Modular optical relay lens system

The present invention relates to a relay lens system, wherein all image aberrations are corrected. This permits modular use with no stand-alone lenses or lenses with great focal distance. The relay lens system comprises at least two subsystems, wherein at least one positive lens and at least one negative lens are placed in at least one subsystem near the intermediate image, and the refractive index N(+) of at least one positive lens and the refractive index N(-) of at least one negative lens and the total focal distance F(FLD) of the lenses of the subsystem near the intermediate image correspond to the following relations: EQU N(+)>N(-)and F(FLD)>0.

FIELD OF THE INVENTION 
The present invention relates to an optical relay lens system, comprising 
at least two subsystems. 
BACKGROUND OF THE INVENTION 
Optical relay systems of this kind are used e.g. in endoscopes or similar 
optical devices. A relay system images the object F1, generated by an 
objective lens into the image F2. Endoscopes comprise also several relay 
systems, mounted one after the other and generating a last intermediate 
image in the focal plane of the ocular lens or, with or without an 
additional lens, on a detector, e.g. film or CCD. 
Relay systems of this kind are known. FIG. 1 shows a simple example, 
wherein all image aberrations may be corrected. However, this system has a 
very pronounced vignetting making it almost unusable for film and 
television applications. Another known system, shown in FIG. 2, allows to 
eliminate the vignetting but does not allow to correct the image field. 
The aberrations multiply with the number of relay systems mounted in the 
optical device. This problem is solved in the state of the art with 
compensating objective and/or ocular lenses. With such lenses, however, a 
different objective or ocular must be used each time, the number of relay 
systems is changed in order to vary the overall length or the image 
orientation. A further disadvantage lies in the fact that the uncorrected 
separations demand narrow tolerances and that the possibility of 
compensating such an objective lens implies a given limit for the number 
of relay systems. 
Further, relay systems are known, e.g. according to German DE 2,619,393-C2 
or German DE 3,534,210-A1, wherein the image field may be corrected and 
the vignetting may be eliminated. A disadvantage of these systems is, 
however, that they comprise uncemented lenses with large focal distances 
which are difficult to center and consequently of elevated cost. They also 
require a very precise mounting. 
The principal object of the present invention is to design a relay lens 
system, wherein all image aberrations may be corrected, which therefore 
permits modular use, and which has no stand-alone lenses or lenses with 
great focal distance. 
SUMMARY OF THE INVENTION 
This object is solved with a relay lens system comprising at least two 
subsystems, wherein at least one positive lens and at least one negative 
lens are placed in at least one subsystem near the intermediate image, and 
the refractive index N(+) of at least one positive lens and the refractive 
index N(-) of at least one negative lens and the total focal distance 
F(FLD) of the lenses of the subsystem near the intermediate image 
correspond to the following relations: 
EQU N(+)&gt;N(-)and F(FLD)&gt;0.