Camera with a positive meniscus lens

In a camera in which a 35 mm photographic roll film is used, a positive convex meniscus lens, the convex side of which faces an object to be photographed, an aperture diaphragm and a flare-stop facing an image side are provided. An image plane is curved toward the lens as the image plane advances away from the optical axis of the lens in the longer side direction of the focal surface. The following inequalities are satisfied: EQU 26.ltoreq.f.ltoreq.32 EQU f.sup.2 /126.ltoreq.F.ltoreq.16 in which f represents the focal distance and F represents the F-number of the lens.

BACKGROUND OF THE INVENTION 
The present invention relates to a compact fixed focus type camera or a 
lens unit having a film, and more particularly relates to a lens that is 
assembled to the camera or the lens unit. 
Conventionally, a 35 mm-roll-film unit having a lens or a fixed focus type 
camera having a simple structure is provided with a lens, the focal 
distance of which is approximately 35 mm. The single lens applied to the 
aforementioned 35 mm-roll-film unit or fixed focus type camera is 
disclosed in Japanese Patent Application Open to Public Inspection Nos. 
199351/1988, 246713/1988 and 106710/1990. In these patent applications, 
examples are disclosed in which an aspherical lens or a spherical lens, 
the focal distance of which is 33 to 50 mm, is combined with a diaphragm. 
However, as the focal distance is 33 to 50 mm in the aforementioned 
examples, the total length of the lens and diaphragm is not less than 30 
mm. Therefore, the depth of the cameras and film units (referred to as a 
camera, hereinafter) available on the market are not less than 35 mm, so 
that it is not easy to carry the cameras in a pocket. Recently, panoramic 
cameras have become popular, and wide-angle lenses by which an object and 
a background can both be photographed are widely used. For that reason, a 
simple camera provided with a wide picture-taking lens, the angle of which 
is larger than 35 mm, is required. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a single lens for 
picture-taking appropriate for a handy camera system, the depth of which 
is not more than 35 mm, and also to provide a camera system by which an 
image of high quality can be obtained although the focal distance is 
reduced to obtain a wide-angle. 
The present invention is to provide a picture-taking lens including a 
positive meniscus lens, convex on the object side, and at least one 
diaphragm provided on the image side, wherein an image is formed by the 
positive meniscus lens on a 35 mm roll film surface that is concavely 
curved on the side facing the lens with regard to the direction of the 
long side of the image, the profile of said meniscus lens satisfying the 
following inequalities: 
EQU 26.ltoreq.f.ltoreq.32 (1) 
EQU f.sup.2 /126.ltoreq.F.ltoreq.16 (2) 
where the focal distance of said meniscus lens is f, and the F-number is F. 
More particularly, the profile of said lens satisfies the following 
inequality: 
EQU 1.5.ltoreq..beta..ltoreq.1.8 (3) 
where the paraxial lateral magnification of the first lens surface is 
.beta.. 
Further, the profile of said lens satisfies the following inequality: 
EQU 1/2(0.8.multidot.h.sub.max).sup.2.multidot. 
(P-F/700).ltoreq..DELTA.(0.8.multidot.h.sub.max).ltoreq.1.3(4) 
where the displacement amount of the curved film surface from the plane is 
.DELTA. at a position located by distance h perpendicular to the optical 
axis of the lens with regard to the direction of the long side of the 
image. In this case, P is Petzval's sum of the lens, F is the F-number, 
and h.sub.max is the maximum distance of the image from the optical axis 
with respect to the direction of the long side of the image. 
A specific profile to satisfy the aforementioned condition (4) is as 
follows: 
When the curved film surface is a portion of a cylindrical surface, the 
radius of curvature of which is R, the following inequality is satisfied: 
EQU 80.ltoreq.R.ltoreq.(P-F/700).sup.-1 ( 5) 
The diaphragm is disposed so that the following inequality can be 
satisfied: 
EQU 0.12f&lt;X&lt;0.18f (6) 
where the distance from the first lens surface to the aperture diaphragm on 
the side that is closest to the image, is defined as X. Further, a 
diaphragm to restrict the off-axis light flux is provided between the 
aperture diaphragm and the lens. 
Preferably, the following inequalities are satisfied: 
EQU 1.45.ltoreq.n.ltoreq.1.65 (7) 
EQU 50.ltoreq..nu.d (8) 
where the refractive index of the aforementioned positive meniscus lens is 
n, and the Abbe's number is .nu.d. 
When the lens is formed in the profile of a convex meniscus and the 
diaphragm is disposed behind the lens, the lens astigmatism can be 
reduced, and the image surface can be brought close to Petzval's image 
surface. This Petzval's image surface is formed in such a manner that: the 
image surface is curved to the lens side more at the sides thereof than at 
the center thereof. When the film surface is curved to the lens side in 
the aforementioned manner, deterioration of the image can be prevented 
even if the film surface is curved. 
Next, the aforementioned inequalities will be explained as follows. 
When the focal distance exceeds the upper limit shown in condition (1), the 
total lens length is extended, so that it is difficult to provide a handy 
camera. 
When the focal distance exceeds the lower limit shown in the condition (1), 
the amount of curvature of the image surface is increased too much. 
Therefore, even when the film surface is curved in the aforementioned 
manner, a large difference is caused between the image quality of the 
short side direction and that of the long side direction. Further, coma 
aberration, astigmatism aberration, chromatic aberration, and distortion 
are increased, so that the image quality of the periphery is remarkably 
deteriorated. 
The condition (2) is established to obtain a fixed focus camera that can 
provide an image of high quality. When the F-number exceeds the upper 
limit of condition (2), the amount of light tends to be insufficient when 
a photograph is taken in a room or in cloudy weather, so that an image of 
an appropriate exposure amount can not be provided. 
The condition (2) is established to obtain an appropriate depth of field. 
In general, when the F number of a lens is F, the focal distance is f, and 
the diameter of a circle of confusion is .epsilon., hyper focal distance 
U.sub.0 can be expressed by the following equation. When the focus is 
adjusted to a subject, the distance of which is U.sub.0, an image, which 
is in focus from the closest distance U.sub.0 /2 to infinite, can be 
obtained. 
EQU U.sub.0 =f.sup.2 /F.epsilon. 
In the case where a 35 mm roll film is enlarged to a print of service size, 
the allowable value of diameter .epsilon. of the circule of confusion is 
probably 0.07, and when the closest distance U.sub.0 /2 is set at not more 
than 0.9 m, the following inequality can be obtained. 
EQU f.sup.2 /(2.times.0.07.times.F).ltoreq.900 
As a result of the foregoing, the lower limit of condition (2) can be 
obtained. That is, when the F-number exceeds the lower limit of condition 
(2), the focusing range is reduced, and further image quality is 
deteriorated by spherical aberration. 
When the paraxial lateral magnification .beta. exceeds the upper limit of 
condition (3), spherical aberration, on-axis chromatic aberration, and 
magnification chromatic aberration are increased. When they exceed the 
lower limit, the Petzval's sum is increased, and as a result, the lens 
length is increased. 
The condition (4) restricts the amount of curvature of the film surface. 
When the amount of curvature exceeds the upper limit of condition (4), the 
difference between the image quality of the long side direction and that 
of the short side direction is increased. When the amount of curvature 
exceeds the lower limit of condition (4), the amount of curvature of the 
film surface is insufficient, so that the image quality is deteriorated in 
the periphery of the long side direction of the image screen. 
When radius R of curvature of the curved film surface exceeds the upper 
limit of condition (5), the amount of curvature of the film surface 
becomes insufficient with respect to the curvature of the image surface. 
Therefore, the film surface is insufficient, so that the image quality is 
deteriorated in the periphery of the long side direction of the image 
screen. 
When radius R of curvature of the curved film surface exceeds the lower 
limit of condition (5), the difference between the image quality of the 
long side direction of the image screen and that of the short side is 
increased, and further the film can not be fed smoothly. 
The condition (6) is established for coma aberration and magnification 
chromatic aberration. When distance X exceeds the upper limit, the 
magnification chromatic aberration is increased, and when distance X 
exceeds the lower limit, introversive coma aberration is increased. 
When refractive index n exceeds the upper limit of condition (7), the 
material cost of glass is increased, and further it becomes difficult to 
grind the lens. As a result, the cost is further increased. When 
refractive index n exceeds the lower limit of condition (7), it is not 
desirable because the spherical aberration is increased. 
When the Abbe's number .nu.d exceeds the lower limit of condition (8), the 
chromatic aberration, and especially the magnification chromatic 
aberration is increased, and it is not desirable because the periphery of 
the image is colored.