JPH0248883B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an inverted Galilean optical finder having a short overall length and having an Albada optical system. In recent years, with the miniaturization of cameras, there has been a demand for not only a smaller master lens for photographing, but also a smaller finder optical system. It goes without saying that the central issue for downsizing the finder optical system is to shorten its overall length. The present invention aims to provide a finder with a short overall length that is comparable in performance to conventional finders. Specifically, it is aimed at a reverse Galilean optical finder with a finder magnification of 0.57 and a half-angle of view of 27 degrees, incorporating the Albata optical system. If an attempt is made to shorten the overall length of this type of finder, the refractive power of each lens must be strengthened in order to provide a predetermined finder magnification, which poses the problem of difficulty in correcting aberrations. In a normal design, when the refractive power of a lens becomes strong and it becomes difficult to correct aberrations, the most common method is to increase the number of lenses and disperse the refractive power among each lens. It is unsuitable for such a finder optical system. In other words, if it is a finder system that only uses the master lens to view the subject, it is possible to increase the number of lenses as described above, but if the finder system has a short overall length as in the present invention, it is possible to use the Albada optical system. When incorporating a half mirror, the number of lenses is increased and the distance between the half mirror and the frame surface, which is already narrow, becomes even shorter, which increases the angle of incidence of the light rays that enter the half mirror, causing the frame of the Albada optical system to become larger. Aberrations and field curvature become significant, making it impractical. The basic configuration of the finder of the present invention employs a type generally used in intermediate-level cameras, consisting of two negative lenses in the front group and one positive lens in the rear group. In this type, the parameters of the optical system are almost determined by the first-order refractive power arrangement to simultaneously establish the finer and Albada systems.
The degrees of freedom that can be used for aberration correction are extremely small.
In the present invention, an aspherical surface is introduced into the lens surface shape in order to ensure this degree of freedom. As shown in the data table below, in the embodiment of the present invention, the overall length of the finder is 17 mm, and in the known examples, it is 31 mm (USP No. 3575082) to 27.6 mm.
(Japanese Patent Application Laid-open No. 50-27539) It is set extremely short compared to the previous one. Regarding aberration correction, the viewfinder system must be corrected over the entire angle of view, but the aberration system only needs to be corrected at the angle of view of the frame, and it is not necessary to correct the aberrations at the entire angle of view. No correction is necessary. Moreover, since the distortion of the frame can be canceled by the shape of the frame, it may be considered as an object of aberration correction. Specifically, the finder optical system of the present invention having the above-mentioned basic configuration includes a negative first lens from the object side and a negative meniscus lens having a strong curvature from the pupil side, as shown in FIG. This is an inverted Galilean optical finder consisting of a second lens and a positive third lens, with the pupil placed behind them.A frame frame is provided on the object side of the third lens to clearly indicate the field of view of the master lens. A so-called Albada optical system is constructed in which the pupil side of the two lenses is a semi-transparent concave mirror, and the pupil side surfaces of each of the first, second, and third lenses are aspherical. Aspherical shape However, c is the paraxial curvature, k is the quadratic surface coefficient, A ₄ , A ₆ , and A ₈ are the higher-order aspherical coefficients, respectively, and y is expressed as the height from the optical axis, and the fourth-order coefficient of y is expressed as the second-order coefficient. Considering the contribution from the surface coefficients, it is expressed as A^ ₄ =k/8c ³ +A ₄ . At this time, for the aspherical surface of the rear surface of the first lens: 1.0×10 ^-5 <A^ ₄ <6.0×10 ^-5 …(1) For the aspherical surface of the rear surface of the second lens (half mirror portion): −3.5×10 ^{− 5} ＜A^ ₄ ＜−1.0×10 ^-5 …(2) The aspheric surface of the rear surface of the third lens satisfies the following condition: 2.0×10 ⁻⁵ ＜A^ ₄ ＜4.5×10 ⁻⁵ …(3). Condition (1) is for the purpose of correcting the field curvature of the finder system by setting the aspherical shape of the rear surface of the first lens so that the off-axis curvature is stronger than the paraxial curvature. This is different from that in which the off-axis curvature is made weaker than the paraxial curvature in order to correct distortion aberration (for example, Japanese Patent Publication No. 41056/1983).
If the aspherical coefficient becomes small beyond the lower limit, the image plane will be tilted significantly in the direction of the positive diopter, and if it becomes large beyond the upper limit, the distortion will become too pincushion-shaped. Condition (2) relates to the rear surface of the second lens, and sets the aspherical shape so that off-axis curvature is weaker than the curvature near the paraxial region.
This surface has an influence on aberration correction for both the Finder system and the Albada system. If the upper limit is exceeded, the occurrence of Albada system coma aberration cannot be suppressed, and if the lower limit is exceeded, the asphericity becomes large. ,
In the Albada system, the image plane tilts greatly toward the negative diopter, and at the same time, in the finder system, the image plane tilts greatly toward the positive diopter, resulting in performance deterioration. Condition (3) relates to the rear surface of the third lens, and the aspherical shape is set so that the curvature is weaker off-axis than near the paraxial region. This surface also affects aberration correction in both the Finder system and the Albada system. Condition (3) is a necessary condition to suppress the occurrence of Albada coma aberration.
When this lower limit is exceeded, the occurrence of external comatic aberration in the Albada system becomes significant, and conversely, when the upper limit is exceeded and the asphericity increases, the image plane collapse in the positive diopter direction becomes large for both the finder system and the Albada system. , the visibility of the viewfinder becomes poor. In addition to the above three conditions, it is desirable to satisfy the following conditions. Regarding the aspherical surface of the rear surface of the first lens, 2.0×10 ^-7 <A^ ₆ <1.5×10 ^-6 ...(4) Here, A^ ₆ is the coefficient of the y ⁶ term in the equation expressing the aspherical surface, and 2 Considering the contribution from the coefficient k of the dimensional surface, it is expressed as A^ ₆ = 1/16 [(1+k) ² -1]c ⁵ +A ₆ . This condition has the purpose of assisting condition (1) and performing image plane correction for the finder system. The influence of this condition is particularly noticeable when the angle of view is large.
The lower limit is the limit for producing this effect, and the upper limit is the limit for preventing barrel distortion from increasing. Examples of the present invention will be described below, and FIGS. 2 to 9 separately show aberration diagrams of the Finder system and Albada system of Examples 1 to 3. However, the aberration diagrams are based on a pupil diameter of 4 mm. Example 1

【table】 However, the seventh side is the eyes.

[Table] f = -570 f _B = -990 Diopter -1.01 Dayopter #2 A^ ₄ = 3.2023 x 10 ^-5 A^ ₆ = 7.1669 x 10 ^-7 #4 A^ ₄ = -2.39128 x 10 ^-5 # 6 A^ ₄ =2.7575×10 ^-5 Example 2

【table】

[Front] However, the 7th side is the pupil. f = -570 f _B = -983 Diopter = -1.02 Dayopter #2 A^ ₄ = 4.305 x 10 ^-5 A^ ₆ = 1.0502 x 10 ^-6 #4 A^ ₄ = -1.7640 x 10 ^-5 #6 A ^ ₄ = 2.7812×10 ^-5 Example 3

【table】 However, the 7th side is the eyes.

[Table] f = -571 f _B = -922 Diopter = -1.08 Dayopter #2 A^ ₄ = 1.819 x 10 ^-5 A^ ₆ = 2.9495 x 10 ^-7 #4 A^ ₄ = -2.166 x 10 ^-5 #6 A^ ₄ =3.392× ^10-5 Example 4

【table】 However, the seventh side is the eyes

[Table] f = -499 f _B = -999 Diopter = -1.0 Dayopter #2 A^ ₄ = 1.465 x 10 ^-5 A^ ₆ = 4.471 x 10 ^-7 #4 A^ ₄ = -3.154 x 10 ^-5 #6 A^ ₄ =2.395× ^10-5

[Brief explanation of drawings]

FIG. 1 is a sectional view of Example 1. Figure 2 is a diagram of Finder system aberrations of Example 1, Figure 3 is a diagram of Albada system aberrations of Example 1, Figure 4 is a diagram of Finder system aberrations of Example 2, and Figure 5 is a diagram of Albada system aberrations of Example 2. 6 is a diagram of Finder system aberration of Example 3, FIG. 7 is a diagram of Albada system aberration of Example 3,
FIG. 8 is a view of finder system aberration of Example 4, and FIG.
The figure is an Albada system aberration diagram of Example 4.

Claims (1)

[Claims] 1. A reverse Galilean optical system finder consisting of a negative first lens from the object side, a negative meniscus second lens with a strong curvature on the pupil side, and a positive third lens, with the pupil located behind. In a finder configured with an Albada system in which a frame frame indicating the field of view is provided on the object side of the third lens and the pupil measurement surface of the second lens is a semi-transparent concave mirror, the pupil side of the first, second and third lenses is The surface is aspherical, and the aspherical shape is However, c is the paraxial curvature, K is the quadratic surface coefficient, A ₄ ,
Assuming that A ₆ and A ₈ are higher-order aspheric coefficients, and y is the height from the optical axis, and A^ ₄ = k/8c ³ + A ₄ , then 1.0 for the aspheric surface on the rear surface of the first lens. ×10 ^-5 <A^ ₄ <6.0×10 ^-5 Regarding the aspherical surface of the rear surface of the second lens -3.5×10 ^-5 <A^ ₄ <−1.0×10 ^-5 Regarding the aspherical surface of the rear surface of the third lens 2.0×10 ^-5 <A^ ₄ <4.5×10 ^-5 A reverse Galilean type Albadaf finder that satisfies -5 <A^ 4 <4.5×10 -5.