JPH0629897B2 - Imaging objective lens - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to an imaging objective lens which is achromatized favorably over a wide range from the ultraviolet region to the near infrared region having a wavelength of about 200 nm.

(Background of the Invention) In ordinary optical glass, light rays in a wavelength range shorter than 350 nm are absorbed, and the transmittance rapidly deteriorates. Therefore, it is difficult to take an image with an ultraviolet ray with a general taking lens. Fluorite (CaF ₂ ) is a substance that transmits light in the ultraviolet region well.
And fused silica (SiO ₂ ) are known. These are both 200n
These substances are often used in a wide wavelength range, especially in an optical system using an ultraviolet ray as a light source, because they have little internal absorption from the wavelength range shorter than m to the infrared range and have excellent transmittance. Furthermore, fluorite (CaF ₂ ) has anomalous dispersion, and when achromatization is performed in combination with fused silica (SiO ₂ ), the generation of a secondary spectrum can be extremely reduced, so that a wide wavelength range can be obtained. It is suitable for the optical system used in.

However, since both fluorite and fused silica have low refractive indices and the difference in Abbe number between the two is small, it is difficult to correct aberrations, and it has been difficult to obtain an objective lens having sufficient performance conventionally. Particularly, 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 Petzval sum. For example, Japanese Patent Publication No. 43-26269. Even in the objective lens disclosed in the official gazette, the Petzval sum is not sufficiently corrected, and as a result, the amount of negative image plane curvature remaining is large.

(Object of the Invention) The object of the present invention is to use fluorite (CaF ₂ ) and fused silica (SiO ₂ ) as a lens material, overcome the above-mentioned difficulties, and have a good aberration balance. An object of the present invention is to provide an objective lens which is well color-corrected for light rays in the visible region and further in the infrared region.

(Outline of the Invention) An imaging objective lens according to the present invention has, in order from the object side, a negative refraction having 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. The first component of power, the second component of biconvex shape with positive refracting power, the third component of biconcave shape with negative refracting power
And a fourth component having a positive refracting power, the focal length of the entire system is f, the refracting power of the _first component is ₁ , the second component and the third component
Assuming that the distance from the component is D and the radius of curvature of the image-side lens surface of the third component is R, −1 <f · ₁ <0 (1) 0.10f <D <0.25f (2) 0.21f <R It satisfies the conditions <0.90f (3).

As described above, fluorite and fused silica are excellent in the transmittance of light in the ultraviolet region, but specifically have the optical characteristics as shown in the table below.

Shall be defined in. Further, n ₉ , n _F, and n _c are the refractive indices for the g line (λ = 435.8 nm), the F line (λ = 486.1 nm), and the C line (λ = 656.3 nm), respectively.

Therefore, chromatic aberration can be corrected by using fluorite for the positive lens and fused silica for the negative lens in view of the Abbe number.

The relationship between the partial dispersion ratio and the Abbe number is shown in FIG. A straight line 1 in the figure shows the tendency of general optical glass. As shown in the figure, in the combination of fluorite (CaF ₂ ) and fused silica (SiO ₂ ), the slope of the partial dispersion ratio vs. Abbe number is small compared to the general optical glass combination, so the secondary spectrum is made smaller. It is possible. Then, assuming that the achromatic condition is a thin-walled contact system, according to the axial chromatic aberration correction theory, the synthetic focal length is f, the focal length of fluorite is f _c , and the focal length of fused silica is f _s. When the Abbe number of stone is V _c and the Abbe number of fused silica is V _s , Achromatism is achieved. However, since the difference in Abbe number is small, each focal length becomes short, and as a result, high-order aberrations are likely to occur.

Next, let the refractive index of fluorite for d line (λ = 587.6 nm) be N
_c, and the refractive index of fused silica and N _s, since it is N c _{<N s,} the thin adhesion system can not satisfy the Petzval condition, positive Petzval sum remains.
According to the theory of the thin-walled adhesive system, this residual amount P is Is.

In the present invention, the first component is configured as a negative refractive power component having 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, and more specifically, By constructing the first component by a negative lens having a concave surface on the object side and a positive lens having a convex surface on the image side in order from the object side,
It forms a flattener as a whole of the first component and has a function of reducing the remaining Petzval sum.

If the upper limit of condition (1) is exceeded, the first component will have a positive refractive power, and the effect of directing the Petzval sum to the positive side will become stronger, and as a result, the Petzval sum of the entire system will not be corrected. Will be enough. The strong negative refractive power of the first component is advantageous for correcting Petzval sum, but the condition
When (1) is out of the lower limit, the divergence action of the first component becomes too strong, and higher-order aberrations occur. In particular, astigmatism is remarkably generated, which makes it difficult to satisfactorily correct aberrations. First
By constructing the component with the negative lens and the positive lens, it is possible to assist correction of axial chromatic aberration and it is also effective for correction of lateral chromatic aberration.

Condition (2) is also for better correcting Petzval sum. The second component to the fourth component have a so-called triplet type lens arrangement of positive, negative and positive, but by setting the distance D between the second component and the third component to an appropriate value,
The negative refractive power of the third component is made stronger, and
Together with (1), it becomes possible to satisfactorily correct Petzval sum. If the lower limit of this condition is not satisfied, the interval between the second component and the third component is too small, and the Petzval sum correction effect is insufficient. If the upper limit is exceeded,
This is advantageous for Petzval sum correction, but as the lens system becomes long, astigmatism tends to occur, making it difficult to obtain a good correction state.

The condition (3) is a condition necessary for maintaining the aberration balance. If the upper limit of this condition is exceeded, the divergent action of the third component on the image-side surface will become small, and spherical aberration and meridional image surface curvature aberration will be undercorrected, and a good correction state will not be obtained. On the other hand, when the value goes below the lower limit, the divergence of the third component on the image-side surface becomes too strong, so that higher-order aberrations become remarkable, and spherical aberrations worsen in particular and good aberration balance is maintained. Becomes difficult.

By the way, in short-distance shooting, aberration variation generally occurs, but in the present invention, when the combined refractive power of the third component and the fourth component is ₃₄ , the condition of −1.1 <f · ₃₄ <0 (4) It is desirable to satisfy. Under this condition, it is possible to maintain a good balance between spherical aberration and the image plane even at a short distance. At a short distance, the exit pupil moves away from the image plane, so for the chief ray reaching a certain image height,
The exit angle becomes smaller, and the light rays passing through the third and fourth components pass closer to the optical axis as the distance becomes shorter. As a result, if the diverging action of the third and fourth components as the rear component is strong, the action of directing the image surface to the negative is enhanced. If the lower limit of this condition is not satisfied, the diverging action of the rear component becomes too strong, and the image surface at a short distance becomes negative, deteriorating the aberration balance, and if the upper limit is exceeded, the rear component converges. Since it has a function, the image plane is positively deviated and the aberration balance is deteriorated.

(Embodiment) In the first embodiment of the present invention, as shown in FIG. 1, the first component L ₁ of the negative refractive power is composed of a biconcave negative lens and a biconvex positive lens in order from the object side. The component L ₂ is composed of a biconvex positive lens with a surface having a stronger curvature facing the object side, and the third component L ₃
Is a biconcave negative lens and the fourth component L ₄ is a biconvex positive lens.

In the second embodiment shown in FIG. 2, the first component L ₁ is composed of a cemented negative lens composed of a biconcave negative lens and a biconvex positive lens. In the third embodiment, as shown in FIG. 3, the first component L
_A divergent air lens is formed between the negative lens and the positive lens which compose ₁ to enlarge the angle of view. In addition, the fourth embodiment shown in FIG.
₄ is composed of a biconvex positive lens with a surface having a stronger curvature facing the image side, and a negative meniscus lens having a convex surface facing the image side separated from the biconvex positive lens. The fifth embodiment of FIG. 5 is the fourth embodiment.
The component L ₄ is composed of a positive lens having a surface having a stronger curvature facing the image side and a negative meniscus lens having a convex surface facing the image side joined to the positive lens.

Tables 1 to 5 below show specifications of the first to fifth examples according to the present invention. In the table, the leftmost number represents the order from the object side, Bf represents the back focus, and Σp represents the Petzval sum of the entire system.

Aberration diagrams of the first to fifth examples are shown in FIGS. 6AB to 10AB, respectively. A in each aberration diagram is an aberration diagram in the infinity photographing state, and B in each aberration diagram is an aberration diagram in the short-distance photographing state with the photographing magnification β = −0.5. The reference wavelength is d line (λ = 587.6 nm) in each case, and in each A diagram showing the aberration at infinity, g line (λ = 435.8 nm), C line (λ = 656.3 nm) and A ′ line. The chromatic aberration of magnification for (λ = 768.2 nm) is shown, as well as the axial chromatic aberration for showing the secondary spectrum. In the off-axis aberration diagram in the infinity photographing state, the scale is represented by the photographing field angle α, and in the off-axis aberration diagram in the short distance photographing state, the scale is indicated by the height H ₀ of the object.

From the above tables and aberration diagrams, it can be seen that the Petzval sum is satisfactorily corrected and all the aberrations are satisfactorily corrected in each of 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 in an extremely wide wavelength range from the ultraviolet region to the infrared region. Further, it is understood that even in an extremely short-distance photographing state where the photographing magnification β = −0.5, the various aberrations are not deteriorated and the good performance is maintained.

As described above, according to the present invention, fluorite (CaF ₂ ) and fused quartz are used.
While using (SiO ₂ ) as a lens material, it has an excellent aberration balance and achieves an objective lens with good color correction for light rays in the wavelength range of about 200 nm from the ultraviolet region to the visible region and further to the infrared region. .

[Brief description of drawings]

FIG. 1 is a lens configuration diagram of a first embodiment according to the present invention, and FIG.
FIG. 4 is a lens block diagram of the second embodiment, FIG. 3 is a lens block diagram of the third example, FIG. 4 is a lens block diagram of the fourth example, and FIG. 5 is a lens block diagram of the fifth example. 6A and 6B
FIGS. 7A and 7B are graphs showing various aberrations in the infinity shooting state and the close-range shooting state, respectively, in FIGS. 7A and 7B, respectively, and FIGS. Figures 8A and 8B are respectively the third
FIGS. 9A and 9B are graphs showing various aberrations in the infinity shooting state and short-distance shooting state, respectively, and FIGS. 9A and 9B are graphs showing aberrations in the infinity shooting state and short-distance shooting state in the fourth example, respectively.
FIGS. 10A and 10B are graphs showing various aberrations of the fifth embodiment in the infinity shooting state and the short-distance shooting state, respectively.
FIG. 1 is a diagram showing the relationship between the partial dispersion ratio and the Abbe number. [Explanation of Symbols of Main Part] L ₁ ... First component L ₂ ... Second component L ₃ ... Third component L ₄ ... Fourth component

Claims (1)

[Claims] 1. A first component of negative refractive power having, 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, a biconvex shape. Second of positive refracting power
A third component having a biconcave shape and a negative refractive power and a fourth component having a positive refractive power, the focal length of the entire system is f, the refractive power of the _first component is ₁ , the second component and the second component When the distance from the third component is D and the radius of curvature of the image-side lens surface of the third component is R, −1 <f · ₁ <0 (1) 0.10f <D <0.25f (2) 0.21f An imaging objective lens characterized by satisfying the respective conditions of <R <0.90f (3).