient in practice. But wishing to investigate the question from a higher 

 mathematical point of view, and being unable from indisposition to go 

 himself into the subject, he charged his step -son. Dr. Sommer, Professor of 

 Mathematics at the Carohnian College of Brunswick, well versed in all the 



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