EXPOSURE AND EXPOSURE DEVICES 



227 



light in a lens is not serious in the majority of cases arising in practical photography 

 and that the latitude of film is sufficient to render absorption and reflection effects of a 

 lens negligible in most cases. 



On the other hand, the absorption and reflection of light which occurs in lenses is 

 important for four practical reasons. Loss of light may be of considerable importance 

 in those cases where very short exposures must be given under very unfavorable light 

 conditions. Secondly, a lens of simple construction may actually transmit more light 

 than a more elaborate lens of greater aperture (smaller /-number). Since the price of a 

 lens is roughly proportional to its maximum aperture, it may actually happen that a 

 simple, inexpensive lens having an /-number of //2 is actually faster than 

 a more complicated and expensive lens having an aperture stop of //1. 5. 

 If speed is the only consideration, the less expensive lens may very easily be 

 the better buy. In the third place, the fact that not all lenses have the same 

 transmission at the same aperture stop gives support to the recommendation of expo- 

 sure-meter manufacturers that the exposures determined from the meter may have to 

 be altered in certain individual cases and that correction factors may have to be 

 determined for each lens or camera. Finally, reflection of light from the surfaces of 

 the individual glass components results in scattered light which decreases the contrast 

 of the negative and may produce a dull print where a sparkling contrasty result was 

 expected. 



Table V. — Absorption and Reflection of Cemented and Uncemented Lenses 



Lens surface 



First... 

 Second. 

 Third.. 

 Fourth . 

 Fifth... 

 Sixth . . . 

 Seventh 

 Eighth. 



Syntor lens 



Percentage of 

 incident light 



100.00 

 93.91 

 88.77 

 84.55 

 80.72 

 76.88 

 73.39 

 68.93 



Percentage of 



transmitted 



light 



94.53 

 88.77 

 84.75 

 80.72 

 77 . 06 

 73.39 

 69.38 

 65.16 



Dagor lens 



Percentage of 

 incident light 



100.00 

 93.71 

 93.48 

 92.65 

 88.81 

 84.37 

 84.19 

 83.43 



Percentage of 



transmitted 



light 



94.51 

 93.68 

 93.47 

 88.81 

 86.12 

 84.36 

 84.16 

 78.86 



In general, as may be seen from Table V,^ more light is lost by reflection from 

 uncemented surfaces than is lost by transmission through a glass component. The 

 loss of light at cemented surfaces due to reflection is generally negligible. It should be 

 noted that the Dagor consists of four uncemented elements, whereas the Syntor con- 

 sists of six cemented pieces of glass. Both lenses were marked as having a maximum 

 aperture of //6.8. But the Syntor transmits about 83 per cent as much light as the 

 Dagor, and so far as light efficiency is concerned, the Syntor at//6.8 is equivalent to 

 the Dagor stopped down to f/7.5. 



Ex-posiire for Near-by Objects. — When the lens is extended beyond its infinity posi- 

 tion for photographing objects near-by, the /-number engraved on the lens is no longer 

 an accurate index of the light-gathering power of the lens, and consequently the indi- 



1 Compiled from data given on p. 81, Neblette, "Photography — Principles and Practice," 1st ed. 



