JP2010197460A - Long focus lens with thermal aberration eliminated - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a long focus lens, selecting glass for eliminating thermal aberration to a large temperature change as the whole lens system while a fluophosphate glass is disposed in a position effective for correcting axial color aberration. <P>SOLUTION: This long focus lens, from which thermal aberration is eliminated, includes: a first lens group G1, a second lens group G2 and a third lens group G3 which are disposed in order from an object side. The first lens group G1 is positive, the second lens group G2 is positive, the third lens group G3 is negative, the first lens group G1 positioned closest to the object includes three lenses disposed positive, negative and positive in order. The third lens group G3 positioned closest to an image includes at least one positive lens, and the first lens group G1 includes at least one lens satisfying a predetermined conditional expression. The at least one positive lens of the third group satisfies a predetermined conditional expression. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a long focus lens from which thermal aberration is removed, and preferably to a long focus lens for removing fluctuations in focus position and resolution (thermal aberration) with respect to temperature change, for a photographic lens using visible light. .

In an infrared optical system using a wavelength longer than 3 μm, usable refractive glass materials are limited to a few, and the refractive index change with respect to temperature changes of these refractive glass materials is large. Therefore, conventionally, a glass material selection method and a design method for power distribution have been disclosed in order to remove the focal shift due to the temperature change of the lens system. (Patent Document 1)
In the present invention, the thermal Abbe number is dealt with, generally expressed as τ, and Δn,

Where ΔT = 40 ° C-20 ° C = 20 ° C and α is the linear expansion coefficient of glass, this amount τ is given by

(Patent Document 2).

Japanese Patent No. 3168221 Japanese Patent Application No. 2008-327449

  In designing a photographing lens that uses visible light, particularly a long focal length lens, correction of axial chromatic aberration is particularly important. To correct this aberration, it is effective to use a fluorophosphate glass having a low refractive index, low dispersion, and positive anomalous dispersion for a group having a high axial ray. However, since the refractive index fluctuations with respect to temperature changes are large in fluorophosphate glass, in order to prevent fluctuations in focal position and resolving power with respect to temperature changes (so-called thermal aberration), the entire lens system has to select glass. Special consideration is required.

  The present invention provides a long-focus lens in which a glass for removing thermal aberration with respect to a large temperature change is selected as a whole lens system while arranging a fluorophosphate glass at a position effective for correcting axial chromatic aberration. is there.

  Examples of the solving means of the present invention are as follows.

(1) A long-focus lens having a first group lens, a second group lens, and a third group lens in order from the object side, wherein the first group lens is positive and the second group lens is The first group lens is positive, the third group lens is negative, and the first group lens located closest to the object side is composed of three lenses arranged in order of positive, negative, and positive, and the third lens located closest to the image side. The group of lenses includes at least one positive lens;
The first group is a conditional expression

Having at least one lens satisfying
The positive lens included in the third lens group is a conditional expression

A long focal lens from which thermal aberration is removed, characterized by satisfying
Where ν is the following formula

D line Abbe number defined by
τ is called thermal Abbe number and Δn is

Where ΔT = 40 ° C-20 ° C = 20 ° C and α is the linear expansion coefficient of glass, this amount τ is given by

Defined by

  (2) The long-focus lens described above, wherein the first group has only one lens satisfying Formula 3.

  (3) The long-focus lens described above, wherein one of the positive lenses in the first group is made of fluorophosphate glass and the negative lens is Kurzflint.

  (4) The long-focus lens as described above, wherein the second group includes two lenses having a weak positive refractive power as a whole and has a lens close to a concentric shape.

  (5) The long-focus lens described above, wherein at least one positive lens included in the third group is lanthanum-based glass.

  (6) The long focus lens according to any one of claims 1 to 4, wherein at least one positive lens included in the third group is barium-based glass.

It is an optical path figure of a present Example. It is a longitudinal aberration figure of a present Example. It is a lateral aberration figure of a present Example. This is a through focus MTF with respect to 50 lp / mm in this embodiment at 25 ° C. (This is the defocus characteristic of MTF related to the spatial frequency of 50 pairs of black and white patterns per 1 mm). This is a through focus MTF with respect to 50 lp / mm in this embodiment at 60 ° C. This is a through focus MTF with respect to 50 lp / mm in this embodiment at 0 ° C. It is an optical path figure of this reference example. It is a longitudinal aberration figure of this reference example. It is a lateral aberration figure of this reference example. This is a through focus MTF for 50 lp / mm in this reference example at 25 ° C. It is a through focus MTF with respect to 50 lp / mm of this reference example at 60 ° C. This is the through focus MTF for 50 lp / mm in this reference example at 0 ° C.

  The thermal aberration of the optical system is caused by the refractive index change Δn / ΔT and the linear expansion coefficient α. Below, the concept of the thermal Abbe number for treating this phenomenon quantitatively is demonstrated. By considering the change rate of the focal length with respect to the temperature change, it is possible to define a “thermal Abbe number” similar to the Abbe number that is a measure of the amount of chromatic aberration.

The reciprocal (power) of the focal length of a thin single lens having a refractive index n and curvature radii R ₁ and R ₂ is given by the following equation.

By differentiating this with temperature T, the following relational expression is obtained.

The amount of change in temperature ΔT and the amount of change in radius of curvature ΔR are given by the following equation, depending on the radius of curvature R with the linear expansion coefficient α.

From this, for i = 1,2,

Holds. When Equation 2 is rewritten, the focal length change with respect to the temperature change is given by the following equation.

Where τ is an amount defined by the following equation.

  Since this definition is similar to the Abbe number ν representing chromatic aberration, this is called the thermal Abbe number. However, in the case of the Abbe number of chromatic aberration, the term corresponding to the linear expansion coefficient α does not appear.

  The relationship between the magnitude of the thermal Abbe number and the change in focal length is the same as in the case of chromatic aberration. That is, the larger the thermal Abbe number, the smaller the focal length change with respect to the temperature change, and the smaller the thermal Abbe number, the larger the focal length change with respect to the temperature change.

  In order to calculate τ defined by Formula 11, ∂n / ∂T is replaced with Δn / ΔT.

  As in the case of achromatism, a theoretical formula for temperature compensation by bringing two thin single lenses into close contact is derived. A thin lens is a model that approximates the lens thickness to zero.

The refractive indices of the two lenses are n _a and n _b , and the respective focal lengths are f _a and f _b . Also, let the thermal Abbe numbers be τ _a and τ _b .

When two lenses are brought into close contact with each other, the reciprocal of the combined focal length f is given by the following equation.

By differentiating this with temperature T, the following equation is obtained.

This equation can be rewritten as Equation 10 representing the thermal Abbe number and the change in focal length as follows.

The temperature compensation condition of the two thin contact lenses is that ∂f / ∂T = 0 is satisfied, that is,

It is. This is an expression in which the Abbe number ν that appears in the achromatic condition when two thin single lenses are brought into close contact with each other is replaced with the thermal Abbe number τ. As already indicated, since the Abbe number and the thermal Abbe number can be replaced as they are, the theory of chromatic aberration holds true for the thermal aberration as well.

The optical system is composed of several thin lens groups, and there is a non-negligible air space between them. The paraxial axial ray height in each lens group is set as h _k . Each lens has no thickness and the power is φ _k . Further, the image position of the entire system is s ′.

The equation that gives the temperature change rate ∂s' / ∂T of the image position is based on the same consideration as the theory of axial chromatic aberration.

Given in. The focal position change with respect to the temperature change is greatly influenced by a lens having a high axial ray, and is proportional to the square of the ray height. Further, it is proportional to the power of the thin lens and inversely proportional to the thermal Abbe number.

According to the above theory of thermal aberration, by appropriately selecting the thermal Abbe number τ _k according to the axial ray height h _k and the power φ _k , the temperature change rate ∂s ′ / ∂T = 0 of the image position I can do it.

Here, thermal characteristics of general optical glass will be described. Many optical glasses have a negative thermal Abbe number,

It is in the range. Fluorosilicate and fluorophosphate glass (OHARA's SFSL5, SFPL51, SFPL53, etc.) effective in correcting chromatic aberration has a negative τ and a small absolute value of −0.1 <τ × 10 ⁻⁶ <− It is in the range of 0.03. These glasses are mainly used for positive lenses, and according to Equation 16, the focal position moves far away from the lens due to the temperature rise compared to other glasses.

Some lanthanum glasses and barium glasses have the opposite sign of τ. For example, SBAL35 (τ × 10 ⁻⁶ = + 1.12) of OHARA, SLAL13 (τ × 10 ⁻⁶ = + 0.40), and the like are applicable. According to Equation 16, when these glasses are used for a positive lens, the focal position moves in a direction approaching the lens. Therefore, according to Equation 16, the focal position change ∂s' / ∂T associated with the temperature change can be reduced to 0 by performing glass selection such that the signs of τ are opposite to each other for two different positive lenses. I can do it.

  The present invention will be described in detail while comparing the examples of the present invention (FIGS. 1 to 6) and the reference examples (FIGS. 7 to 12) that do not satisfy the conditions described in the claims of the present invention. 1 and 7, the same members are denoted by the same reference numerals.

  The long-focus lens 1 of the embodiment and the long-focus lens 1 ′ of the reference example have almost the same configuration, and the performance at room temperature (25 ° C.) is almost the same. In FIG. 1, the long focus lens 1 has three groups (G1, G2, G3) of positive, positive, and negative in order from the object side.

  The first group G1 is composed of a first lens L1, a second lens L2, and a third lens L3, each having a positive, negative, and positive three-element separation configuration. One of the positive lenses (the third lens L3) is a fluorine On-axis chromatic aberration is corrected by using phosphoric acid glass and a negative lens (second lens L2) by Kurzflint. If the other positive lens (first lens L1) is also made of fluorophosphate glass, it is difficult to correct thermal aberration in the direction in which the focal point moves away from the lens due to temperature rise. If the first group G1 does not contain any fluorophosphate glass, the generation of thermal aberration is reduced, but the correction of axial chromatic aberration is insufficient, and a high resolving power over the entire visible light range is ensured. I can't. Accordingly, it is necessary and sufficient for the glass satisfying Equation 20 in claim 1 to be only one in the first lens group G1 in order to minimize the thermal aberration while effectively correcting the chromatic aberration.

  The second group G2, which has a force away from the first group G1, is composed of a fourth lens L4 and a fifth lens L5 having a weak positive refractive power as a whole, and has a lens close to a concentric shape, Effective in correcting astigmatism and barrel distortion. Moreover, the third group G3 having negative refractive power as a whole is composed of the sixth lens L6 and the seventh lens L7, and is arranged at the end of the lens system, so that the tele ratio is increased and the long focus lens is obtained. The entire lens system can be stored compactly.

  The difference between the example and the reference example is whether or not the third group G3 is configured to compensate the focal shift with respect to the temperature change of the fluorophosphate glass in the first group G1. As described above, in the first group G1, in order to minimize the thermal aberration while correcting the chromatic aberration, it is necessary and sufficient to limit the glass satisfying Equation 20 to only one sheet. However, when the other group is formed of ordinary optical glass that satisfies Equation 17, thermal aberration correction cannot be performed as the entire system, and focus shift due to temperature change is unavoidable. In the reference example, the positive lens (seventh lens L7 ′) included in the third group G3 ′ is configured by a normal lens that satisfies Expression 17. According to FIGS. Is a problem.

  In the embodiment of the present invention, a lanthanum crown having a thermal characteristic of τ> 0 opposite to normal is used for the positive lens (seventh lens L7) included in the third group G3. This glass selection method makes it possible to cancel the thermal aberration of the first group G1 fluorophosphate glass whose focal position moves away from the lens as the temperature rises by the reverse thermal aberration generated by the third group G3. As a whole, it is possible to realize a lens system in which the thermal aberration is corrected.

Table 1 shows lens data of this example. However, the glass material indicates the refractive index with respect to the d-line, the Abbe number, the glass name of OHARA, and the thermal Abbe number based on its physical property values.

Table 2 shows lens data of this reference example.

Claims (6)

A long-focus lens having a first group lens, a second group lens, and a third group lens in order from the object side, wherein the first group lens is positive and the second group lens is positive The third group lens is negative, and the first group lens located closest to the object side is composed of three lenses arranged in order of positive, negative, and positive, and the third group lens located closest to the image side Contains at least one positive lens,
The first group is a conditional expression

Having at least one lens satisfying
The positive lens included in the third lens group is a conditional expression

A long focal lens from which thermal aberration is removed, characterized by satisfying
Where ν is the following formula

D line Abbe number defined by
τ is called thermal Abbe number and Δn is

Where ΔT = 40 ° C-20 ° C = 20 ° C and α is the linear expansion coefficient of the glass, this amount τ

Defined by   2. The long focus lens according to claim 1, wherein the first lens group has only one lens that satisfies Formula 20.   3. The long focus lens according to claim 1, wherein one of the positive lenses in the first group is made of fluorophosphate glass and the negative lens is Kurzflint.   The long focal point according to any one of claims 1 to 3, wherein the second group includes two lenses having a weak positive refractive power as a whole and has a lens close to a concentric shape. lens.   The long focus lens according to claim 1, wherein at least one positive lens included in the third group is lanthanum-based glass.   5. The long focus lens according to claim 1, wherein at least one positive lens included in the third group is barium-based glass.

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