JPS6190115A - Objective lens for forming image - Google Patents

Abstract

PURPOSE:To improve aberration balance and to make excellent color corrections in a wide wavelength range by composing a lens system of the 1st element which has negative refracting power, the 2nd biconvex element with positive refracting power, the 3rd biconcave element with positive refracting power, and the 4th element with positive refracting power, and allowing the lens to meet specific requirements. CONSTITUTION:The 1st negative-refracting-power element L1 which has a object- side concave surface on the object side and an image-side concave surfaces on the object side, the 2nd biconvex element L2 with positive refracting power, the 3rd biconcave element L3 with negative refracting power, and the 4th element L4 with positive refracting power. Then, inequalities in a figure hold, where (f) is the focal length of the whole system, phi1 the refracting power of the 1st element L1, D the gap between the 2nd element L2 and the 3rd element L3, and R the radius of curvature of the image-side lens surface of the 3rd element L3.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention provides an imaging objective lens that has good color/i'i L over a wide range from the ultraviolet region with a wavelength of about 200 nm to the near infrared region. Regarding.

(Background of the Invention) In ordinary optical glass, light in a wavelength range shorter than 350 nm is absorbed and the transmittance deteriorates rapidly. Therefore, with a general photographic lens, it is difficult to cause shadow loss with ultraviolet light. Fluorite (fluorite) is a substance that transmits UV light well.
CaFz) and fused silica (Sift) are known.

Both of these materials have low internal absorption D and excellent transmittance from wavelengths shorter than 200 nm to the infrared region, so these materials are often used in optical systems that use UV light as a light source over a wide wavelength range. There is. In addition, fluorite (CaF
t) has anomalous dispersion D, fused silica (SiO□)
When combined with achromatism, the generation of secondary spectra can be significantly reduced, making it suitable for optical systems used in a wide wavelength range.

However, both fluorite and fused silica have low refractive indexes, and the difference in their numbers is also small, making it difficult to correct aberrations.
Conventionally, it has been difficult to obtain objective lenses with sufficient performance. In particular, since the refractive index of fluorite used as a positive lens is lower than that of fused silica used as a negative lens, it is extremely difficult to correct the Petzval sum. Even in the objective lens disclosed in the publication, the Petzval sum was insufficiently corrected, resulting in a large residual amount of negative field curvature.

(Object of the invention) The object of the invention is to use fluorite (CaFt) and fused silica (Si).
(h) as a lens material, it overcomes the above difficulties and has a good aberration balance, and has good color correction for light rays from the ultraviolet region with a wavelength of about 200 nm to the visible region and even the infrared region. Our objective is to provide objective lenses.

(Summary of the Invention) An imaging objective lens according to the present invention has, in order from the object side, an object side surface having a concave surface facing the object side and an image side surface having a concave surface facing the object side. 1st component of power, 2nd component of positive refractive power in biconvex shape, t't 1i1 in biconcave shape
Hitting the third component of i refractive power and the fourth component of 1F refractive power,
Let r be the sub-focal length of the entire system, φ1 be the refractive power of the first component, D be the distance between the second component and the third component, and R be the radius of curvature of the image-side lens surface of the third component. When, -1〈'・φ, <O(1) 0.1Of< D <0.25r(2) 0.21f
<R<0.90f (3) is satisfied.

As mentioned above, fluorite and fused silica have excellent transmittance of ultraviolet light, and specifically have optical properties as shown in the table below.

shall be defined as Also, n, , nr and nc
are the g-line (λ=435.8n+a) and the F-line (λ-
486.1 nm) and C line (λ=656.3 nm).

Therefore, from the relationship of Atbe's number, chromatic aberration can be corrected by using fluorite for the positive lens and fused silica for the negative lens.

The relationship between the partial dispersion ratio and the Abbe number is shown in FIG. 11. The straight line l in FIG. 1 shows the tendency of general optical glasses. As shown in the figure, compared to the combination of general optical glasses, fluorite (CaF)
z) and fused silica (Sift), the slope of the partial dispersion ratio to the Abbe number (lit) is small, so it is possible to reduce the secondary spectrum. According to the axial chromatic aberration correction theory for a close-contact system, the composite focal length is f, and the focal length of fluorite is f6.
, the focal length of fused silica is fs, and the number of fluorite points is V
6.? Achromatization is achieved when the Abbe number of 8-fused silica is ▪ and S is S. However, since the difference in number is small, each focal length is short, and as a result, higher-order aberrations are likely to occur.

Next, when the refractive index of fluorite for the d-line (λ-587.6 nm) is N c + >a and the refractive index of fused silica is N, then NC<N! Therefore, the Petzval condition cannot be satisfied in a thin-walled close-contact system, and a positive Petzval sum remains.

According to the theory of thin-walled close contact system, this residual ¥P is It is.

In the present invention, the first component is configured as a negative refractive power component having an object-side surface with a concave surface facing the object side and an image-side surface with a concave surface facing the object side. The first
By configuring the component from the object (!1) to a Page lens having a concave surface on the object side and a positive lens having a convex surface on the image side in order, the first component as a whole forms a flattener and the residual Petzval sum. If the upper limit is exceeded in condition 6 (1), which has the effect of reducing the D , as a result, the Petzval sum of the entire system is insufficiently corrected.

A strong negative refractive power in the first component is advantageous for correcting the Petzval sum, but if condition (1) is outside the lower limit, the divergence effect in the first component becomes too strong and high-order Aberrations occur. In particular, the occurrence of astigmatism becomes noticeable, making it difficult to properly correct the aberration.

By configuring the first component with a negative lens and a positive lens, it helps to correct axial chromatic aberration, and is also effective in correcting lateral chromatic aberration.

Condition (2) is also for better correcting the Petzval sum. The second to fourth components have a so-called triplet lens arrangement of positive, negative, and positive, but each time the distance between the second and third components is set to an appropriate value and 4 degrees,
L, the S'j refractive power of the 13th component is made stronger, and the Petzval sum is corrected satisfactorily in combination with condition (1).

If the lower limit of this condition is exceeded, the distance between the second component and the third component is too small, and the complementary IL effect of the Petzval sum is insufficient. In addition, if the upper limit is exceeded, although it is advantageous for correcting the Petzval sum, point aberration tends to occur because the lens system becomes long, and it is difficult to achieve a good correction state. Become.

Condition (3) is the condition ('1) necessary to maintain aberration balance.
It is. If the upper limit of this condition is exceeded, the divergence effect of the third component on the image side surface will be small (D), spherical aberration and meridional field curvature aberration will be insufficiently corrected (D), and a good correction state will not be obtained. On the other hand, if the lower limit is exceeded, the divergence effect on the image side surface of the third component becomes too strong, resulting in noticeable higher-order aberrations (D), especially spherical aberrations, and making it difficult to maintain a good aberration balance. becomes difficult to maintain.

By the way, in close-range photography, aberration fluctuations generally occur, but in the present invention, when the combined refractive power of the third component and the fourth component is φ, 4, -1,1<f・φi4<0
It is desirable to satisfy the condition (4). With this condition D, it is possible to maintain a good balance between the spherical aberration and the image plane even at short distances. At short distances, the exit pupil moves away from the image plane, so if we look at the chief ray that reaches a certain image height, the rays that pass through D, the third and fourth components, which have small exit angles, will move toward the optical axis at short distances. It will pass through a nearby location. As a result, if the divergence effect of the third and fourth components as rear components is strong, the effect of directing the image plane toward the negative side increases. If the lower limit of this condition is exceeded, the divergence effect of the rear component is too strong, resulting in a negative image plane at close distances, and the aberration balance deteriorates, and if the upper limit is exceeded, the rear component becomes too strong. Since it has a converging effect, the image plane is positively biased and the aberration balance deteriorates.

(Embodiment) In the first embodiment of the present invention, as shown in FIG. 1, the first component L1 of negative refractive power is composed of a biconcave negative lens and a biconvex positive lens in order from the object side, The third component D,
The fourth component is composed of a biconcave negative lens, and 4 is composed of an l+lil convex positive lens.

In the second embodiment shown in FIG. 2, the first component D is composed of a negative lens made up of a biconcave negative lens and a biconvex positive lens. In the third embodiment, as shown in FIG. 3, a diverging air lens is formed between the negative lens and the positive lens constituting the first component. be. Further, in the fourth embodiment shown in FIG.
4 is a biconvex positive lens with a surface of stronger curvature facing the image side, and +IL (!l'l) is separated from this by a biconvex positive lens with a surface of stronger curvature facing +IL (!l'l with f[
It is composed of a meniscus lens. The fifth embodiment shown in FIG. 5 is composed of a positive lens having a fourth component with a surface of stronger curvature facing the image side, and a negative meniscus lens cemented with this lens having a convex surface facing the image side. be.

Tables 1 to 5 below show specifications of the first to fifth embodiments of the present invention. In the table, the leftmost number is 111 from the object side.
1, B is the pack focus, and ΣP is the Petzval sum of the entire system.

Table 1 (First Example) F number 4 Angle of view 2ω = 23.4°Bf =
73.723 ΣP = 0.00249 φ, = −0,00083 φsa” −0,00885 Table 2 (Second Example) F number 4 Angle of view 2ω = 23.4゜nf =
72.453 Σp = 0.00248 φ, = -0,00084 φ, 4 = -0,00922 Table 3 (Third Example) F number 4 Angle of view 2ω = 32.2°Bf =
79.142 ΣP = 0.00271 φ, = -0,00305 φko, = -0,01053 Table 4 (4th example) f = 100 F number 4.5 Angle of view 2ω = 23.4°R
(= 82.765 ΣP = 0.00191 φ Summer −−0.00447 φ1.−−0.00695 Table 5 (Fifth Example) f = 100 F number 3.76 Angle of view 2ω = 23.4°B
f -78,764 ΣP = 0.00281 φ, = -0,00593 φ, , = -0,00483 The first aberration diagrams for the first to fifth embodiments are shown in Figures 6AB to 6AB, respectively. Shown in Figure 108B.

A in each aberration diagram is like shooting at infinity! D is an aberration diagram of the lens, and B of each aberration diagram is an aberration diagram in a close-range photographing state at a photographing magnification β-0.5. In both cases, the reference wavelength is the d-line (λ”'587.
(in+n)D, and each A diagram showing aberrations at infinity includes g-line (λ=435.8nm), C-line (λ=656°3Bm), and centerline (λ=768.2nm). In addition to showing the lateral chromatic aberration of , it also showed the axial chromatic aberration to show the secondary spectrum. In the off-axis aberration diagram for the infinity photographing state, the scale is represented by the photographing angle of view α, and in the off-axis aberration diagram for the close-range photographing state, talnPt is represented by the height Ho of the object.

It can be seen from the tables and aberration diagrams D that the Petzval sum is well corrected and all aberrations are well corrected in all the examples according to the present invention.

In particular, it is clear that chromatic aberration is well corrected and that it has excellent imaging performance over an extremely wide wavelength range from the ultraviolet region to the infrared region. Furthermore, it can be seen that good performance is maintained with little deterioration in various aberrations even in an extremely close-distance photographing state where the photographing magnification β=-0.5.

(Effects of the Invention) As described above, according to the present invention, while using fluorite (CaF) and fused silica (SiO□) as lens materials, the aberration balance is good, and from the ultraviolet region with a wavelength of about 200 nm, For light rays in the visible region and even in the infrared region.

In contrast, an objective lens with good color correction is achieved.

[Brief explanation of the drawing]

FIG. 1 is a lens configuration diagram of the first embodiment according to the present invention, and the second
FIG. 3 is a lens configuration diagram of the second embodiment, FIG. 3 is a lens configuration diagram of the third embodiment, FIG. 4 is a lens configuration diagram of the fourth embodiment, and FIG. 5 is a lens configuration diagram of the fifth embodiment. Figures 6A and 6B
The figures are various aberration diagrams for the infinity photography state and the close-range photography state in the first embodiment, respectively, and FIGS. 7A and 7B are various aberration diagrams for the infinity photography state and the close-range photography state in the second embodiment, respectively. Figures 8A and 8B [2I are various aberration diagrams for the infinity ti shadow state and close-up shooting state in the third embodiment, respectively, and Figures 98 and 9B are respectively for the infinity shooting state and the close-up shooting state in the fourth embodiment. The various aberration diagrams of the recent Ml 11 shadow state, Figure 10A, and Figure 1OB are shown in Figure 5, respectively.
FIG. 11 is a diagram showing the relationship between the partial dispersion ratio and the Abbe number. [Explanation of symbols of main parts] Ll...first component L...second component,...third component], 4...fourth component

Claims (1)

[Claims] In order from the object side, an object-side surface with a concave surface facing the object side;
A first component of negative refractive power that also has a concave surface facing the object side and an image side surface, a second component of positive refractive power that is biconvex, a third component of negative refractive power that is biconcave, and a third component of positive refractive power. , the focal length of the entire system is f, and the refractive power of the first component is φ_1
, when the distance between the second component and the third component is D, and the radius of curvature of the image-side lens surface of the third component is R, -1<f・φ_1<0(1) 0.10f<D An imaging objective lens characterized by satisfying the following conditions: <0.25f(2) 0.21f<R<0.90f(3).