JPH05264895A - Wide-angle lens - Google Patents

Abstract

PURPOSE:To obtain an inexpensive and small wide-angle lens for a monitoring camera with a sufficiently practical aperture of an F number 2 other than the use in a particularly dark environment, while having a few lens component, a sufficiently long back focal length and a fully wide-angle with a viewing angle of nearly 90 deg.. CONSTITUTION:This lens is composed of four lens components in which a negative meniscus lens with a convex face on the object side, a biconvex single lens, a biconvex single lens and a negative lens with an intensive refracting surface on the object side are successively arranged from an object side and satisfies respective conditions below; 2.1<(d2+d3)/f<3.3, -2.1<f1/f<-1.6, 1.65<n1. Where d2 is spacing between the first lens and the second lens, d3 is thickness of the second lens, f1 is focal length of the first lens, f is composite focal length, n1 is refractive index of the first lens.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention mainly relates to a wide-angle lens suitable for mounting on a small surveillance camera or the like.

[0002]

2. Description of the Related Art Generally, a wide-angle lens is required as a lens to be mounted on a surveillance camera or the like in order to see or photograph a wide range. In addition, an optical member such as a color separation system is often arranged between the lens system and the image sensor, and in that case, the lens system requires a long back focus.

As a lens for a small camera, for example,
There is a lens described in JP-A-63-75718. Although this lens is a lens for a small TV camera, it has an angle of view of about 40 despite using as many as five lenses.
The angle of view is so narrow that it cannot be said that the angle of view is sufficient for use in surveillance cameras. Further, as a lens that realizes a sufficient angle of view, there is a lens described in JP-A-1-265216 by the present applicant. This lens realizes a wide angle of view of 80 ° or more, but also uses five lenses. Further, although this lens has a large aperture of F1.4 to 1.6, when it is used in an environment not particularly dark, or when the sensitivity of the image sensor is improved, a highly sensitive image sensor can be used. In view of the above, a low-cost lens having a simple structure is desired even if the aperture becomes slightly smaller.

[0004]

SUMMARY OF THE INVENTION The present invention has a four-lens structure with a small number of lenses, has a sufficiently long back focus, and has a wide angle of view as close as 90 °.
Except for the number 2 and especially when it is used in a dark environment, it is intended to provide a lens having a sufficiently practical aperture at a low cost.

[0005]

The wide-angle lens of the present invention comprises:
In order from the object side, the first lens is a negative meniscus lens whose convex surface faces the object side, the second lens is a biconvex single lens, and the third lens
The lens is composed of a biconvex single lens, and the fourth lens is composed of four negative lenses having a strong refracting surface on the object side, and each of the following conditions is satisfied. (1) 2.1 <(d ₂ + d ₃ ) / f <3.3 (2) −2.1 <f ₁ / f <−1.6 (3) 1.65 <n ₁ However, d ₂ : Distance between first lens and second lens d ₃ : Lens thickness of second lens f ₁ : Focal length of first lens f: Combined focal length of entire system n ₁ : Refractive index of first lens

[0006]

The lens of the present invention employs a so-called retrofocus type in which a lens group having a negative focal length is arranged in the first group as a basic lens structure. Furthermore, by making the negative power of the first group relatively strong, a sufficiently long back focus is obtained, and negative distortion aberration of a size that causes no practical problem is generated, and the peripheral portion of the screen due to the distortion is generated. By using compression, a wide angle has been realized despite the small number of lenses of four.

Each conditional expression will be described below. Conditional expression (1) appropriately generates negative distortion, and
This is for maintaining a sufficient back focus. d ₂
When + d ₃ is larger than the upper limit, the negative distortion becomes too large to be practically used, and it becomes difficult to secure the light amount around the screen. Conversely, d ₂ +
When d ₃ is smaller than the lower limit, distortion is too small to generate a sufficient angle of view, and the back focus becomes short. Like the conditional expression (1), the conditional expression (2) is for appropriately generating negative distortion and maintaining a sufficient back focus. When the upper limit of this expression is exceeded and the refractive power of the first lens becomes large, the negative distortion is too large to be practically used. On the other hand, when the value is smaller than the lower limit, the occurrence of distortion is too small to secure a sufficient angle of view and the back focus becomes short. The conditional expression (3) is for keeping the distortion appropriately, and if the refractive index of the first lens is decreased outside this condition, the negative distortion becomes particularly large.

[0008]

EXAMPLES Examples of the lens of the present invention will be shown below. In the embodiment of the present invention, the aberration is corrected with the cover glass included, and the cover glass is shown together. The lens of this example secondarily satisfies the following conditions. _{(4) 1.6 <(r 1} + r 2) / (r 1 -r 2) <2.
2 (5) 20 <ν ₃ −ν ₄ where r _{1 is the} radius of curvature of the first lens on the object side r _{2 is the} radius of curvature of the first lens on the image side ν ₃ , is the Abbe number of the third lens ν _{4, is the} second The Abbe number of 4 lenses is shown. Conditional expression (4) is a condition mainly for favorably correcting astigmatism. If the upper limit of this expression is exceeded, the astigmatism generated in the first lens will become large and the meridional image plane will become over. On the other hand, when the value goes below the lower limit, the meridional image surface becomes under, and negative distortion becomes large. Conditional expression (5) is a condition for favorably correcting chromatic aberration.
If this condition is not satisfied, it will be difficult to satisfactorily correct axial chromatic aberration and lateral chromatic aberration simultaneously.

In the data tables of the following examples, f is the focal length of the entire system, r is the radius of curvature of each lens surface, d is the lens thickness or lens spacing, nd is the refractive index, νd is the Abbe number, 2ω
Indicates the angle of view. The back focus is represented by fB, and the cover glass thickness is included in fB in terms of air length.

Example 1

Example 2

Embodiment 3

Embodiment 4

[0014]

The wide-angle lens of the present invention is composed of only four lenses, as seen in the examples and aberration diagrams thereof.
Despite having an angle of view reaching almost 90 degrees, an appropriate distortion was left, various aberrations were corrected in a well-balanced manner, and a wide-angle lens with high practicality could be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of a wide-angle lens according to a first embodiment of the present invention.

FIG. 2 is an aberration curve diagram of Example 1 of the wide-angle lens of the present invention.

FIG. 3 is an aberration curve diagram of Example 2 of the wide-angle lens of the present invention.

FIG. 4 is an aberration curve diagram of Example 3 of the wide-angle lens of the present invention.

FIG. 5 is an aberration curve diagram of Example 4 of the wide-angle lens of the present invention.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] December 14, 1992

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

[Figure 1]

[Fig. 2]

[Figure 3]

[Figure 4]

[Figure 5]

Claims (1)

[Claims] 1. From the object side, in order from the object side, the first lens component is a negative meniscus lens whose convex surface faces the object side, the second lens is a biconvex single lens, the third lens is a biconvex single lens, and the fourth lens. Is a wide-angle lens 2.1 <(d ₂ + d ₃ ) / f <3.3 − which is composed of four negative lenses having a strong refracting surface on the object side and satisfies the following conditions. 2.1 <f ₁ / f <−1.6 1.65 <n ₁ However, d ₂ : the distance between the first lens and the second lens d ₃ : the lens thickness of the second lens f ₁ : the first lens Focal length f: Composite focal length of the entire system n ₁ : Refractive index of the first lens