JPS6123525B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an optical system containing an image rotating prism used in a low magnification range of about 1x to 10x. Lens systems with image rotation prisms that use trapezoidal prisms and are used in high magnification ranges of 20x or higher have been known, but in these lens systems, the imaging system on the magnification side is used exclusively for magnification. There is a tendency for aberration correction to be performed in the imaging system on the reduction side. Therefore, if you use this method to design an optical system with an image rotation prism of 10 times or less, there is a risk that the residual aberrations in the magnifying side imaging system will become too large to be corrected by the reducing side imaging system alone. This will come out. In addition, although it is a small number, there are 12~
There is also a 13x magnification, but the 1x magnification corrects aberrations only near the axis, and the 12 to 13x magnification only uses lens groups before and after the image rotator (trapped prism). Since aberrations are corrected,
The entire system becomes relatively large. Therefore, at present, this type of lens system for low magnification range is hardly used, and it can be said that it is hardly used.For example, when enlarging a small negative to the service size of a bill with a color roll printer, 35
When a small size and a half size are mixed, it is necessary to change the installation direction of the negative or photographic paper, or to rotate the image direction by placing a mirror in the middle, which is an inconvenient operation. It was. The purpose of the present invention is to provide a new optical system containing an image rotating prism for low magnification range in order to eliminate such inconveniences. A trapezoidal prism placed in parallel is surrounded by an imaging system at the front and back, and
In an optical system with an image rotation prism arranged so that the light beams between the imaging system and the trapezoidal prism are almost parallel, : Composite focal length of the entire system _f : Composite focal length _R of the imaging system on the magnification side : Composite focal length S _f of the imaging system on the reduction side: On-axis spacing S between the imaging system on the enlargement side and the trapezoidal prism S _R : On-axis spacing t between the imaging system on the reduction side and the trapezoidal prism _p : Axial thickness of the trapezoidal prism n _p : Refractive index of the trapezoidal prism α : Base angle NA of each trapezoidal prism _R : Numerical aperture M seen from the reduction side : Enlargement magnification L : Base length of the trapezoidal prism (i ) M・_R −0.03< _f <M・_R +0.03 (ii) L<S _f +t _P +S _R <L+0.4 In addition to satisfying the following conditions, the refractive index of the composition glass of each positive lens included in the imaging system on the magnifying side and the reducing side is n _CP1・n _CP2 ......n _CPN (however, N is the total optical (iii) 1.70<n _CP1 +n _CP2 +……+n _CPN
/N. The present invention will be described below based on one embodiment (a case where both the magnification-side imaging system and the reduction-side imaging system are configured with three groups). As shown in FIG. 1, the optical system of this embodiment includes, in order from the magnification side, a first group C ₁ consisting of a positive single lens;
, a second group C ₂ of a meniscus single lens with a convex surface facing the magnification side located behind it, and a second group C 2 of meniscus lenses located behind it with a convex surface facing the magnification side.
An imaging system on the magnification side is composed of a third group C ₃ consisting of a cemented lens, and a fourth group of trapezoidal prisms C ₄ located behind it and arranged so that its bottom surface is parallel to the optical axis. Behind that, there is a fifth group _C5 consisting of a two-piece meniscus cemented lens with the concave surface facing the magnifying side, and a sixth group _C6 and a seventh group _C7 each consisting of a positive single lens. It is formed from a reduced-side imaging system. Now, when a diverging light beam or a convergent light beam passes through a trapezoidal prism, aberrations are rotationally asymmetric with respect to the optical axis, and the spherical lens systems before and after the prism cannot eliminate this rotationally asymmetric aberration. Therefore, in the optical system of the present invention, the optical paths before and after the trapezoidal prism are arranged so that the beams passing through the trapezoidal prism become almost parallel. That is, as long as the optical system satisfies the relationship _f = M· _R , the light beam passing through the trapezoidal prism becomes parallel, and the rotationally asymmetric aberration described above does not occur. Therefore, in the present invention, the above-mentioned condition (i) is provided to ensure this relationship. In this case, if the upper and lower limits of the condition are exceeded, this relationship collapses and the rotationally asymmetric aberrations cannot be corrected in the front and rear imaging systems. Therefore, if the condition (i) above is satisfied, and assuming that a circular diaphragm is installed in the optical path between the third group _C3 and the fifth group _C5 , its radius will be _R × NA _R. . Therefore, if you try to make a rectangular diaphragm with the outside diameter of a prism whose vertical cross section is square, such as a trapezoidal prism, one side of the vertical cross section will be √
- By using a trapezoidal prism with a square cross section of _R × NA _R , the area can be made equal to that of a circular aperture. In the present invention, we focused on this point and used the external shape of the trapezoidal prism _C4 as the diaphragm of the optical system without installing a dedicated diaphragm. In this case, the base length L of a prism that has one side with a value of √・_R × NA _R and whose trapezoid base angles are α on each side is as shown in Figure 2, L = 2 l ₁ + 2 l ₂ ………(1) Then, l ₁ is l ₂ is However, i means that the rays parallel to the bottom of the prism come from the bottom.

The angle of incidence i' when the light is incident coincident with the optical axis at the height of [Formula] is given by the angle of refraction with respect to the angle of incidence. Here, since i=π/2-α and n _P sini′=sini=cosα, substituting sini′=1/n _P cosα into the above equation (2), we get It will be expressed as Therefore, the base length L of the trapezoidal prism _C4 is becomes. Therefore, the prism cannot enter the optical system unless the above-mentioned S _f +t _P +S _R (which corresponds to d ₇ + d ₈ + d ₉ in this embodiment) is longer than the bottom surface of the trapezoidal prism C ₄ . This was made possible by providing condition (ii). In this case, increasing the value of S _f +t _P +S _R is effective in reducing the curvature of field, but if it increases beyond the upper limit, the Petzval sum increases and the amount of peripheral light becomes insufficient. Further, the lower limit is the minimum value when the trapezoidal prism _C4 is inserted into the lens system. By the way, when the trapezoidal prism _C4 enters between the magnification-side imaging system and the reduction-side imaging system, the above-mentioned S _f +t _P +S
Since the value of _R becomes a very large value, if the refractive index of the composition glass of each positive lens in the lens system is made small, the Petzval sum becomes large. Condition (iii) is a condition for preventing this, and if this inequality collapses, the Petzval sum of the entire system cannot be reduced. Next, specific examples of the present invention will be shown. Example = 100, numerical aperture NA _R seen from the reduction side = 0.12
Enlargement magnification M = 3.36, angle of view 2ω18° as seen from the reduction side
40′, base angle of trapezoidal prism = 45° = 261.246 _R = 77.473 L = 60.132 R: Radius of curvature of each lens refractive surface counted sequentially from the magnifying side. d: Axial thickness or air spacing between the refractive surfaces of each lens, counted sequentially from the magnifying side. n: refractive index of the glass composition of each lens, counted sequentially from the magnifying side. V: Abbe number of the composition glass of each lens counted sequentially from the magnifying side.

[Table] As shown in FIG. 3, all aberrations in this example were well corrected, which shows how effective each condition of the present invention is. Incidentally, when enlarging an image, the reduction side naturally becomes the object world, but it goes without saying that in this case as well, a good aberration correction state is exhibited, just as when the light beam is introduced from the enlargement side. As described above, when the present invention is used, it is possible to obtain an optical system containing an image rotating prism for a low magnification region that is unprecedented and excellent.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram of an optical system of an embodiment based on the present invention, Fig. 2 is an explanatory diagram of a trapezoidal prism used in the optical system, and Fig. 3 shows each aberration of an embodiment based on the above embodiment. It is an aberration curve diagram. C ₁ ... 1st group, C ₂ ... 2nd group, C ₃ ... 3rd group
group, C ₄ ... 4th group trapezoidal prism, C ₅ ... 5th group
Group, C ₆ ... 6th group, C ₇ ... 7th group, R ₁ to R ₁₆ ...
Radius of curvature of each lens refractive surface, _d1 to _d15 ... axial thickness or air gap between each lens refractive surface, L... bottom surface length of the trapezoidal prism.

Claims (1)

[Scope of Claims] 1. A trapezoidal prism whose bottom surface is placed parallel to the optical axis is surrounded by imaging systems at the front and rear, respectively, and
In an optical system with an image rotation prism arranged so that the light beams between the imaging system and the trapezoidal prism are almost parallel, the combined focal length of the entire system is F=100, and the combined focal length of the imaging system on the magnification side is Ff. = 261.246 Composite focal length of the imaging system on the reduction side Fr = 77.473 Numerical aperture as seen from the reduction side NAr = 0.12 Magnification magnification M = 3.36 Base length of the trapezoid prism L = 60.132 Low angle of each trapezoid prism α = 45° reduction side The angle of view seen from the lens is 2ω = 18°40', R: radius of curvature of each lens refractive surface counted sequentially from the magnifying side. d: Axial thickness or air spacing between the refractive surfaces of each lens, counted sequentially from the magnifying side. n: refractive index of the glass composition of each lens, counted sequentially from the magnifying side. v: Atupe number of the glass composition of each lens, counted sequentially from the magnifying side. An optical system containing an image rotating prism, characterized in that it is represented by [Table] [Table] when [Table] [Table]