Fig. 10 The same oblique view into the “Aviogon’’ of equal focal length. The diaphragm plane 
seems to turn towards the observer. More light passes through than with a front diaphragm. 
Depending upon the refracting power of the lens group in front of the diaphragm, the 
entrance pupil becomes an enlarged or reduced virtual image of the diaphragm. It is 
apparent that the ratio of the entrance pupil size to the diaphragm size in the meridian 
section depends upon the inclination of the pencil of rays. To obtain a minimum light 
loss, the size of the entrance pupil for a strongly inclined pencil of rays must be as large 
as possible compared to that for a pencil along the optical axis (Figures 9 and 10). This 
is actually possible by a proper selection of the lens forms which Steinheil obtained 
for the first time in 1901 with his objective (Figure 1). He obtained the desired effect 
by using a strong negative meniscus lens for the front element. 
The small light loss in the distortion free objectives with interior diaphragm of Stein- 
heil, Russinov and Bertele should demonstrate that the false hypothesis of “natural 
loss of illumination according to cosine 4 «” should be replaced by a more correct con 
cept.