JPH03139606A - Real image type finder - Google Patents

Abstract

PURPOSE:To obtain the real image type finder which is usable even with high power and to preclude aberration deterioration due to a ghost resulting from reflection on a large-curvature surface and a position shift by using resin for at least one lens constituting a cemented lens. CONSTITUTION:This finder has a 1st positive lens group I which is constituted by cementing two negative and positive resin lenses together, a 2nd negative lens group 2 which consists of a negative and a positive lens, a 3rd positive lens group III which is constituted by cementing two positive and negative resin lenses, a 4th negative meniscus lens group IV, a Porro prism V which reflects an image, and a positive ocular VI. Thus, a pair of lenses 1 and 2 are used as one cemented lens and at least one of the lenses 1 and 2 is made of resin so that a large-curvature surface is their cemented surface. Consequently, the reflection on this surface is suppressed and the real image type finder with a small chromatic aberration is obtained by using the inexpensive resin.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an improvement in the finder of a camera such as a lens shutter camera or a video camera in which a photographing optical system and a finder optical system are configured separately. .

[Prior Art] In this type of camera, a reverse Galilean finder that visually recognizes a subject using a virtual image has been widely used.

The finders used in these cameras are required to be more compact as camera bodies become smaller. Also,
More and more cameras are using zoom lenses with high variable magnification in their photographic systems, and in these cases, the finder system is also required to have a zoom finder with high variable magnification and less variation in aberrations due to zooming.

Therefore, in recent years, there has been a trend toward the use of real-image type finders, which have a brighter field of view, higher magnification, and can clearly recognize the field frame than reverse Galilean type finders.

For example, as a conventional real image finder,
-156018 publication or JP-A-64-6551
There is one described in Publication No. 9.

[Problems to be Solved by the Invention] However, the real image finder described in the above publication does not sufficiently correct chromatic aberration, and as the magnification is increased, coloring becomes noticeable. I can't.

When correcting chromatic aberration using a glass lens, there is a method using a glass material with a high refractive index and low dispersion, or a method using a glass material with a low refractive index,
One possible method is to use an achromatic lens that is a combination of a positive lens made of a low-dispersion inexpensive glass material and a negative lens made of a high-dispersion glass material.

However, in the former case, the cost will be high, and in the latter case, the power of the positive lens will be strong, so the radius of curvature of the lens surface will be small, causing total internal reflection on the rear side, and causing damage to the lens frame. The disadvantage is that the sensitivity of aberrations to assembly errors is very high.

[Object of the invention] This invention was made in view of the above problems, and
At low cost, even if the power of the positive lens is increased, total reflection does not occur on the rear surface of the positive lens, and chromatic aberration can be sufficiently corrected without increasing the sensitivity of aberrations to errors when assembled to the lens frame. The purpose is to provide a real image finder.

[Means for Solving the Problems] In order to achieve the above object, a real image finder according to claim 1 of the present invention uses at least one set of laminated lenses, and at least one lens constituting the laminated lenses. Claim 2 is characterized in that the lens material of is bound with resin, among the lenses constituting the laminated lens,
Claim 3 is characterized in that at least one lens is made of a material with high ultraviolet transmittance.Claim 4 is characterized in that lenses with low UV cut are arranged before and after the laminated lens. A fifth aspect of the present invention is characterized in that at least one of the lenses constituting the laminated lens is provided with a receiving portion for receiving the adhesive that protrudes from the lens. A sixth aspect of the present invention is characterized in that it has a fixing structure that suppresses longitudinal force on the bonded lens in the optical axis direction.

[Function] According to the configuration of claim 1, by using the surface with a strong curvature as the bonding surface, reflection on this surface is suppressed, and a real image finder with small chromatic aberration can be realized while using an inexpensive resin material. can be provided.

Note that thermosetting adhesives cannot be used on resin lenses because they deteriorate and cause discoloration when heated. Therefore, an ultraviolet curing adhesive is used, and the optical resin material contains an ultraviolet cut agent to prevent deterioration over time caused by ultraviolet rays during boat fishing.

For this reason, if both lenses to be laminated are made of resin, if an ultraviolet curing adhesive is used for lamination, the adhesive may not be completely cured and the lenses may peel off. .

According to the second aspect of the invention, the adhesive layer can be irradiated with ultraviolet rays from at least the side of the lens with high ultraviolet transmittance, and it can be cured even if an ultraviolet curable adhesive is used.

According to the structure of claim 3, since ultraviolet rays do not reach the bonded lens when assembled as a finder, deterioration over time does not occur even if a resin lens containing no ultraviolet blocking agent is used.

According to the structure of claim 4, even if the adhesive protrudes from the periphery of the lens when the lenses are bonded together, the receiving part receives the adhesive, thereby preventing the protruding adhesive from interfering with assembling the lens to the frame. It never happens.

According to the configuration of claim 5, the optical axes of the lenses can be easily aligned with each other by the positioning structure.

According to the configuration of claim 6, the lenses that are positioned and bonded together by the fixing structure can be held as they are without using a special jig.

[Example] The present invention will be explained below based on the drawings.

(First Embodiment) FIGS. 1 to 3 show a first embodiment of a real image finder according to the present invention.

As shown in FIG. 1, the finder according to this embodiment includes a positive objective optical system composed of lenses 1 to 6, and a positive eyepiece optical system composed of a porroprism 8 and a lens 9. There is. In FIG. 1, the Porro prism 8 for inverting an image is shown as an expanded glass block.

The objective optical system consists of a positive lens group consisting of a resin lens 1.2 bonded together, a negative second lens group consisting of a negative lens 3 and a positive lens 4, and a resin lens 5.6 bonded together. It has a positive third lens group consisting of a positive third lens group, and forms an image 7 just in front of the Porro prism 8 of the eyepiece optical system.

This finder is a zoom finder that corrects changes in diopter as well as changes magnification by moving the second and third lens groups in the direction of the arrows.

In the case of a real-image finder, the objective optical system forms the light beam from the subject, and then the eyepiece optical system converts it into an afocal image. Therefore, in order to maintain a high finder magnification and shorten the overall length of the finder, It is necessary to shorten the focal lengths of both the objective optical system and the eyepiece optical system.

Here, if the focal length of the eyepiece optical system is fe (mm), the axial aberration of the objective optical system is 6 (mm), and the axial aberration of the entire finder is △ (dpt, ), then the relationship Δ=1000 x δ/fe2 Therefore, if the focal length of the eyepiece optical system is shortened, axial aberrations in the entire finder system will increase.

Furthermore, in the case of a zoom finder, variations in chromatic aberration and spherical aberration occur due to changes in the magnification of the moving group.

In particular, since the zoom finder shown in Example 1 has a large magnification ratio of 2.6 times, fluctuations in various aberrations also tend to increase.

In the first embodiment, the chromatic aberration and spherical aberration are suppressed by using a positive and negative lens group with strong positive power, and the magnification is changed by using a positive and negative lens group as a movable third lens group. This suppresses fluctuations in various aberrations due to

The specific numerical structure of this lens is shown in Table 1, and its aberrations are as shown in FIGS. 2 and 3.

FIG. 2 shows aberrations at the wide-angle end (short focal length), and FIG. 3 shows aberrations at the telephoto end (long focal length).

In the table, r represents the radius of curvature of the lens, d represents the lens thickness and air gap, n represents the refractive index of the lens, and ν represents the Abbe number of the lens.

In addition, the first surface, third surface, fifth surface, tenth surface,
The 15th surface is an aspherical surface. For an aspherical surface, the distance from the tangent plane of the aspherical vertex of the coordinate point on the aspherical surface whose height from the optical axis is Y is x1 The curvature (1/r) of the spherical apex is 01 The conic coefficient is 4 The aspherical coefficients of the 8th to 8th orders are expressed as A4 to A8. These coefficients are shown in Table 2. Note that the radius of curvature of the aspherical surface in Table 1 is the radius of curvature of the apex of the aspherical surface.

In addition, in FIGS. 2 and 3, spherical aberration SA is shown by a solid line, and sine condition SC is shown by a broken line, and axial chromatic aberration,
For lateral chromatic aberration, d-LINE (588nm), g-
LINE (436nm), C-LINE (656nm)
) are shown. Astigmatism is shown in the sagittal direction S by a solid line and in the meridional direction M by a broken line.

In the case of the first embodiment, P is applied to the positive lens of the laminated lens.
Both MKA and negative lenses use polycarbonate and resin. Therefore, it is easy to obtain any lens shape, and if a method such as injection molding is used, it can be mass-produced at a much lower cost than glass.

r 18.773 12.852 18.695 -11.352 3.096 5.574 12.017 9.880 5.000 6.878 12.373 -12.000 10.285 20.308 17.710 1st Table 1.50 4.30 0.80-5.23 1.50 1.55 2.05 8.47-0.50 1.50 2.33 0.50-4.04 7.50 12, 72 27 , 30 0.20 2.55 1.49186 1.58547 1.58547 1.49186 1.49186 1.49186 1.49186 1.49186 1.49186 29.9 57.4 57.4 57.4 29.9 57.4 57.4 57.4 57.4 -0,84198400 on the first page of Table 2 +01 A4-0.000
00000E+0OAs” 0.00000000E+
0OAe-0,00000000E+OO On the third side=-0,99918000E+OOA4= 0.0O
OOOOOOOE+0OAe-0,79509400E-
08 Aa= 0.00000000E+OO 5th side=-0,89834000E+OOA4= O,0O
OOOOOOOE+0OAe=-0,48704300E
-04Ae= O,0OOOOOOOOE+OO on the 10th side -0,14451500E+OI A4= 0
．． 00000000E+0OAs=0.1475190
0E-04 As-0,0OOOOOO00E+OO On the 15th side=-0,61570500E+OI A4=O,0
OOOO000E+0OAa=-0,26725800
E-05Aa=0.00000000E+OO In addition, both the positive and negative lenses that make up the bonded lens for aberration correction have large powers, and the radius of curvature of the bonded surface is particularly small, so the bonded lens can be separated. If the 11 12- lens is arranged with Therefore, high accuracy is required.

Note that the positive lens 2.6 of the bonded lens uses PHMA, which does not contain a UV cut agent and has high UV transmittance, so when bonding using a UV curable adhesive, the positive lens 2.6 By irradiating ultraviolet rays from the lens side, the adhesive can be reliably cured and bonded firmly.

Further, if ultraviolet rays reach these positive lenses 2.6 when assembled as a finder, discoloration etc. will occur due to deterioration over time. Therefore, the lens 1 on the subject side of these lenses is made of polycarbonate containing an ultraviolet cut agent, and the eyepiece optical system 8.9 on the photographer's side is made of PMMA containing an ultraviolet cut agent. As a result, after being assembled as a finder, ultraviolet rays are absorbed by the front and rear lenses and do not reach the positive lens 2.6, so that no discoloration or the like due to deterioration over time occurs.

(Second Embodiment) FIG. 4 shows a second embodiment of the real image finder according to the present invention.

The specific numerical structure of this lens is shown in Table 3, and its aberrations are as shown in FIGS. 5 and 6.

FIG. 5 shows aberrations at the wide-angle end, and FIG. 6 shows aberrations at the telephoto end.

Note that the first, second, fourth, ninth, and sixteenth surfaces in the lens are aspherical surfaces, and their coefficients are shown in Table 4.

(Margin below) 13- 14- r 13.529 10.447 4.941 6.738 9.100 7.241 4.080 -8,430 -8.710 11.190 0 0 10.200 0ri12. 402 42.685 Table 3 2.06 5.51~1,37 1.40 2.02 2.04 1.12~10.32 1.30 2.85 6.10~1.04 7.50 13 , 04 0.55 0.30 27, 70 0.20 2.50 1.49186 1.49186 49186 1.58547 1.49186 1.49186 1.51633 1.49186 1.49186 57.4 57.4 57. 4 29.9 57.4 57.4 64.1 57.4 57.4 On the first page of Table 4 = O,0OO00000E+OOA4=-0,8
2700000E-04As=-0,21900000
E-05Ae=0.42500000E-07 -0,61600000E+OOAa” 0.955 on the second side
00000 to 04As=-0,25070000E-
05As-0,43300000 to 07 4th side=-0,10250000E+OI A4=0.00
000000E+OOAθ= O,0OOOO0000E
+0OAe=0.0OOOOOOOOE+00 On the 9th side=-0,30920000g+01 Aa=O,0O
OOOOOOOE+0OAa= 0.39000000g
-05AII=O,0OOOO000KOIO on the 16th side=-0,62400000E+01 Aa= 0.2
6300000g-03Ae =-0,2770000
0E-05As-0,0OO00000E+O0 (Third Embodiment) FIG. 7 shows a third embodiment of the real image finder according to the present invention.

The specific numerical structure of this lens is shown in Table 5 = 15
-6- The aberrations are as shown in FIGS. 8 and 9. FIG. 5 shows aberrations at the wide-angle end, and FIG. 6 shows aberrations at the telephoto end.

Note that the first, third, tenth, and eleventh surfaces in the lens are aspherical surfaces, and the aspherical coefficients are as shown in Table 6.

Surface number r 1 -21.292 2 32.878 3 21.660 4 -7.789 5 -12.047 6 14.0?2 7 10.481 8 15.440 9 ω 10 12.765 11 78.585 Table 5 d n ν 1.80 1.49186 57.432, 51
~5.38 5.00 1.49186 57.42.00
1.80518 25.45.99~24.47 3.00 1.49186 57.422.70 42.00 1.49186 57.42.50 5.40 1.49186 57.4 On the first page of Table 6 =-0,34786192E+OI A4=O,0O
000000E+00As”-0,41446806g
-06As=0.44376580E-08 On the third side -0,17642562E+01 A4=-0,
42529237-104A6-0.30444362
g-06 As” 0.12533558g-07 1st O surface: 0.30562449E+OOA4= 0.798
80176E-04As” 0.10716974g-
06As=0.60035087E-08 On the 11th side=0.00000000E+OOA4” 0.234
61768E-03As” 0.44353360E-
06Ae" 0.19874932E-07 (Fourth Embodiment) FIG. 10 shows the fourth embodiment of the real image finder according to the present invention.
This shows an example.

The specific numerical structure of this lens is shown in Table 7, and its aberrations are as shown in FIGS. 11 and 12. Fig. 11 shows aberrations at the wide-angle end, and Figs. 1217-18- show aberrations at the telephoto end.

Note that the first, third, tenth, and eleventh surfaces in the lens are aspherical surfaces, and the aspherical coefficients are as shown in Table 8.

Surface number r 1 -30.997 2 37.168 3 31.229 4 -8.709 5 -13.646 6 23.094 7 -38.215 8 ■ 9 c.

10 12.785 11 78.585 Table 7 1.80 34, 27-3.41 5, 00 2.00 26, 43-49.93 3.50 5.98 42, 00 2.50 5.40 1 .49186 1.49188 1.78182 1.49186 1.49186 1.49186 57.4 57.4 26.6 57.4 57.4 57.4 (Hereafter the margin) Table 8, 1st page=-〇, 13583084E+02 A4-0.0O
O00000E+0OAe=-0,30601014E
-07Ae= 0.23170581E-09 on the third side; -0,17491622E+OI A+=-0,4
2876354E-04Aa-0,54144487g
-06 Aa=0.19052631E-07 On the 10th side=-0,42281364E-01Aa-0,340
93967E-04As=0.47647145E-0
7 Aa=0.27093303E-08 On the 11th side=0.00000000E+OOA4=0.152
06411E-03As Knee 0.64469250E
-07Ae=0.61256717E-08 (Modifications of laminated lenses) FIGS. 13 to 17 show modified examples of laminated lenses that can be applied to each of the embodiments described above.

When two bonded lenses are bonded together, the adhesive may protrude and the lens may not be placed in the correct position relative to the lens frame. 9- = 20- Therefore, in the case of glass lenses used for boat fishing, the work of melting off the adhesive that has protruded with a solvent after bonding is performed. However, in the case of resin lenses, the lens itself is attacked by the solvent, causing problems such as clouding, so it is not possible to remove the protruding adhesive by the same means as with glass lenses.

As shown in FIG. 13(A), by making the diameter of one lens 10 larger than the diameter of the other lens 20, it is possible to prevent the adhesive 30 from entering between the frame 40 and the lens 10.20. Although methods to try to prevent this are commonly used, if a large amount of adhesive protrudes, the method shown in Figure 13 (B
), it becomes an obstacle to positioning. Furthermore, in order to minimize the amount of adhesive that protrudes, the amount of adhesive must be accurately controlled, which impairs work efficiency.

Therefore, if a groove 11 is provided on the inner surface of the large diameter lens 10 as shown in FIG. 14(A), even if a large amount of adhesive 30 protrudes during bonding, the first
As shown in Figure 4 (B), the adhesive 30 that has protruded out is in the groove 11.
The lens 10.20 does not become an obstacle when the lens 10.20 is assembled into the frame 40.

Furthermore, when the lenses 10.20 shown in FIG. 15(A) are bonded together, optical axes of the bonded lenses 10.20 do not match as shown in FIG. 15(B). decreases significantly. Therefore, in the bonding work, the centering work of aligning the optical axes of the lenses is extremely important.

Conventionally, it has been common to use a jig such as a bell chuck to perform centering work, but this method is time-consuming and
In addition to impairing mass production, in the case of resin lenses, there is a problem that the lens surface is easily scratched by the jig.

FIG. 16 shows a means for solving these problems. That is, as shown in FIG. 16(A), a convex portion 12 is formed on one lens 10, and a concave portion 21 into which the convex portion 12 fits is formed on the other lens 20.

These uneven portions 12.21 are formed so that the optical axes of both lenses coincide when they are fitted, and function as a positioning structure.

21 - 22 By adopting such a structure, when bonding, the optical axis can be aligned by simply fitting the convex part 12 into the concave part 21 as shown in FIG. 16(B), without using any jig. can be glued together while accurately aligning.

A bonded lens is hardened after the above-mentioned centering work, but the optical axis may shift until the adhesive is completely hardened.

Therefore, as shown in FIG. 17(A), protrusions 13 and 22 are provided as a fixing structure on the peripheral edge of each lens 10 and 20, and these protrusions are attached using U-shaped stops 14. By fixing as shown in (B), it is possible to prevent the optical axis from shifting during curing.

Note that when the configuration shown in FIG. 17 is adopted, the positions between the lenses will not shift even if the adhesive is not completely cured. Therefore, for example, even when both lenses in the bonded lens contain an ultraviolet blocking agent, an ultraviolet curing adhesive can be used. Furthermore, the lenses can be fixed only by the fixing structure, and the space between the lenses can be filled with a fluid such as oil that does not have an adhesive effect.

3- [Effects] As explained above, the real image finder of the present invention has the following advantages by using a plastic laminated lens.
Chromatic aberration is sufficiently corrected and can be used even at high magnification.
In addition, it is possible to prevent significant aberration deterioration due to ghosts and positional deviations caused by reflections on surfaces with a steep curvature.

[Brief Description of the Drawings] Figures 1 to 3 show a first embodiment of a real-image finder according to the present invention. Figure 1 is a lens configuration diagram, and Figure 2 shows aberrations at the wide-angle end. 3 are aberration diagrams at the telephoto end. Figures 4 to 6 show a second embodiment of the real image finder according to the present invention, where Figure 4 is a lens configuration diagram, Figure 5 is an aberration diagram at the wide-angle end, and Figure 6 is a telephoto view. It is an aberration diagram at the edge. Figures 7 to 9 show a third embodiment of the real image finder according to the present invention, in which Figure 7 is a lens configuration diagram, Figure 8 is an aberration diagram at the wide-angle end, and Figure 9 is a telephoto view. It is an aberration diagram at the edge. 24- Figures 10 to 12 show a fourth embodiment of the real image finder according to the present invention, in which Figure 10 is a lens configuration diagram, Figure 11 is an aberration diagram at the wide-angle end, and Figure 12 is a diagram of the aberration at the wide-angle end. is an aberration diagram at the telephoto end. FIG. 13(A)(B) to FIG. 17(A)(B) show modified examples of the laminated lens. 1 to 6...Objective optical system 8.9...Eyepiece optical system 25-3rd = 9.9° w = 9.9' Chromatic aberration Opu. Lateral chromatic aberration 6 Figure 1: 418.3 w = 9.8° w = 9.8° w = 9.8° Chromatic aberration Lateral chromatic aberration Astigmatism Distortion 9th 1st knee 134013.2 w = 8 F +8 -8 - 0.5 0.5 Chromatic aberration -0,10,1 Lateral chromatic aberration Astigmatism Distortion 12 Figure 1: 43050884.0 w=8 Chromatic aberration Lateral chromatic aberration Astigmatism Distortion Figure 17 (A) 13 2 Figure 17 ( B) 013 2 0 Procedural amendment (voluntary) 1. Indication of the case 1999 Patent Application No. 278987 2. Name of the invention Real image finder 3. Person making the amendment Relationship with the case Patent applicant address Itabashi, Tokyo 2-36-9 Maeno-cho, Ward Name
(052) Asahi Optical Industry Co., Ltd. 4, Agent address: 1-14-3 Monzennakacho, Koto-ku, Tokyo Office
Planet 6th floor 5, Specification subject to amendment 6, Contents of amendment z d ≥ 1, (1) In the 7th line of page 4 of the specification, insert the following sentence in a new line after "There is a defect." do. "Also, considering the compactness of the camera, the shorter the overall length of the finder system, the better; however, a finder system with high magnification and a short overall length increases the power of both the objective and eyepiece groups, resulting in monochromatic aberration and spherical aberration. In particular, in the case of a zoom finder, if the overall length is short, the moving distance of the moving group will be shortened, so in order to increase the zoom ratio, the power of the moving group must be increased, resulting in chromatic aberration, (2) In line 10 of page 4 of the specification, the phrase ``is'' is replaced by ``is,'' which has a short overall length, a high magnification or zoom ratio, and has a high chromatic aberration. The purpose is to provide a viewfinder that can be sufficiently corrected, and further corrected. (3) Specification IJ4 page #! Line 13 - No. 14 on the same page
Chromatic aberration can be sufficiently corrected without changing the line.
Correct the statement with ``kotonai.'' (4) In the specification page 24, line 4 to line 5 of the same page, “
``Chromatic aberration...and...'' was replaced by ``It is possible to provide a finder with a short overall length and a high magnification or high variable magnification ratio with sufficiently corrected chromatic aberration.Moreover,
” he corrected. Written amendment to the above procedures (voluntary) June 14, 1990 1999 Patent Application No. 278987 2, Name of the invention Real image finder 3, Relationship with the case of the person making the amendment Patent applicant address Maeno, Itabashi-ku, Tokyo Town 2-36-9 Name (052) Asahi Kogaku Kogyo Co., Ltd. 4, Agent Address 1-14-3 Monzennakacho, Koto-ku, Tokyo Office
Planet 6th Floor 3 - (1) From page 7, line 13 to page 8, line 8 of the specification [The finder according to this embodiment is...a zoom finder. '' should be corrected as follows. ``As shown in Figure 1, the finder according to this embodiment consists of a positive lens group consisting of two negative and positive resin lenses bonded together, a negative lens and a positive lens. A negative second lens group ■, which is a negative lens group ■, a positive third lens group ■ consisting of two positive and negative resin lenses bonded together, a fourth lens group ■, which is a negative meniscus lens, to invert the image. The objective optical system composed of the first to fourth lens groups has a positive refractive power and forms an image just before the Porro prism V. The objective optical system composed of the Porro prism V and the eyepiece lens (2) has positive refractive power.In FIG. 1, the Porro prism (2) is shown as an expanded glass block. This finder consists of the second lens group ■ and the third lens group ■
This is a zoom finder that changes the magnification and corrects for changes in diopter by moving the and the arrows in the direction of the arrows. (2) r14 10.200 27.70 1.49 on page 15, line 16 of the specification
186 57.4J, r14 10.200 27.13 1.49
Corrected to 186 57.4J. (3) rA8-0.27093303E-08J on page 20, line 13 of the specification,
Correct as rAe=0.27083303E-08J. (4) Amend Figure 1 as attached. that's all

Claims (6)

[Claims] (1) A real image finder comprising at least one set of laminated lenses, the material of at least one lens constituting the laminated lenses being resin. (2) The real image finder according to claim 1, wherein at least one of the lenses constituting the laminated lens is made of a material with high ultraviolet transmittance. (3) The real image finder according to claim 1, characterized in that lenses with low ultraviolet transmittance are arranged before and after the laminated lens. (4) The real image finder according to claim 1, wherein at least one of the lenses constituting the bonded lens is provided with a receiving portion for receiving the adhesive that protrudes. (5) The real image finder according to claim 1, wherein the laminated lens has a positioning structure that aligns the optical axes of the lenses constituting the laminated lens. (6) The real image finder according to claim 1, wherein the laminated lens has a fixing structure that fixes each lens constituting the laminated lens from the front and back in the optical axis direction.