Triplet lens system

A triplet lens system comprising, in order from the object side, a first positive meniscus lens component having a convex surface on the object side, a second biconcave lens component, a third biconvex lens component and a stop; and configured so as to correct aspherical aberration and coma at the same time by using at least one aspherical surface on the second and the third lens components.

BACKGROUND OF THE INVENTION 
a) Field of the invention 
The present invention, as can be best viewed in FIG. 1A, relates to a 
triplet lens system 1 which is suited for use with lens shutter cameras 2, 
etc., and has a stop 3 disposed on the image side of the lens system. 
b) Description of the prior art 
A large number of lens shutter cameras have conventionally adopted triplet 
lens systems owing to a fact that each of these lens systems can have 
favorable optical performance by using a small number of lens components 
and can easily be configured compact by. 
Most of the conventional examples of the triplet lens systems belong to 
lens systems which have stops disposed after the lens systems since these 
lens systems are compatible with lens barrels having simple structures and 
advantageous not only for moving lens components, but also for exposure 
control. 
For the triplet lens systems, each of which is composed of a small number 
of lens components and has low design freedom, however, there is a certain 
limit in configuring a photographic lens system which is composed only of 
spherical lens components, and has a wide field angle, a large aperture, a 
compact size and high optical performance. In particular, the triplet lens 
system which is composed only of the spherical lens components allows the 
optical performance thereof to be remarkably degraded, or produces coma 
flare and aggravates spherical aberration at the same time at intermediate 
and marginal angles within a range of a field angle thereof. 
As conventional examples of the triplet lens systems which use aspherical 
lens components for solving these problems, there are known the lens 
systems disclosed by Japanese Patents Kokai Publication No. Sho 59-34,510, 
Kokai Publication No. Sho 59-160,119, Kokai Publication No. Sho 59-160,120 
and Kokai Publication No. Sho 63-96,620. 
The triplet lens systems disclosed by these patents have relatively large 
apertures, wide field angles, compact compositions and favorable optical 
performance which are obtained by using the aspherical lens components. 
However, these photographic lens systems have F numbers on the order of 
3.5 and cannot be said to be sufficient in brightness thereof. 
Further, the triplet lens systems disclosed by Japanese Patents Kokai 
Publication No. Sho 59-160,119, Kokai Publication No. Sho 59-160,120 and 
Kokai Publication No. Sho 63-96,620, out of the above-mentioned 
conventional examples, select small departures from the reference sphere 
of the aspherical surfaces, thereby having insufficient functions to 
correct aberrations, or undercorrecting coma. 
Furthermore, the triplet photographic lens systems disclosed by Japanese 
Patent Kokai Publication No. Sho 59-34,510 uses an aspherical surface on a 
first lens component thereof. However, such an aspherical surface used on 
the first lens component remarkably undercorrects spherical aberration 
though it has a function to correct coma, whereby the triplet lens system 
allows spherical aberration to be remarkably produced on the negative 
side. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide a triplet lens 
system which comprises three lens components of three lens elements using 
an aspherical surface, or aspherical surfaces, and has a wide field angle, 
a large aperture and a compact size. 
The triplet lens system according to the present invention comprises, in 
order from the object side, a first positive meniscus lens component 
having a convex surface on the object side, a second biconcave lens 
component, a third biconvex lens component and a stop, uses at least one 
aspherical surface on the second lens component and the third lens 
component, and is configured so as to satisfy the following conditions (1) 
and (2): 
EQU 1.times.10.sup.-5 &lt;.vertline.x.sub.1 /f.vertline.&lt;1.times.10.sup.-2 ( 1) 
EQU 0.5&lt;r.sub.5 /f&lt;1.1 (2) 
wherein the reference symbol x.sub.1 represents a departure from a 
reference sphere of at least one of the aspherical surfaces as measured at 
a maximum effective diameter thereof, the reference symbol r.sub.5 
designates a radius of curvature on an object side surface of the third 
lens component and the reference symbol f denotes a focal length of the 
triplet lens system as a whole. 
When an attempt is made to configure a triplet lens system so as to have an 
F number on the order of 2.8, it is general that an axial marginal ray is 
enhanced and rays passing through portions in the vicinities of margins of 
lens components are strongly refracted, whereby remarkable spherical 
aberration is produced. When an additional attempt is made to configure 
the triplet lens system so as to be compact by shortening a total length 
thereof, the lens components have strengthened refractive powers and 
produce coma, thereby degrading image qualities on marginal portions of an 
image surface of the triplet lens system. 
On the other hand, an aspherical lens component has a high function at a 
marginal portion thereof, and serves for correcting the axial marginal ray 
and off-axial rays which have large heights. 
The triplet lens system according to the present invention is configured so 
as to correct spherical aberration and coma at the same time which 
conventionally posed the problems. For correcting these aberrations at the 
same time by using an aspherical surface or surfaces, it is effective to 
dispose the aspherical surface or surfaces as a third surface and/or 
subsequent surfaces. When an aspherical surface is disposed as a first or 
second surface, the off-axial rays pass through a marginal portion of a 
lens component having the aspherical surface, whereby the function of the 
aspherical surface is effective only for the off-axial rays and the 
aspherical surface cannot correct spherical aberration. When an aspherical 
surface is used as a third surface or a subsequent surface, in contrast, 
the function of the aspherical surface is favorably distributed between 
the axial ray and the marginal rays, thereby making it possible to 
configure a photographic lens system which is capable of correcting both 
spherical aberration and coma, or has high optical performance. 
For this reason, the triplet lens system according to the present invention 
uses an aspherical surface or aspherical surfaces as one or some of 
surfaces of the second lens component and the third lens component. At 
least one of these aspherical surfaces is configured so as to satisfy the 
above-mentioned condition (1) so that the function of the aspherical 
surface will be maximum. In other words, so far as at least one of the 
aspherical surfaces has a departure from a reference sphere thereof which 
is within the range defined by the condition (1), it is possible to obtain 
the maximum effect of the function of the aspherical surface, or correct 
spherical aberration and coma favorably. Since a lens system which is of 
the triplet type like the lens system according to the present invention 
and is bright, or has an F number on the order of 2.8, cannot be composed 
only of spherical lens component, it is necessary to use an aspherical 
surface or aspherical surfaces to compose such a lens system and the 
condition (1) need be satisfied for favorably correcting spherical 
aberration and coma. If the condition (1) is not satisfied, it will be 
impossible to correct coma. If the lower limit of 1.times.10.sup.-5 of the 
condition (1) is exceeded, coma will be undercorrected. If the upper limit 
of 1.times.10.sup.-2 of the condition (1) is exceeded, aberrations of high 
orders will be produced. Speaking of spherical aberration, it will be 
undercorrected if the lower limit of the condition (1) is exceeded or 
overcorrected if the upper limit of the condition (1) is exceeded. 
By the way, it is more desirable to modify the condition (1) so as to have 
a lower limit of 1.times.10.sup.-4. 
Further, the triplet lens system according to the present invention is 
configured so as to satisfy the condition (2) for maintaining coma at a 
favorable level. The condition (2) defines a radius of curvature for an 
object side surface of the third lens component. So far as r.sub.5 /f is 
within the range not exceeding the upper limit of the condition (2), the 
third lens component can have a certain degree of refractive power, 
whereby the triplet lens system can have a short back focal length or a 
short total length. If r.sub.5 /f exceeds the lower limit of 0.5 of the 
condition (2), the object side surface of the third lens component will 
strongly refracts rays, thereby remarkably curving coma. By the way, it is 
more desirable to modify the condition (2) so as to have an upper limit of 
1.0. 
In the next place, it is desirable to configure the triplet lens system 
according to the present invention so as to satisfy the following 
conditions (3) and (4): 
EQU 0.5&lt;f.sub.1 /f&lt;0.95 (3) 
EQU -3.5&lt;f.sub.12 /f&lt;-0.8 (4) 
wherein the reference symbol f.sub.1 represents a focal length of the first 
lens component, and the reference symbol f.sub.12 designates a total focal 
length of the first lens component and the second lens component. 
The condition (3) defines a refractive power for the first lens component, 
whereas the condition (4) defines a total refractive power for the first 
lens component and the second lens component. Since it is possible to 
suppress Petzval's sum at a small level and maintain good flatness of an 
image surface by distributing refractive powers adequately between the 
first lens component and the second lens component as defined above, it is 
desirable to configure the triplet lens system so as to satisfy the 
conditions (3) and (4). 
The condition (3) is required for correcting mainly spherical aberration 
and astigmatism. If the lower limit of 0.5 of the condition (3) is 
exceeded, spherical aberration will be tilted on the negative side and 
barrel form distortion will be produced in a large amount. In this case, a 
sagittal image surface and a meridional image surface will be remarkably 
tilted on the positive side, and astigmatic difference will be increased. 
If the upper limit of 0.95 of the condition (3) is exceeded, spherical 
aberration will be overcorrected. 
The condition (4) is required for maintaining Petzval's sum at an adequate 
value and reducing astigmatism. If the lower limit of -3.5 of the 
condition (4) is exceeded, Patzvel's sum will have a large positive value, 
thereby making it impossible to suppress curvature of field. If the upper 
limit of -0.8 of the condition (4) is exceeded, Petzval's sum will have a 
small value and the sagittal image surface will be overcorrected but, at 
high orders, the meridional image surface will be remarkably tilted on the 
negative side, whereby astigmatic difference will be enlarged and can 
hardly be corrected. 
For correcting spherical aberration and coma more favorably, it is 
desirable to configure the triplet lens system according to the present 
invention so as to satisfy the following condition (5): 
EQU 1.times.10.sup.-6 &lt;.vertline.x.sub.2 /f.vertline.&lt;1.times.10.sup.-3( 5) 
wherein the reference symbol x.sub.2 represents a departure from the 
reference sphere of the aspherical surface as measured at a point located 
at 1/2 of the maximum effective diameter thereof. 
For correcting spherical aberration and coma much more favorably, it is 
desirable that the aspherical surface satisfies the condition (1) and the 
above-mentioned condition (5) at the same time. Speaking concretely, the 
condition (5) has an effect similar to that of the condition (1), or 
spherical aberration and coma can be corrected favorably so far as x.sub.2 
is within the range defined by the condition (5).