JPS6125126B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a Tessar type lens, and is particularly suitable for a Tessar type lens with a rear aperture. The rear aperture Tetsusar type lens is extremely useful in terms of downsizing the entire camera and making the lens barrel structure cylindrical. Looking at this from the standpoint of miniaturization, for example, even if a lens has an angle of view of 59 degrees or more and is bright enough to reach an F number of 2.8, if its focal length is f, then the focus will be from the first surface of the lens. The total length to the surface can be shortened to within 1.16f. However, in a Tetsusar type lens with a shortened overall length, the front surface of the intermediate biconcave lens, which has the function of correcting spherical aberration in the middle part of the incident height, has an excessive refractive effect on the light beam at the maximum incident height part of the off-axis light beam. Therefore, it has the disadvantage that coma and flare are larger at peripheral angles of view than at intermediate angles of view. Therefore, various improvements have been made to overcome this drawback. The main means of improvement is to increase the refractive index of the first lens.
By making it larger than about 1.78, the occurrence of spherical aberration at the first surface, which is the surface where the maximum spherical aberration occurs, is suppressed,
The objective was to reduce coma and flare by weakening the spherical aberration correcting effect of the front surface of the biconcave lens. On the other hand, if the refractive index of the first lens is made larger than 1.78, the Abe number of the selected glass material will be 45 or more if the selected glass material is selected from among commonly available glass types, which has the negative effect of making it difficult to correct chromatic aberration of magnification. accompanies. An object of the present invention is to satisfactorily correct chromatic aberration even when a glass material with an Abbe number greater than 48 is selected for the first lens, and also to improve correction of other aberrations. Therefore, a positive meniscus lens, a biconcave lens, and a positive lens consisting of a negative meniscus lens and a biconvex lens are placed in order, and ri is successively set to the radius of curvature,
When di is the sequential lens thickness or surface spacing, N ₄ is the refractive index of a biconvex lens, ν ₁ is the Atsube number of a positive meniscus lens, ν ₂ is the Atsube number of a biconcave lens, and f is the focal length, the following conditional expressions are written. Fulfill. (1) 0.34f< r ₁ <0.37 f (2) 0.95f< r ₂ <1.3 f (3) 1.3f< |r ₃ | <1.6 f, r ₃ <0 (4) 0.31f< r ₄ <0.37 f (5) 1.6f< r ₅ <2.7 f (6) 0.74f< | r ₇ | <0.84 f (7) 0.1f< d ₁ <0.12 f (8) 0.05f< d ₃ <0.065f (9) 0.1f<d ₅ +d ₆ <0.13 f (10) 1.78 <N ₄ (11) 48 <ν ₁ (12) ν ₂ < 30 Next, we will explain the significance of the limits of each condition, but each of the For reference, FIG. 1 is attached to show in which part of the aberration curve in the aberration diagram the aberration characteristically appears. First, if the upper limit of (1) is exceeded, the back focus becomes long, and the purpose of shortening the total length cannot be achieved. If the lower limit is exceeded, the spherical aberration will be insufficiently corrected, and the bulge in the middle portion of the spherical aberration will become excessive. When the upper limit of equation (2) is exceeded, the negative astigmatism becomes excessive, and when the lower limit is exceeded, the positive spherical field curvature at the periphery of the screen becomes excessive. If the upper limit of equation (3) is exceeded, the negative astigmatism becomes excessive and spherical aberration is insufficiently corrected, and if the lower limit is exceeded, coma flare becomes noticeable from the center to the periphery of the screen. If the upper limit of equation (4) is exceeded, the spherical aberration will be insufficiently corrected, and if the lower limit is exceeded, the spherical aberration of the annular zone will be overcorrected. When the upper limit of equation (5) is exceeded, the positive meridian and field curvature at the periphery of the screen become excessive, and when the lower limit is exceeded, the negative astigmatism becomes excessive. When the upper limit of equation (6) is exceeded, the introverted coma aberration increases from the center to the periphery of the screen, and when the lower limit is exceeded, the negative astigmatism becomes excessive and at the same time, the extroverted coma aberration increases at the periphery of the screen. . When the upper limit of equation (7) is exceeded, correction of spherical aberration becomes insufficient, and when the lower limit is exceeded, negative astigmatism becomes excessive. When the upper limit of equation (8) is exceeded, inward coma aberration at the periphery of the screen increases, and when the lower limit is exceeded, negative astigmatism becomes excessive. If the upper limit of equation (9) is exceeded, the positive meridian and field curvature will be excessive at the periphery, and if the lower limit is exceeded, the negative astigmatism will be excessive and the extroverted coma will be excessive from the center to the periphery. Aberrations occur. When the range of equation (10) is exceeded, the Petzval sum increases, and the negative spherical field curvature at the center of the screen becomes excessive. If the range of equation (11) is exceeded, it becomes difficult to correct lateral chromatic aberration. If the range of equation (12) is exceeded, it becomes difficult to correct longitudinal chromatic aberration. Numerical examples of rear aperture Tetsusar type lenses are described below, but the focal length of each example is standardized to f = 1, R is the radius of curvature of the lens surface counted sequentially from the subject side, and D is the subject Nd is the d-line refractive index of the medium counted sequentially from the subject side, and νd is the Atsube number of the medium counted sequentially from the subject side. Further, FIG. 2 shows the cross-sectional shape of the lens, and FIGS. 3 to 6, which show aberration curves, show the aberration correction in Examples 1 to 4 in order.

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[Table] In addition, the third-order aberration coefficients of Examples 1 and 3 are listed below.

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[Table] SA is the spherical aberration coefficient, CM is the comatic aberration coefficient, AS is the astigmatism coefficient, PT is the Petzval sum, and DS is the distortion coefficient.

【table】 [Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the tendency of an aberration curve and aberration. Figure 2 is a sectional view of the lens. FIG. 3, FIG. 4, FIG. 5, and FIG. 6 are aberration curve diagrams of each example. In the figure, R is a lens surface, D is a lens thickness or surface spacing, and M is a meridional focal line and a sagittal focal line.

Claims (1)

[Claims] 1. In a Tetsusar type lens consisting of a positive meniscus lens, a biconcave lens, and a positive lens made by laminating a negative meniscus lens and a biconvex lens, ri is the radius of curvature, di is the lens thickness or surface spacing, and N is When ₄ is the refractive index of a biconvex lens, ν ₁ is the Atsube number of a positive meniscus lens, ν ₂ is the Atsube number of a biconcave lens, and f is the focal length, (1) 0.34f<r ₁ <0.37f (2) 0.95 f＜r ₂ ＜1.3f (3) 1.3f＜｜r ₃ ｜＜1.6f, r ₃ ＜0 (4) 0.31f＜r ₄ ＜0.37f (5) 1.6f＜r ₅ ＜2.7 f (6) 0.74f＜｜r ₇ ｜＜0.87f (7) 0.10f＜d ₁ ＜0.12f (8) 0.05f＜d ₃ ＜0.065f (9) 0.10f＜d ₅ +d ₆ ＜0.13f (10) 1.78＜ N ₄ (11) 48<ν ₁ (12) ν ₂ <30 A Tetsusar-type lens characterized by satisfying each of the above conditions.