JPS6131446B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a medium telephoto lens with a brightness of about 1:4, for example, a 6x7 size 200mm, a 6x
This applies to 4.5 size lenses around 150mm. The lens type of the present invention is the so-called Ernostar type. Conventionally, this type uses glass materials with a refractive index of 1.5 to 1.6 and an Atsbe number of about 55 to 65 for the first and second lenses, which limits the ability to remove chromatic aberrations and has problems with imaging performance. . The purpose of the present invention is to reduce the overall length to a telephoto ratio.
The ultra-low dispersion glass, which is kept at about 0.96 to 0.92, but is particularly effective for super-telephoto lenses, is used in the Ernostar type, which is a medium-telephoto type, to create a lens with good axial and lateral chromatic aberration. It is about providing. First, to explain the lens configuration of the present invention, in order from the object side, the first lens is a positive lens having a surface with a large curvature that is convex to the object side, the second lens is a positive meniscus lens that is convex to the object side, and the third lens is a positive meniscus lens that is convex to the object side. A negative lens having a concave surface with a large curvature on the image side, the fourth lens is a third lens.
In a lens consisting of 4 elements in 4 groups, which is a positive lens placed at a large distance from the lens, the following (1) ~
This is a high-performance telephoto lens that satisfies conditions (8). (1) 0.3F＜F ₁ , ₂ ＜0.4F (2) 1.2F＜F ₁ , ₂ , ₃ ＜2.0F (3) 0.1F＜d ₁ +d 2 +d ₃ +d ₄ + _{d 5 ＜} 0.2F (4) 0.13F＜r ₆ ＜0.20F (5) 0.15F＜d ₆ ＜0.3F (6) 68＜(ν ₁ +ν ₂ )/2 (7) 1.49＜(n ₁ +n ₂ )/2＜1.58 (8) 30<(ν ₃ +ν ₄ )/2<45 where F: Focal length of the entire system F ₁ , ₂ ... _i : Combined focal length up to the i-th lens d _j : Thickness of the j-th lens or lens spacing r _j : j-th radius of curvature ν _i : Atsube's number m _i of the i-th lens: d-line refractive index of the i-th lens Next, each of the above conditions will be explained. Condition (1) is related to conditions (2) and (3), and the telephoto ratio is approximately 0.96 to 0.92, and the focal length distribution necessary to form the framework of a telephoto lens with well-corrected aberrations. This is a condition that indicates. When the upper limit is exceeded, _d4 must be made considerably large to maintain compactness, and it becomes difficult to correct chromatic aberration and other aberrations. Also, when the lower limit is exceeded, the first and second lenses will be under strain, so a positive lens with a large refractive index must be used.
The sum of Petzval tends to become small, which causes difficulties in blending glass, etc., and causes a decrease in performance. Condition (2) is a condition that determines the power of the third lens under condition (1). When F ₁ , ₂ , and ₃ are longer than the upper limit, the telephoto ratio increases, so in order to make it compact, the fourth lens must be placed as close as possible, which reduces coma aberration, This results in an imbalance of aberrations such as distortion. Moreover, when it is short beyond the lower limit, the power of the fourth lens becomes too small, and various aberrations generated in the first to third lenses cannot be corrected in a well-balanced manner. Condition (3) is mainly a condition for keeping the Petzval sum normal and maintaining the telephoto ratio within the desired range. When it is larger than the upper limit, it is good for reducing the telephoto ratio, but the spherical aberration of the peripheral light beam increases. Moreover, when it is smaller than the lower limit, the total length becomes longer and the Petzval sum also increases. Therefore, in both cases, the desired angle of view cannot be satisfied due to aberrations. Condition (4) is a condition for suppressing as much as possible over-correction of the spherical aberration of the short-wavelength side peripheral light beam generated on the r ₆ surface. When the upper limit is exceeded, even if the spherical aberration of the short-wavelength side marginal light beam is good, the spherical aberration of the reference wavelength is extremely under-corrected, which is inappropriate. On the other hand, when the lower limit is exceeded, the spherical aberration of the peripheral light beam on the short wavelength side becomes excessively corrected, which is unsuitable. Condition (5) is a condition regarding the position of the fourth lens. When _d6 is large beyond the upper limit, the diameter of the fourth lens becomes too large in order to maintain an appropriate amount of peripheral light, which is undesirable in terms of frame construction. Furthermore, when d ₆ is small beyond the lower limit, coma aberration occurs and its correction becomes difficult, which is undesirable. Condition (6) is a condition indicating the existence range of ν obtained by using ultra-low dispersion glass. Compared to the conventional case where (v ₁ +v ₂ )/2≈60, it can be seen that the occurrence of chromatic aberration in the first and second lenses is small. If the lower limit of condition (6) is exceeded, the occurrence of chromatic aberration will not be much different from that of conventional lenses, and the objective of the present invention will be missed, which is inappropriate. Condition (7) is a condition indicating the range of the average refractive index of the first and second lenses. When exceeding the upper limit,
When ultra-low dispersion glass is used for one of the first and second lenses, the other must be made of a glass material with a value of 1.66<n, but there is no glass material that satisfies this requirement and has a large Atsbe number, which is the purpose of this application. This is not suitable for obtaining a lens with good axial and lateral chromatic aberration conditions. Moreover, when the lower limit is exceeded, the curvature of the first and second lenses becomes large, making it difficult to correct spherical aberration, and in order to achieve good aberration correction, the total length becomes large. Condition (8) is a condition for appropriately correcting axial and lateral chromatic aberrations in the lens system of the present invention determined by conditions (1) to (7). Examples of the present invention will be shown below. Here, F is the focal length, f _B is the back focus, ω is the half angle of view,
r is the radius of curvature, d is the lens thickness or lens spacing, n
is the refractive index of the d-line, and ν is the Abbe number.

【table】

【table】

【table】

【table】 [Brief explanation of the drawing]

1 and 3 are lens system configuration diagrams corresponding to Examples 1 and 2, respectively, and FIGS. 2 and 4 are aberration curve diagrams corresponding to Examples 1 and 2, respectively.

Claims (1)

[Claims] 1. In order from the object side, the first lens is a positive lens having a surface with a large curvature that is convex toward the object side, the second lens is a positive meniscus lens that is convex toward the object side, and the third lens is a positive lens that is convex toward the object side. In a lens consisting of 4 elements in 4 groups, in which a negative lens has a large concave curvature surface, and the 4th lens is a positive lens arranged at a large distance from the 3rd lens, the following (1) to (8) are applied. A high-performance telephoto lens that satisfies the requirements. (1) 0.3F＜F ₁ , ₂ ＜0.4F (2) 1.2F＜F ₁ , ₂ , ₃ ＜2.0F (3) 0.1F＜d ₁ +d 2 +d ₃ +d ₄ + _{d 5 ＜} 0.2F (4) 0.13F＜r ₆ ＜0.20F (5) 0.15F＜d ₆ ＜0.3F (6) 68＜(ν ₁ +ν ₂ )/2 (7) 1.49＜(n ₁ +n ₂ )/2＜1.58 (8) 30<(ν ₃ +ν ₄ )/2<45 where F: Focal length of the entire system F ₁ , ₂ ... _i : Combined focal length up to the i-th lens d _j : Thickness of the j-th lens or lens spacing r _j : j-th radius of curvature ν _i : Atsube number n _i of the i-th lens: d-line refractive index of the i-th lens