Fig. 17 - Close-range distortion 
The lens is clearly an improvement of the classical Hypergon, where the outside negative 
meniscus have been mainly added to increase the object field without reducing the aperture to the 
lowest values. In fact the lens has 1:6.3 relative aperture, 15 cm focal length, 36° as shown in 
Fig. 18. The lens, first computed for infinity, was then "balanced" as far as the optical 
aberrations are concerned also for its use at a close range. Exactly the lens must operate from 
infinity as far as a 2 m distance. The focusing at the different distances is obtained changing the 
distance between the lens and the image plane where the photographic plate is placed. 
  
This method is better than adding some additional lenses, even if it needs a refined 
mechanics. The lens changement should give the following drawbacks : the inconvenience of 
having a certain lens outfit according to the distances and the serious mechanical problem of 
truing these lenses by hand within a strict tolerance. In fact we know that the lack of truing of the 
optical system is particularly dangerous as regards to the tangential distortion besides it makes 
worse the image along all the field. 
The curves concerning the distortion to infinity and to the minimum range of 2 m were 
given for the lens in question. The straight line curves give the absolute distortion (referred to 
the focal length or to the Gauss magnification), you may note they are very different. The dashed 
curve give the 'calibrated' distortion resulting quite well balanced in both cases. 
4.2. Principal point stability 
  
Santoni solved the mechanical problem of changing the distance between the lens and the 
focal plane to focus close range objects carrying out tests on two different devices. 
The first was formed by a threaded ring changing the distance between the focal plane 
overcoming the resistance of 4 springs. Of course the lens is prevented from rotating on its 
optical axis so as some possible asymmetries do not change their position. 
On the contrary the second foresees a pipe supporting and guiding the lens and some 
different thickness spacers for several pre-established distances. 
The screw device is more expensive but offers a more practical use. 
and iris diaphragm) is connected to the camera body by a cylindrical slide in which it can run n 
Figure 19 shows a schematical section of this realization : the lens (with central shutter © 
without turning. 
The lens, O held by the threaded ring A, is driven away from the focal plane by four 
45 
  
  
S