JP6062160B2 - Optical element design method - Google Patents

Description

  The present invention relates to an optical element that constitutes a photographing lens used in an imaging apparatus such as a digital camera or a video camera, and more particularly to an optical element that prevents generation of a ring-shaped reflection image due to internal reflection.

  In the design and manufacture of a photographing lens used in a photographing apparatus, internal reflection that causes reflection or scattering on the outer peripheral surface or chamfered portion of an optical element arranged in a lens barrel becomes a problem.

  When the light beam that has caused internal reflection reaches the image plane, the image is captured as a ghost or flare, which is one of the causes of lowering the image quality. Even if the light beam that caused internal reflection does not reach the image plane, the light beam returns to the object side, and the outer peripheral surface or chamfered part of the optical element that caused the internal reflection is conspicuous when viewed from the object side. Sometimes. As described above, even when the photographed image itself is not adversely affected, it is assumed that the quality of the product as an optical device that dislikes internal reflection is impaired, and countermeasures are required.

  In order to prevent degradation of image quality caused by light rays that have caused internal reflection reaching the image surface, a light shielding member is provided so that the light rays that have undergone internal reflection do not reach the image surface, and the outer peripheral surface or chamfer of the optical element is provided. Measures are taken to suppress internal reflection by increasing the diameter of the lens so that light does not strike the part, or by applying a light-absorbing material to the outer peripheral surface or chamfered part of the optical element (for example, patent documents) 1).

  Also, in order not to impair the quality of the product due to internal reflection, a light-absorbing material is applied to the outer peripheral surface or chamfered portion of the optical element to suppress internal reflection so that the outer peripheral surface or chamfered portion of the optical element is not noticeable. A countermeasure is taken (for example, Patent Document 2). However, in an optical element having a high refractive index (approximately 1.8 or more), the effect of preventing the inner reflection of the light-absorbing paint is likely to deteriorate, so that there are cases where countermeasures are insufficient only by application of the light-absorbing material. know.

  In general, when the photographing lens is observed from the object side, it can be seen that the outer peripheral portion of the optical element and the mirror chamber holding the optical element are arranged in a ring shape. These measures are usually taken by devising the shape and surface treatment so as not to stand out, and consideration is given so as not to impair the quality of the optical device. However, when there is a chamfered portion (hereinafter referred to as a conical chamfered portion) having a shape corresponding to the side surface of the truncated cone having the apex on the image side on the outer peripheral surface of the optical element, the light beam reflected by the chamfered portion is reflected on the object side. Therefore, when the photographing lens is observed from the object side, a white ring-shaped reflection image may be generated (shaded portion X in FIG. 5). In such a white ring-shaped reflection image, a one-surface reflection image generated by the light reflected on the conical chamfered portion returning directly to the object side, and the light reflected on the conical chamfered portion on the other surface in the optical element. It is known that there is a two-surface reflection image generated by further reflecting and returning to the object side.

  In addition, as a shape of such an optical element in which a two-surface reflection image is likely to be generated, a conical chamfered portion is provided on the outer periphery of the optical element, and the optical element has a concave surface on the image side on the image side. is there. Also, the optical element constituting the photographic lens is arranged closer to the object side, and there is a difference in outer diameter from the optical element arranged on the object side, so that the conical chamfered portion is easily observed. Surface reflection images are likely to occur. Furthermore, even when a glass material having a high refractive index is used, it can be said that a two-surface reflection image is likely to be generated because the effect of preventing internal reflection of the light-absorbing paint is likely to be reduced and total reflection is likely to occur. Due to the shape and arrangement of the optical element, the light beam reflected by the conical chamfered portion is incident on the image-side surface of the optical element at an incident angle greater than the critical angle, and is totally reflected by the image-side surface and is reflected on the object side. It is known that a white ring-shaped two-sided reflection image is generated by returning the light beam to.

JP 2001-33677 A JP 2009-282488 A

  As a countermeasure against the problem of generating the above-described two-surface reflection image, there is a method of setting the angle of the conical chamfered portion so that the incident angle of the light beam on the image-side surface is less than the critical angle. However, the generation of the two-surface reflection image, which is a problem of the present invention, varies depending on each photographing lens, and is not a problem that can be solved by setting the angle of the conical chamfered portion to a constant angle.

  On the other hand, although there is a measure to set the angle of the conical chamfered part to an angle much larger than the angle at which the two-surface reflected image is generated, such as making the angle of the conical chamfered part as close as possible parallel to the optical axis (Fig. 4). In addition, it is impossible to avoid newly adverse effects such as an increase in the size of the optical element and the photographing lens and a deterioration in workability.

  That is, the problem to be solved by the present invention is that even if a light beam incident from the object side is reflected by the conical chamfer and incident on the image side surface, the incident angle of the light beam on the image side surface is the critical angle. It is an object of the present invention to provide an optical element having a shape that prevents total reflection by being less than that and prevents generation of a two-surface reflection image that occurs when a photographing lens is observed from the object side.

An invention that is a means for solving the above-described problem is a method for designing an optical element that constitutes a photographing lens used in an imaging apparatus, and has a concave surface R2 on the image side, and the end of the concave surface R2 is chamfered. This is a method for designing an optical element having a step of setting the angle of the chamfered portion T so as to satisfy both formulas using the following conditional expressions (1) and (2).
(1) n1> n2
(2) 0 <θ1 <{arcsin (n2 / n1) −arccos (h / R)} / 2 + π / 4
θ1: Angle formed by the chamfered portion T and the optical axis h: Ray height R at the position where the light beam HL reflected by the chamfered portion T is incident on the concave surface R2: Radius of curvature of the concave surface R2 n1: Refractive index n2 of the optical element: Optical element Refractive index of the medium filling the periphery of

  According to the present invention, even if a light beam incident from the object side is reflected by the chamfered portion and incident on the image side surface, the incident angle of the light beam on the image side surface becomes less than the critical angle, so that total reflection is achieved. It is possible to provide an optical element having a shape that prevents the occurrence of a two-surface reflection image that occurs when the photographing lens is observed from the object side.

It is sectional drawing of the optical element of this invention. It is a lens block diagram of the photographic lens of Example 1 which concerns on this invention. It is sectional drawing which chamfered the 4th lens G4 of Example 1 which concerns on this invention at 45 degrees. It is sectional drawing which chamfered 4th lens G4 of Example 1 which concerns on this invention at 5 degrees. It is the external view which looked at the imaging lens of Example 1 which concerns on this invention from the object side.

  The optical element of the present invention is an optical element that constitutes a photographing lens used in an imaging apparatus such as a digital camera or a video camera, and has a concave surface R2 on the image side, and a chamfered portion that chamfers an end of the concave surface R2. It is characterized by having T.

The optical element of the present invention satisfies the following conditional expressions (1) and (2).
(1) n1> n2
(2) 0 <θ1 <{arcsin (n2 / n1) −arccos (h / R)} / 2 + π / 4
θ1: Angle formed by the chamfered portion T and the optical axis A h: Light ray height R at the position where the light beam HL reflected by the chamfered portion T is incident on the concave surface R2: Radius of curvature of the concave surface R2 n1: Refractive index n2 of the optical element: Optical Refractive index of the medium filling the periphery of the element

  Conditional expression (1) prescribes a condition in which light rays incident in parallel from the object side and reflected by the chamfered portion T are totally reflected on the concave surface R2, so that the medium fills the refractive index n1 of the optical element and the periphery of the optical element. This defines the relationship with the refractive index n2.

  Conditional expression (2) is incident on the chamfered portion T parallel to the optical axis, and in order to calculate a condition where the incident angle of the light beam reflected by the chamfered portion T to the concave surface R2 is less than the critical angle at which total reflection does not occur. , N1, n2, h, and R are defined.

  When the upper limit of conditional expression (2) is exceeded, the incident angle of the light beam reflected by the chamfered portion T on the concave surface R2 exceeds the critical angle, causes total reflection, and returns to the object side, and the lens is observed from the object side. At this time, a two-surface reflection image of the chamfered portion T is reflected.

  If the lower limit of conditional expression (2) is not reached, θ1 is less than 0 °, so that the chamfered portion T is not a chamfered portion having a shape corresponding to the side surface of the truncated cone having the apex on the image side.

  Here, the process of deriving conditional expressions (1) and (2) will be described. FIG. 1 shows an optical element constituting a photographic lens used in an image pickup apparatus, which has a concave surface R2 on the image side and a chamfered portion T that chamfers an end of the concave surface R2. The cross section of is shown. In FIG. 1, there is an image plane on which the photographic lens connects images on the right side, and an object that is the subject of the photographic lens is on the left side.

  In FIG. 1, T is a chamfered portion, R2 is a concave surface on the image side, HL is a light beam incident on the chamfered portion T parallel to the optical axis, TL is a concave surface at a position where the light beam HL reflected by the chamfered portion T is incident on the concave surface R2. The tangent of R2, A is the optical axis. In FIG. 1, θ1 is an angle formed by the chamfered portion T and the optical axis A, θ2 is an angle formed by the tangent TL and the optical axis A, θc is an incident angle of the light beam HL reflected by the chamfered portion T to the concave surface R2, h is the light ray height at the position where the light beam HL reflected by the chamfered portion T is incident on the concave surface R2. In FIG. 1, the hatched portion represents a cross section of the optical element, and the outer peripheral portion of the optical element is assumed to be parallel to the optical axis A. Furthermore, the radius of curvature of the concave surface R2 is R, the refractive index of the optical element is n1, and the refractive index of the medium filling the periphery of the optical element is n2.

  Among the variables listed here, h, R, n1, and n2 are determined during the optical design of the taking lens, and θc and θ2 change according to θ1, so that only θ1 can be changed in solving the problem. Become.

  Here, as shown in FIG. 1, it is assumed that the light beam HL incident on the chamfered portion T of the optical element is incident parallel to the optical axis. In the first place, it is known that the light beam that causes the white ring-shaped two-surface reflection image observed when the photographing lens is observed from the object side is the light beam HL substantially parallel to the optical axis. Therefore, in order to simplify the derivation, it is assumed that the light beam HL incident on the chamfered portion T of the optical element is incident parallel to the optical axis.

  When a light beam HL parallel to the optical axis is incident on the chamfered portion T from the object side, it is reflected at the chamfered portion T, totally reflected by the concave surface R2, and returned to the object side again. The light beam HL that follows such a path is observed as a white ring-shaped two-surface reflection image when the photographing lens is observed from the object side. In order to eliminate such a two-surface reflection image, it is only necessary to prevent the light from entering the chamfered portion T, but it is necessary to provide an outer diameter that is significantly larger than the effective light beam diameter of the optical element. This is not a realistic measure because it leads to an increase in size. Therefore, it is necessary to derive a condition that the light beam HL reflected by the chamfered portion T is not totally reflected by the concave surface R2.

Therefore, focusing on the point where the light beam reflected by the chamfered portion T enters the concave surface R2, the incident angle of the light beam HL at this point is θc, and the angle between the tangent TL of the concave surface R2 at this point and the optical axis is θ2. The following formulas (a) and (b) are established.
(A) θ2 = arccos (h / R)
(B) θc = −π / 2 + 2θ1 + θ2

Further, the reason why the light beam reflected by the chamfered portion T is not totally reflected by the concave surface R2 is that θc is less than the critical angle of total reflection, and therefore the following equation (c) is established.
(C) θc <arcsin (n2 / n1)
However,
(1) n1> n2

If θ2 and θc are eliminated from the above equations (a), (b), and (c), the following equation (d) is established.
(D) −π / 2 + 2θ1 + arcos (h / R) <arcsin (n2 / n1)

Here, since the following formula (e) holds for θ1, the formulas (1) and (2) are obtained from the formulas (d) and (e).
(E) 0 <θ1
(1) n1> n2
(2) 0 <θ1 <{arcsin (n2 / n1) −arccos (h / R)} / 2 + π / 4

  Next, with respect to the optical element of the present invention, the effect of reducing the two-surface reflection image will be described using an actual photographing lens as an example. In the following description, the lens configuration is described in order from the object side to the image side.

  FIG. 2 is a lens configuration diagram of the photographing lens according to Example 1 of the present invention. The photographing lens according to the present embodiment is an interchangeable lens used in a single-lens reflex camera, and in order from the object side to the image side, a first lens group L1 having a positive refractive power and a second lens group having a negative refractive power. The zoom lens includes L2, a third lens unit L3 having a positive refractive power, and a fourth lens unit L4 having a positive refractive power.

  The first lens unit L1 includes a cemented lens DB1 including a negative lens G1 having a convex surface facing the object side, a positive lens G2 having a convex surface facing the image side, and a positive meniscus lens G3 having a convex surface facing the object side. The

  The second lens unit L2 includes a fourth lens G4 in which an aspheric surface is formed by a photocurable resin layer G4a on the object side of a base G4b that is a negative meniscus lens, a negative lens G5 having a concave surface facing the object side, It is composed of a cemented lens DB2 composed of a positive lens G6 having a convex surface on the side, a negative lens G7 having a concave surface on the object side, and G8 having a convex surface on the object side. The second lens unit L2 moves toward the object side along the optical axis during focus adjustment from infinity to a short distance.

  The third lens unit L3 includes a cemented lens DB3 including a positive lens G9 having a convex surface facing the object side, a positive lens G9 having a convex surface facing the object side, and a negative meniscus lens G10 having a concave surface facing the object side. A cemented lens DB4 composed of a negative lens G11 having a concave surface facing the surface, a positive meniscus lens G12 having a convex surface facing the object side, and a positive meniscus lens G13 having a convex surface facing the object side. In addition, the cemented lens DB4 has an anti-vibration function that suppresses image blur by shifting in the direction perpendicular to the optical axis when camera shake occurs.

  The fourth lens unit L4 includes a positive lens G14 having a convex surface directed toward the object side, a positive lens G15 having a convex surface directed toward the object side, a negative lens G16 having a flat surface on the object side and a concave surface directed to the image surface side, It is composed of a positive lens G17 having a convex surface on the side. Further, the convex surface R1 formed by the photocurable resin layer G4a, the convex surface on the object side of the positive meniscus lens G12, and the both surfaces of the positive lens G15 each have a predetermined aspherical shape.

  Here, attention is focused on the fourth lens G4 of Example 1 according to the present invention. The fourth lens G4 is a composite optical element having a convex surface R1 formed with an aspheric surface by the photocurable resin layer G4a on the object side and a concave surface R2 formed with the base material G4b on the image side, and the convex surface is directed to the object side. It has a negative meniscus shape.

  As can be seen from FIG. 2, the fourth lens G4 has a smaller outer diameter than other optical elements located on the object side, and the chamfered portion T is at a position lower than the effective ray height of the convex surface R1, and negative refraction. Since it has force, it has a shape that allows light to easily hit a wide range of the chamfered portion T when light enters from the object side.

  Furthermore, since the refractive index n1 of the base material G4b is as high as 1.883, even if a light-absorbing paint is applied to the outer peripheral surface as in the case of an optical element of a general photographing lens, the reflectance is The light beam incident on the object side and reflected by the chamfered portion T is easily reflected by the concave surface R2 and easily returns to the object side.

FIG. 3 is a cross-sectional view of the fourth lens G4 chamfered at a standard chamfer angle of 45 °. In this case, the incident angle θc when the light beam reflected by the chamfered portion T enters the concave surface R2 is as follows. Note that R = 14.42 and h = 0. This is to confirm whether or not total reflection does not occur in order to examine the conditions under which total reflection is most likely to occur, that is, the maximum value of the incident angle θc. The chamfered portion T is parallel to the optical axis from the object side. This is because the point on the concave surface R2 where the incident angle θc that changes depending on the height of the incident light beam takes the maximum value is a point where the concave surface R2 and the optical axis intersect.
θc = −90 ° + 2θ1 + θ2
= -90 ° + 2θ1 + arccos (h / R)
= −90 ° + 2 × 45 ° + arccos (0 / 14.42)
= 90 °

Here, the critical angle θm at which the light incident on the concave surface R2 is totally reflected is as follows.
θm = arcsin (n2 / n1)
= Arcsin (1.000 / 1.883)
= 32.1 °

  Therefore, since the incident angle θc is larger than the critical angle θm, it can be seen that total reflection occurs. Therefore, when the fourth lens G4 is chamfered at a standard chamfer angle of 45 °, a white ring-shaped two-surface reflection image is observed from the object side.

Therefore, the angle θ1 of the chamfered portion T is obtained using the following equations (1) and (2). Here, h = 0 is set for the same reason as before.
(1) n1> n2
(2) 0 <θ1 <{arcsin (n2 / n1) −arccos (h / R)} / 2 + π / 4
0 <θ1 <(32.1 ° -90 °) / 2 + 45 °
0 <θ1 <16.1 °

Here, the angle of the chamfered portion T is set to 15 ° in consideration of individual differences in products. At this time, the incident angle θc when the light beam reflected by the chamfered portion T enters the concave surface R2 is as follows.
θc = −90 ° + 2θ1 + θ2
θc = −90 ° + 2 × 15 ° + arccos (0 / 14.42)
= 30 °

  Therefore, since the incident angle θc is larger than the critical angle θm, total reflection does not occur. Therefore, when the angle of the chamfered portion T is 15 °, even if the lens is observed from the object side, a white ring-shaped two-surface reflection image is not observed from the object side, and the appearance is not good. Absent.

  Thus, by using the above-described equations (1) and (2), the angle θ1 of the chamfered portion T where no two-surface reflection image is generated can be obtained.

L1 1st lens group L2 2nd lens group L3 3rd lens group L4 4th lens group G4 4th lens S Aperture stop I Image surface TL Tangent HL Ray A Optical axis R1 Object side surface R2 Image side surface θ1 Chamfer Angle θ2 formed by T and optical axis Angle θc formed by tangent TL and optical axis Incident angle h of light beam HL reflected by chamfered portion T to concave surface R2 Position where light beam HL reflected by chamfered portion T is incident on concave surface R2 Ray height

Claims (1)

A method for designing an optical element constituting a photographing lens used in an imaging device,
A concave surface R2 on the image side;
An optical element having a step of setting the angle of the chamfered portion T so as to satisfy both equations using the following conditional expressions (1) and (2) when the chamfered portion T chamfers the end of the concave surface R2 Design method.
(1) n1> n2
(2) 0 <θ1 <{arcsin (n2 / n1) −arccos (h / R)} / 2 + π / 4
θ1: Angle formed by the chamfered portion T and the optical axis h: Ray height R at the position where the light beam HL reflected by the chamfered portion T is incident on the concave surface R2: Radius of curvature of the concave surface R2 n1: Refractive index n2 of the optical element: Optical element Refractive index of the medium filling the periphery of

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