JPH0455810A - Wide angle zoom lens of large back focal length - Google Patents

Abstract

PURPOSE:To obtain the zoom lens of a longer back focal length and wider angle by satisfying specific conditions among the focal distance of the entire system at the wide angle end, the focal distance of a 2nd lends group and the imaging magnification at the telephoto end of the 2nd lens group. CONSTITUTION:This zoom lens has, successively from an object side,the 1st lens group having a negative refracting power and the 2nd lens group having a positive refracting power and executes zooming by changing the air spacing between the 1st lens group and the 2nd lens group. The conditions of equations I, II are satisfied among the focal distance of the entire system at the wide angle end, designated as Fw, the focal distance of the 2nd lens group, as F2 and the imaging magnification at the telephoto end of the above-mentioned 2nd lens group, as beta2T. The zoom lens which attains the wider angle of about 80 deg. angle of view at the wide angle end and the considerably large back focal length is obtd.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention provides a zoom lens, particularly a wide-angle zoom lens that is suitable for video cameras and has a half angle of view of about 40' at the wide-angle end. .

[Prior art]

2. Description of the Related Art In recent years, as video cameras have become more widespread, there has been a demand for smaller photographic lenses with a high zoom ratio. Conventional general zoom lenses are composed of four components, namely a focus lens, a variator lens, a compensator lens, and a relay lens, and the angle of view at the wide-angle end is mostly within the standard or semi-telephoto range. There is. When it is desired to change the focal length to a wider angle side, a method of attaching a front conversion lens is adopted, but this tends to increase the size of the lens system and degrade image quality.

Wide-angle zoom lenses are popular for 35 mm film size, but the F number is F2.8.
It is difficult to say that the zoom lens has a large aperture ratio, and the zoom lens for general video cameras has a fairly long back lens because it is necessary to place a cover glass such as COD, an infrared cut filter, and an optical low-pass filter in the back focus. The premise is to secure focus. For example, C.C.
When the screen diagonal length of D is l, the back focus length is 1
．． Some lengths range from around 51 to 21.

In comparison, a 35 mm film has a back focus of about 0.81, where l is the diagonal of the film size, making it unsuitable for video cameras.

By the way, it is composed of, in order from the object side, a first lens group having a negative refractive power and a second lens group having a positive refractive power,
Japanese Patent Application Laid-Open No. 63-96621 is known as a zoom lens that ensures a relatively long back focus.

[Problem to be solved by the invention]

However, the above-mentioned publication has a relatively long back focus (although it discloses a zoom lens suitable for video cameras, the half angle of view at the wide-angle end is about 29°, which is not a sufficient wide-angle angle of view). I couldn't say that I was getting the most out of it.

In general, in a so-called retrofocus type zoom lens consisting of a negative lens and a positive lens, when trying to widen the angle of view, it is necessary to strengthen the refractive power of the negative lens group or widen the distance between the principal points of the negative lens group and the positive lens group. is an effective method, but
On the other hand, in order to obtain a constant angle of view at the wide-angle end, various aberrations tend to worsen and the back focus length tends to become short.

In view of these problems, it is an object of the present invention to provide a zoom lens with a long back focus length and a wide angle of view.

[Configuration to solve the problem]

The wide-angle zoom lens with a long back focus of the present invention has, in order from the object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and the first lens group has a second lens group having a positive refractive power.
In a zoom lens that performs zooming by changing the air distance between the lens group and the second lens group, Fw is the focal length of the entire system at the wide-angle end.

When the focal length of the second lens group is F2 and the imaging magnification at the telephoto end of the second lens group is β, 3.70.
<Fg/Fw=4.60. --- (1)0
．． 93<lβ2Tl<1.30...(
2) is characterized by satisfying the following conditional expression.

〔Example〕

The present invention will be described in detail below based on the drawings. (2) indicates a first lens group having negative refractive power, (2) indicates a second lens group having positive refractive power, and SP indicates an aperture. When zooming from the wide-angle side to the telephoto side, the first lens group and the second lens group
The lens groups are moved according to the trajectories shown by the arrows.

The back focus at the wide-angle end is set long while maintaining a wide-angle ratio, and the above-mentioned conditional expressions (1) and (2) are satisfied in order to maintain better optical performance.

Conditional expression (1) indicates the ratio of the focal length of the second lens group to the focal length of the entire system at the wide-angle end, and is intended to reduce the occurrence of various aberrations while ensuring a certain amount of back focus. .

Exceeding the upper limit is effective in securing back focus, but it becomes difficult to achieve a wide angle ratio, and in order to obtain the desired angle of view, the negative refractive power of the first lens group must be strengthened. There is a need. This increases the curvature of field and increases the amount of coma aberration, making it difficult to correct it.

On the other hand, if the lower limit is exceeded, the focal length of the second lens group becomes too small and it becomes difficult to obtain a predetermined back focus.

Conditional expression (2) is a conditional expression regarding the lateral magnification of the second lens group at the telephoto end, and is intended to obtain good optical performance while reducing the size of the lens system.

If the upper limit is exceeded, the first lens group will tend to move back and forth when zooming from the wide-angle end to the telephoto end, and the amount of back-and-forth movement will increase, and the wasted space will increase, especially at the telephoto end. Since it is necessary to increase the negative refractive power of the first lens group relative to the refractive power of the entire lens system, the negative Petzval sum increases and the curvature of field increases. On the other hand, if the lower limit is exceeded, the amount of movement of the first lens group increases near the wide-angle end, resulting in an increase in the total length at the wide-angle end, which is not preferable.

By the way, video zoom lenses like the one in this example generally require a mechanism to electrically open and close the aperture, so it is difficult for the mechanism to move the aperture in the optical axis direction during zooming. This will lead to complications, so it is desirable to fix the aperture SP in the optical axis direction. In this embodiment, while the aperture is fixed in the optical axis direction, the moving inflection point of the first lens group is set near the telephoto end, as shown in conditional expression (2) above. A high zoom ratio is achieved while reducing wasted space.

Although the object of the present invention can be achieved by satisfying the numerical ranges of conditional expressions (1) and (2) above, in order to obtain further optical performance, it is preferable to satisfy the following conditional expressions.

In other words, when the average refractive index of the positive lenses constituting the second lens group is taken as a sieve, the conditional expression is 1.65≧n2P≧1.45 - (3), and the conditional expression (3) is Indicates the average value of the refractive index of the positive lens in the group. If the upper limit is exceeded, it becomes difficult to correct the negative Petzval sum generated in the first lens group in the second lens group, resulting in increased field curvature and coma. It becomes difficult to suppress the amount of aberrations generated.

Furthermore, if the lower limit is exceeded, the curvature of each positive lens in the second lens group becomes strong, making it difficult to correct spherical aberration.

Next, numerical examples relating to the present invention will be shown below.

In the numerical examples, R4 is the radius of curvature of the first lens surface from the object side, Di is the thickness and air gap of the first lens from the object side, and Ni and νi are the radius of curvature of the first lens surface from the object side. Indicates the refractive index and Atsube number of glass. However, the last two surfaces are glass materials for the face plate, filter, etc. in the CCD. Table 1 shows values corresponding to the conditional expressions of the present invention for each numerical example.

Table 1 [Effects of the Invention] As explained above, according to the present invention, the angle of view at the wide-angle end is 8.
We were able to provide a zoom lens that is capable of widening the angle (nearly 0°) and has a fairly long back focus.

[Brief explanation of the drawing]

Figures 1, 3, 5, and 7 are cross-sectional views of lenses of numerical examples 1 to 4, Figures 2, 4, 6, and 8, respectively.
The figures show various aberration diagrams of Numerical Examples 1 to 4, respectively. still,
In the aberration diagrams, (A), (B), and (C) show various aberration diagrams at the wide-angle end, intermediate, and telephoto end, respectively. Also, g is glA
, , d indicates the d-line, 6M indicates the meridional image plane, and ΔS indicates the sagittal image plane. F~θ/2', lf w-zs, :3B'7-25.33" FNO/237 W・2285'1 W-22,85°FNO/2.1) θ 1. #23t?, ε0 Low 39.8''' FNθ/3. 〃 W=/Yuj″'W・B,!;' FNθ/1.B5 w= 3'?, 5゜pp47z, /41 W = 25./ -25J' Procedure correction () (self-motivated)

Claims (3)

[Claims] (1) In order from the object side, a first lens group having a negative refractive power and a second lens group having a positive refractive power are provided, and the first lens group
In a zoom lens that performs zooming by changing the air distance between the lens group and the second lens group, the focal length of the entire system at the wide-angle end is Fw, and the focal length of the second lens group is Fw.
_2, the imaging magnification of the second lens group at the telephoto end is β
When _2_T, 3.70<F_2/Fw=4.6
0 0.93<|β_2_T|<1.30 A wide-angle zoom lens with a long back focus is characterized by satisfying the following conditional expression. (2) The wide-angle zoom lens according to claim 1, further comprising a diaphragm fixed in the optical axis direction between the first lens group and the second lens group. (3) When the average refractive index of the plurality of positive lenses constituting the second lens group is @n_2_P@, 1.65≧@n
_2_P@≧1.45 The wide-angle zoom lens according to claim 1 or 2, characterized in that it satisfies the following conditional expression:_2_P@≧1.45.