JPH0360407B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a zoom lens using a plastic lens, particularly a high-performance and compact zoom lens suitable for use in a small color camera for VTR. (Prior art) In order from the object side: a positive first lens component with a focusing function, a negative second lens component that moves with zooming and mainly changes magnification, and a negative second lens component that moves with zooming to mainly keep the image position constant. A zoom lens is known that includes a negative third lens component that has the function of maintaining the image plane, and a positive fourth lens component that has the function of forming an image on the image plane as a master lens. On the other hand, it is clear that using plastic lenses in zoom lenses is extremely effective in reducing the weight of the entire lens system and increasing mass productivity, but due to the physical property limitations of plastics, the following difficulties arise. Ta. (1) The types of optical plastic materials with excellent transparency are limited, and the degree of freedom in optical design is limited. (2) The change in refractive index due to temperature is larger than that of inorganic glass materials, and it is difficult to correct this in terms of design. (3) Even if the above two difficulties are overcome, if an attempt is made to obtain performance equivalent to or better than a conventional design using only glass lenses, the entire lens system will become larger and its compactness will be compromised. (Purpose of the Invention) The present invention uses a plastic lens in the zoom lens having the above-known positive/negative/negative/positive configuration to sufficiently reduce fluctuations in the focal position due to temperature changes, and at the same time, it The objective is to obtain a zoom lens that is about the same size as the lens and has about the same performance. (Structure of the Invention) In general, the refractive index of optical plastic materials decreases as the temperature rises, and the focal length of a plastic lens becomes longer in the case of a positive single lens, and its absolute value increases in the case of a negative single lens. It tends to become. However, if we stack several positive and negative single lenses like this and make their combined refractive power equal to 0, then assuming that the degree of refractive index change due to temperature of each lens is the same, the refraction due to temperature will be It is clear that the change in force is also equal to zero. In this invention, a total of four plastic lenses are used for the third and fourth lens components, and the above principle is applied between these lenses to keep the sum of the refractive powers of these lenses below a certain value. By doing so, fluctuations in the focal position due to the temperature of the entire system are suppressed. Of course, in an actual zoom lens, the plastic lenses are arranged apart from each other, and the composite refractive power is often not completely zero, but it is sufficient that this condition is approximately satisfied. In addition, when a plastic lens is used as the first lens component for focusing, the focal position of the entire system can be changed by moving the first lens component back and forth by the change in the focal position of the first component due to temperature change. can be maintained at a constant position, there is no need to perform optical design correction regarding the first lens component. As plastic lenses are introduced into the third and fourth lens components, the effect of lowering the refractive index of the negative lens is particularly large, and the Petzval sum of the entire system tends to decrease. Additionally, chromatic aberrations tend to be undercorrected overall. In the present invention, this point is particularly corrected by using a glass having a low refractive index and low dispersion as the positive lens in the fourth lens component. Specifically, such a zoom lens includes, in order from the object side, a first lens component that has a positive focal length and is used for focusing, a second lens component that has a negative focal length and is used for zooming, and The third lens has a negative focal length and is used to correct the movement of the image plane due to zooming.
lens component, the fourth lens component 4, which has a positive focal length and is fixed during zooming as a master lens component.
The first lens component is made up of three lenses: a negative meniscus lens L ₁ with a convex side facing the object side, a positive lens L ₃ with a convex side facing the object side, and a positive lens L ₃ with a convex side facing the object side. The two lenses on the object side are sometimes bonded together to form a bonded positive lens. Second
The lens components consist of 3 lenses in 2 groups: a meniscus negative lens L ₄ with a concave side facing the image side, a biconcave lens L ₅ and a positive lens L ₆ with a convex side facing the object side. The component consists of one negative meniscus lens L ₇ with a concave side facing the object side, and the fourth lens component is a positive meniscus lens L ₈ with a convex side facing the image side.
Biconvex lens L ₉ , Biconvex lens L ₁₁ , Biconvex lens L ₁₂ ,
Negative lens L ₁₃ with concave surface facing the image side, biconvex lens
L ₁₄ , positive lens L ₁₅ with convex surface facing the object side, 8 groups 8
Each lens L ₇ , L ₈ , L ₁₁ , and L ₁₃ above is a plastic lens, f _w : focal length at the wide-angle end of all threads f _i : of the i-th lens from the object side. Focal length n _i : Refractive index of the i-th lens from the object side ν _i : Abbe number of the i-th lens from the object side -0.22<fw (1/f ₇ +1/f ₈ +1/f ₁₁ +1/ _f13 )<-
It is configured as a zoom lens that satisfies the following conditions: 0.18...(1) n ₁₀ < 1.5...(2) 65 < ν ₁₀ ...(3). Condition (1) is a condition for compensating for changes in focal position due to temperature changes due to the use of a plastic lens; if the upper limit is exceeded, the plastic lens as a whole will strengthen its positive refractive power, causing a rise (fall) in temperature.
Sometimes there is a large change in the direction of the back orcus becoming longer (shorter). On the other hand, when the lower limit is exceeded, the back focus changes significantly in the direction of becoming shorter (longer) when the temperature rises (falls). Condition (2) is based on the reduction in the Petzval sum of the entire system due to the use of plastic lenses with a low refractive index for the negative lenses L ₇ and L ₁₃ , which have conventionally often been made of glass with a relatively high refractive index. This is a condition to suppress it. Condition (3) is a condition for correcting the axial chromatic aberration that occurs in the master lens system due to the positive lenses L ₈ , L ₁₁ , and L ₁₃ being made of plastic. Under-axial chromatic aberration remains. When the zoom lens of this invention is used for a VTR small color camera, the zoom ratio is about 6,
Moreover, it is required to have a large aperture with an F number of 1.4 to 1.2. However, as mentioned above, in order to maintain an appropriate Petzpar sum, the refractive index of the positive lens decreases overall, and as a result, the radius of curvature of the refractive surface decreases, especially when trying to increase the aperture. Although the spherical aberration becomes noticeably undervalued, by using an aspherical surface with a large amount of deformation from a spherical surface in the plastic lens, extremely good aberration correction is possible even in a large aperture lens with an F number as high as 1.2. For this reason, it is desirable that the following conditions be satisfied as a secondary condition. The shape of the aspherical surface is expressed by the following equation. Here, X is a coordinate with the apex of the aspherical surface as the origin and moving along the optical axis from the object side to the image side, and h is a coordinate with the apex of the aspherical surface as the origin and in the direction perpendicular to the X axis. c represents the paraxial curvature of this surface. If we expand this equation and take the term h to the 6th power, we can transform it as follows. X=h ² /2r+h ⁴ /8r ³ +h ⁶ /16r ⁵ + (k/8r ³ +A ₄ ) h ⁴ + (2k+k ² /16r ⁵ +A ₆ ) h ⁶ where r=1/c is the paraxial surface It is the radius of curvature. In the formula, the fourth and fifth terms represent the amount of deformation from a spherical surface with radius r, and have a close relationship with the third-order aberration and fifth-order aberration, respectively, occurring on this surface. In particular, regarding third-order aberrations, when the number of third-order aspherical surfaces is defined as ≡(N'-N) {8A ₄ +k/r ³ }, the spherical aberration S coma aberration C, astigmatism A and distortion aberration occurring on this surface It is known that D undergoes changes by ΔS=h ⁴ ΔC=h ³ ΔA=h ^{2 2} ΔD=h ³ , respectively. Here, h and h indicate the heights at which the paraxial axial ray and the paraxial principal ray pass through this aspheric surface, respectively. The zoom lens of this invention uses an aspherical surface on the object side surface of the lens _L11 , but since this surface is located very close to the aperture, the effect of using the aspherical surface is mainly expressed in the spherical aberration S. be exposed. Also, the lens
On the image-side surface of _L13 , both h and h have positive and relatively large values, so the effect of making this surface aspherical mainly appears in coma C and astigmatism A. And ₁₁ : The 3rd aspheric coefficient of the object side surface of the 11th lens L ₁₁ from the object side ₁₃ : The 3rd order aspheric coefficient f of the image side surface of the 13th lens L ₁₃ from the object side: When the focal length of the second lens component is −0.2<f ³ _{w 11} <0...(4) 0<f ³ _{w 13} <0.1...(5) 1.2<|f〓|/fw<1.4...( 6) It is desirable to satisfy the following conditions. Condition (4) is due to the introduction of condition (2) in order to maintain a good Petzval sum of the entire system as described above.
This condition is mainly for correcting the under spherical aberration that occurs in the lens L ₁₀ , and since ₁₁ <0, ΔS < 0, and it can be seen that it has the effect of suppressing the under spherical aberration. If the upper limit is exceeded, under spherical aberration will occur significantly in the entire zoom range,
On the other hand, if the lower limit is exceeded, the spherical aberration will be overcorrected, making it impossible to achieve a large aperture of up to F number 1.2. Condition (5) is necessary due to the use of a plastic lens with a low Abbe number and low refractive index as the lens L ₁₃ , and it is necessary to correct chromatic aberration and maintain an appropriate Petzval sum for the entire system while using plastic for the lens L ₁₃ . If an attempt is made to maintain this value, the curvature of the image-side surface of the lens _L13 tends to become stronger, and the occurrence of astigmatism and coma aberration becomes significant in the entire zoom range. Condition (5) is a condition for suppressing this; if the lower limit is exceeded, the meridional image plane will be greatly excessively tilted, and the occurrence of extroverted comatic aberration will become significant. On the other hand, if the upper limit is exceeded, the correction will be over-corrected and both the meridional image plane and coma will be under-corrected. Condition (6) is a basic condition for making the entire lens system compact; if the upper limit is exceeded, the amount of movement of the second lens component increases, making it impossible to make the entire lens system compact. If the lower limit is exceeded, the negative refractive power becomes excessive, and aberration fluctuations due to zooming become large, making it impossible to obtain a high-performance zoom lens. (Example) Examples of the present invention will be shown below. Examples 1 and 2 are for small color cameras with a screen size of 2/3 inch, and Examples 3 and 4 are for a small color camera with a screen size of 1/3 inch.
This is a zoom lens for a 2-inch VTR compact color camera, and Examples 1 and 3 use a plastic lens in the first lens component. Lens cross-sectional views of each embodiment are shown in FIGS. 1 to 4, and the plastic lens is indicated by hatching in the cross-section. The value of focal position change with temperature in the table does not include the effect of focal position change due to the plastic lens in the first lens component. In Example 2, the third and fifth surfaces, and in Example 3, the third and fifth surfaces.
Although the 11th surface, the 3rd surface, the 5th surface, and the 12th surface in Example 4 are aspherical, these surfaces do not necessarily have to be aspherical in the present invention. Furthermore, in Examples 3 and 4, glass plates equivalent to low-pass filters L and F are included in the fourth lens component, and cover glasses C and G are included in all Examples, and aberration correction is performed using these glass plates. This is done for the entire system including

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[Table] (Effects of the Invention) As described above, the zoom lens of the present invention incorporates a plastic lens, which makes it possible to reduce the weight of the lens system and improve mass production. We were able to suppress changes in position. Furthermore, when increasing the aperture, by using an aspherical surface for the plastic lens, various aberrations can be well corrected despite the large aperture of F1.2 to F1.4, as shown in the aberration curve diagram. This makes it possible to obtain a compact zoom lens with a compact design.

[Brief explanation of drawings]

1, 2, 3, and 4 are lens sectional views of Examples 1, 2, 3, and 4 of the zoom lens of the present invention, and FIG. 5, FIG. 6, FIG. 7, and FIG. Similarly, FIG. 8 is an aberration curve diagram of Examples 1, 2, 3, and 4.

Claims (1)

[Claims] 1. In order from the object side, a first lens component has a positive focal length and is used for focusing, a second lens component has a negative focal length and is used for zooming, and a negative focal length. It consists of four lens components: a third lens component for correcting the movement of the image plane due to zooming, and a fourth lens component that has a positive focal length and is fixed during zooming as a master lens component. The component is a meniscus negative lens L ₁ with the convex side facing the object side,
A positive lens L with a convex side facing the object side and a positive lens _L3 with a convex side facing the object side.The two lenses _on the object side remain separated or are bonded together, and the bonded positive lens 3 elements or 2 elements in 3 groups, sometimes referred to as a lens
The group consists of three elements, and the second lens component is a negative meniscus lens _L4 with a concave side facing the image side, a double concave lens _L5 , and a positive lens _L6 with a convex side facing the object side. The group consists of three elements, the third lens component consists of one meniscus negative lens L ₇ with a concave side facing the object side, and the fourth lens component consists of a meniscus positive lens L ₈ with a convex side facing the image side, a biconvex lens L ₉ , double convex lens
L ₁₁ , biconvex lens L ₁₂ , negative lens L ₁₃ with a concave surface facing the image side, biconvex lens L ₁₄ positive lens L ₁₅ with _a convex surface facing the object side _. ,
Each lens L ₁₁ and L ₁₃ is a plastic lens, f _w : focal length of the entire system at the wide-angle end f _i : focal length of the i-th lens from the object side n _i : i-th lens from the object side Refractive index ν _i of the lens: −0.22<fw (1/f ₇ +1/f ₈ +1/f ₁₁ +1/f ₁₃ )<− when the Atsube number of the i-th lens from the object side
A zoom lens using a plastic lens characterized by satisfying the following conditions: 0.18 n ₁₀ < 1.5 65 < ν ₁₀ .