Real-image finder having variable power

A real-image finder having variable power includes an objective lens system of positive power, a condenser lens, and an ocular lens system. The objective lens system includes a first positive lens group, a second negative lens group, and a third positive lens group from an object side towards an image side. The second lens group is moved toward an image side and the third lens group is moved toward an object side, when the magnification changes from a wide angle side to a telephoto side. Thus, the distance between the first and second lens groups increases, the distance between the second and third lens groups decreases, and the distance between the third lens group and the condenser lens increases. The real-image finder meets the requirements defined by the relationships 0.5<.vertline.m.sub.2w .vertline.<1.0; 1.1<.vertline.m.sub.2T .vertline.; 0.5<.vertline.m.sub.3w .vertline.1,0; and, 1.1<.vertline.m.sub.3T .vertline., where "m.sub.2w " and "m.sub.2T " designate the lateral magnification of the second lens group on the wide angle and telephoto sides, and "m.sub.3w " and "m.sub.3T " designate the lateral magnification of the third lens group on the wide angle and telephoto sides.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a real-image finder having an increased 
zoom ratio, which can be used for a compact camera, etc. 
2. Description of Related Art 
The zoom ratios of almost all conventional variable power real-image 
finders for compact cameras are less than three times. An objective lens 
system is usually made up of a zoom lens system having two lens groups, a 
negative lens group and a positive lens group. 
SUMMARY OF THE INVENTION 
It is the aim of the present invention to provide a small real-image finder 
having a zoom ratio of more than three times, which is realized by an 
improved arrangement of the objective lens system and power distribution 
thereof. 
To achieve the object mentioned above, according to the present invention, 
a real-image finder is provided having variable power comprising an 
objective lens system of positive power, a condenser lens, and an ocular 
lens system, in this order from the object side. The objective lens system 
comprises at least three lens groups including a first lens group of 
positive power, a second lens group of negative power, and a third lens 
group of positive power arranged in this order from the object side. The 
second and third lens group are moved toward an image side and an object 
side, respectively, when the magnification changes from wide angle to 
telephoto. The distance between the first lens group and the second lens 
group increases, the distance between the second lens group and the third 
lens group decreases, and the distance between the third lens group and 
the condenser lens increases. The real-image finder meets the requirements 
defined by the following relationships (1) through (4) below: 
(1) 0.5&lt;.vertline.m.sub.2 W .vertline.&lt;1.0 
(2) 1.1&lt;.vertline.m.sub.2 T .vertline. 
(3) 0.5&lt;.vertline.m.sub.3 W .vertline.&lt;1.0 
(4) 1.1&lt;.vertline.m.sub.3 T .vertline. 
where 
m.sub.2 W represents a lateral magnification of the second lens group at 
the wide angle end, 
m.sub.2 T represents a lateral magnification of the second lens group at 
the telephoto end, 
m.sub.3 W represents a lateral magnification of the third lens group at the 
wide angle end and 
m.sub.3 T represents a lateral magnification of the third lens group at the 
telephoto end. 
Preferably, the real-image finder further satisfies the following 
relationships; 
(5) 1.1&lt;.vertline.m.sub.2 T .vertline.&lt;1.7 
(6) 1.1&lt;.vertline.m.sub.3 T .vertline.&lt;1.7 
In a preferred embodiment, upon varying the magnification, the second and 
third lens groups are moved without moving the first lens group. Namely, 
the first lens group does not move when the magnification is varied. This 
simplifies the mechanical structure of the lens barrel. 
Preferably, the surfaces of the first lens group and the second lens group, 
that are opposed at a predetermined spatial distance defined by an image 
side surface of the first lens group and an object side surface of the 
second lens group, are aspherical surfaces. The aspherical surface of the 
first lens group can be made of a convex surface facing the image side, 
and having positive surface power which is decreased toward the peripheral 
edge thereof. The aspherical surface of the second lens group can be made 
of a concave surface facing the object side and having negative surface 
power which is decreased toward the peripheral edge thereof. 
The second lens group can be made of a single concavo-concave, i.e. 
concavo-concave lens of negative power having aspherical surfaces on both 
sides. These aspherical surfaces belonging to the negative concavo-concave 
lens have negative surface power which decreases toward the peripheral 
edge thereof. 
The first positive lens group, the second negative lens group, and the 
third positive lens group, of the objective lens system can each be made 
of a single lens.