JPH0511603B2 - - Google Patents

Description

[Detailed description of the invention]

Object of the Invention (Field of Industrial Application) The present invention relates to a zoom lens, particularly a zoom lens suitable for electronic still cameras and VTR cameras. (Prior art) Although zoom lenses used in electronic still cameras or VTR cameras are required to have large apertures, there is also a strong demand for lower costs, and for this reason, there is a strong desire to reduce costs without deteriorating imaging performance. Efforts are being made to reduce the number of constituent lenses. Furthermore, zoom lenses for electronic still cameras that have a reflective mirror for SLR viewfinders at the rear of the lens require a long back focus.
Attempting to obtain the necessary back focus with a conventional zoom lens results in disadvantages such as an increase in the size of the zoom lens. JP-A-60-11812 has a similar purpose to this invention, but it has a large number of lenses.
Also, as a similar structure, JP-A-58-
There are No. 59407 and No. 188611, both of which have large aperture ratios for use in electronic still cameras or VTR cameras. (Problems to be solved by this invention) This invention has a small number of lenses, is compact, has a large aperture suitable for use in electronic still cameras, VTR cameras, etc., and has a reflective mirror for single-lens reflex viewfinders behind the lens. The object of the present invention is to obtain a zoom lens having sufficient backfocus for use in a zoom lens. Structure of the Invention (Means for Solving the Problem) As shown in the cross-sectional view in FIG. This is a four-component zoom lens consisting of a negative second component for the purpose of zooming, a third component that is a positive single lens that corrects focal shift due to zooming, and a fixed positive fourth component, where the fourth component is a positive lens. Consisting of 3 lenses and 2 negative lenses, the arrangement of the two negative lenses is m _1st within the 4th component from the object side.
When m is _{the second} , it is composed of 5 elements in 5 groups arranged so that m ₁ + m ₂ is 5 or 6. ψ〓: Power of the 4th component ψ〓 _-1 : Object in the 4th component Power of the negative lens on the side 〓 ₊ : Average value of the Atpe number of the positive lens in the 4th component 〓 _- : When taking the average value of the Atpe number of the negative lens in the 4th component 0.3＜1ψ〓 _-1 1/ψ 〓＜1.8 ……(1) 13＜〓 ₊ −〓 _- ……(2) is satisfied. As shown in FIGS. 1 and 17, the fourth component is arranged in order from the object side: a biconvex lens, a negative lens with a strongly concave surface facing the object side, a negative lens with a strongly concave surface facing the image side,
When consisting of a positive lens with a strongly convex surface facing the image side and a positive lens with a strongly convex surface facing the object side, ψ〓 _a : Power of the air lens consisting of the second and third surfaces from the object side of the fourth component d〓 ₄ : 2nd lens and 3rd lens from the object side of the 4th component
When the axial air distance between the lens and the lens is 0.3<1ψ〓 _a 1/ψ〓<1.3 (ψ〓 _a <0) ……(3) 0.1＜d〓 ₄ ψ〓 ……(4) conditions It is desirable to meet the requirements. In addition, as shown in FIG. 5, the fourth lens component is
In order from the object side: a positive lens with a strongly convex surface facing the object side, a negative lens with a strongly concave surface facing the object side, a positive lens with a strongly convex surface facing the image side, a negative meniscus lens with a strongly concave surface facing the image side, and both lenses. In the case of a convex lens, R〓 ₅ : When the radius of curvature of the fifth refractive surface from the object side in the fourth component is 0.3＜1ψ〓 _a 1/ψ〓 (ψ〓 _a <0) ...(5) 1/(R〓 ₅・ψ〓)＜1.5...It is desirable to satisfy the condition of (6). Furthermore, as shown in Figures 9 and 13, the fourth lens component consists of, in order from the object side, a negative meniscus lens with a strongly concave surface facing the object side, two positive lenses with a strongly convex surface facing the image side, and the like. When the lens consists of a biconvex lens and a negative lens, or a negative meniscus lens with a strongly concave surface facing the image side and a positive lens, it is desirable to satisfy the following condition: 1/(R〓 ₅・ψ〓)＜1.5...(7) . (Function) The above m ₁ +m ₂ being 5 or 6 is a condition for obtaining a long back focus for a reflector for a single-lens reflex finder. Conventionally, relay lenses consisting of three positive lenses and two negative lenses have been known to have a positive, positive, negative, negative, positive, or positive, negative, positive, positive, negative configuration from the object side. The above m ₁ +m ₂ is 7. A small value means that the negative lens is placed on the object side, which is advantageous for obtaining a long back focus. Condition (1) determines the power range of the object-side negative lens in the fourth lens component, and it is difficult to obtain a long back focus beyond the lower eye. The upper limit relates in particular to spherical aberration. In the fourth lens component, the spherical aberration is over-corrected with this negative lens, and the spherical aberration is under-corrected with the group behind it. However, if the power of this negative lens becomes strong beyond the upper limit, the balance of such spherical aberration becomes unbalanced, and the annular spherical aberration becomes significantly underdeveloped. Condition (2) is for favorably correcting chromatic aberration by using a glass material with a relatively small Abbe number for the negative lens in the fourth component. Considering the two lenses on the object side of the fourth component as the front group and the three lenses on the image side as the rear group, since m ₁ + m ₂ above is 5 or 6, the front group and rear group are respectively This means that each lens includes a negative lens. Therefore, by using a glass material with a relatively small Abbe number for each negative lens, both longitudinal chromatic aberration and lateral chromatic aberration can be corrected without difficulty. If the conditions are exceeded, both the axial chromatic aberration and the lateral chromatic aberration at the telephoto end will be significantly undervalued. Condition (3) is for improving spherical aberration, comatic aberration, etc. If the lower limit is exceeded, the extroverted coma becomes large with respect to the off-axis upward light beam. Further, when the upper limit is exceeded, the spherical aberration generated at the third surface becomes large and negative, and it becomes difficult to correct this. Condition (4) assumes a long back focus, and attempting to maintain a long back focus while departing from this condition causes deterioration of spherical aberration. If condition (5) is not met, the extroverted coma becomes large with respect to the off-axis upward beam. If conditions (6) and (7) are not met and the radius of curvature of the fifth surface becomes positive and small, the spherical aberration at the periphery becomes under-corrected and cannot be corrected. (Example) Examples of the present invention will be shown below. In the table, R is the radius of curvature of the surface, D is the surface spacing, n is the refractive index, and ν is the Abbe number. In each example, aberrations are corrected with a glass block placed on the image side of the lens.

【table】

[Table] d ・〓 =0.43
Four

【table】

[Table] 1/(R ・〓 )=−0.11
Five

【table】

[Table] 1/(R ・〓 )=−0.13
Five

【table】

[Table] 1/(R ・〓 )=0.13
Five

【table】

[Table] d ・〓 =0.53
Four
Effects of the Invention As seen from each embodiment and its aberration curve diagram, the zoom lens of the present invention has three lenses in two groups for the first component and the second component, one lens in each group for the third component, and one lens in each group for the third component.
We were able to create a zoom lens that is compact, bright at around F1.4, and has various aberrations well corrected, while having a small number of lenses (5 elements in 5 groups). Moreover, as shown in Examples 1, 2, and 5, it was possible to provide a long back focus that allows a reflector for a single-lens reflex finder to be placed behind the lens.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of Example 1 of the zoom lens of the present invention, FIGS. 2, 3, and 4 are aberration curve diagrams thereof, and FIGS. 5, 6, 7, and 8 are Example 2
9, 10, 11, and 12 are the cross-sectional views and aberration curve diagrams of Example 3, FIG. 13, and 1.
4, 15, and 16 are the 17th figures of Example 4.
18, 19, and 20 are a cross-sectional view and an aberration curve diagram of Example 5, respectively.

Claims (1)

[Scope of Claims] 1. In order from the object side, a positive first component for focusing, a negative second component for zooming, and a third positive single lens that corrects focal shift due to zooming.
It is a four-component zoom lens consisting of a fixed positive fourth component, and the fourth component consists of three positive lenses and two negative lenses, and the two negative lenses are arranged within the fourth component from the object side. When m _1st and m _2nd , it is composed of 5 elements in 5 groups arranged so that m ₁ + m ₂ is 5 or 6, ψ〓: power of the fourth component ψ〓 _-1 : Power of the negative lens on the object side in the 4th component 〓 ₊ : Average value of the Atpe number of the positive lens in the 4th component 〓 _- : When taking the average value of the Atpe number of the negative lens in the 4th component 0.3<1ψ A zoom lens characterized by satisfying the _following conditions: 〓 _-1 1/ψ〓＜1.8 13＜〓 ₊ −〓 -