6 
In the E4, the controls for adjusting image sharpness in fulfilment 
of the lens equation and the Scheimpflqg condition are both fully 
automatic. For maintaining the relative distances for satisfying 
the lens equation, a new servo system with indirect connection to 
the negative carriage, shown diagramatically in Fig.15, has been 
developed; it is not influenced by the working load. 
The lens carriage (4) and the negative carriage (5) are guided 
precisely vertically on two sturdy columns (2). For introduction 
of the enlargement settings, the spindle (l) is driven electrically 
or by the footdisk to move the lens carriage to the distance 
f + X 2 ^ rDm ^he Projection table. A change of this distance of 
A x 9 requires a change of the distance between lens and negative 
of Ax, = - (—• A x , in order to satisfy the lens equation 
2 x 2 
x l ’ x 2 = V The CDn ^ rDl system taking care of this condition 
functions as follows: 
Mounted on the lens carriage is an elbow lever (6), the roller B 
of which bears on the inversor curve (3) under tension of a spring. 
The shape of the curve is such that, at each change of the enlarge 
ment setting, the other end A of the elbow lever is raised or 
lowered by exactly x^ in relation to the lens plane, i.e. by the 
same amount as the negative plane must be moved. Although it would 
be possible to control the movement of the negative carriage direct 
ly through the lever, this is not desirable because of the in 
fluence on the control system resulting from the load. The negative 
carriage is therefore not supported by the end A of the elbow lever, 
but on a stout spindle (17) which is subjected only to axial 
loading. This spindle is driven by an electrical regulator circuit. 
At the input end of the circuit is a differential transformer (8), 
the coil of which is fixed to the negative carriage while its core 
(7) is supported by the end A of the elbow lever. Uhen the coil is 
exactly in the central position in relation to the core, the regu 
lator circuit is passive. Uith a change of the enlargement setting 
(i.e. a movement of the lens carriage), the core of the differential 
transformer is moved vertically by the elbow lever. This causes 
a small voltage difference which corresponds to the amount and di 
rection of the movement. This signal voltage must be amplified and