22 



HANDBOOK OF PHOTOGRAPHY 



of all light falling on it within 20 or 30° of the normal. This value ranges from 0.04 

 (4 per cent) for n = 1.5 up to 0.067 for n = 1.7. Consequently, taking a mean 

 reflecting power of 0.05, the transmission of one glass-air surface is about 0.95, and if 

 a lens has -p glass-air surfaces, the over-all transmission will be (0.95)^. Values of 

 this are given in Table VI. 



Now most of this reflected light emerges back again through the front of the lens, 

 but 5 per cent of it is again reflected by each surface as the light passes through it, 

 with the result that each double internal reflection causes about 0.25 per cent of the 

 incident light to go back into the camera as unwanted light. If the theory is worked 

 out fully including the effects of further multiple internal reflections, it is found that 

 for light entering the lens along its axis, the total light transmitted is given by the 

 fraction (1 — r)/[l -\- {;p — \)r\ whereas it was seen above that the useful light is 

 only (1 — i-y. In these formulas r is the fraction reflected at each surface, and p is 

 the number of surfaces. The difference between these two amounts represents the 

 intensity of the unwanted light, which reaches alarming proportions in a lens con- 

 taining five or six separate elements (see last column, Table VI) and explains why a 

 picture taken by a simple landscape lens in a cheap camera is often much more 

 contrasty than the same picture taken with a complex anastigmat. Each double 

 internal reflection, of course, forms an image of the source somewhere, but as these 

 ordinarily fall verj^ far from the plate, they are recorded as general illumination and 

 not as specific images. However, it does occasionally happen that one or more of 

 these doubly reflected images falls on or nearly on the plate, where it forms a most 

 annoying "ghost image." Ghosts of this type are most likely to occur when photo- 

 graphing a bright object such as the sun or an artificial source of light against a dark 

 background. Occasionally a faint image of the iris itself is formed, after two internal 

 reflections, on or almost on the plate, causing a "flare spot" to appear in the center of 

 the picture. 



Table VI. — Stray Light in Lenses (/• = 0.05) 



The true transmission of the glass itself sometimes becomes significant, especially 

 in the near ultraviolet region of the spectrum, if extra-dense flint glasses have been 

 included in the lens. This maj^ become noticeable in enlargers used with ordinary 

 developing papers which have a large portion of their sensitivity range in the near 

 ultraAdolet part of the spectrum. No fear need be entertained as to the infrared 

 transmission of a lens, for optical glasses transmit to a wavelength of 2.5 m whereas 

 no photographic plate has ye% been made which is sensitive beyond 1.2 n in the 

 infrared. 



Bubbles and scratches in a lens generally act merely as direct obstructions to 

 light and, unless of unusually serious magnitude, are insignificant. However, it 

 should not be forgotten that enough light may be diffracted hy a scratch to cause 



