JPH0218511A - Zoom lens - Google Patents

Abstract

PURPOSE:To obtain the small-sized, lightweight, and low-cost lens which has an about two zoom ratio by composing the lens system of five lens elements in two groups, using at least three plastic lenses, and providing one aspherical surface to each group. CONSTITUTION:The lens system consists of a 1st group which is composed of a 1st lens with a positive convex surface on an object side, a 2nd lens with a negative concave surface on the object side, and a 3rd biconcave lens and has positive refracting power on the whole and a 2nd group which is composed of a 4th meniscus lens with a positive concave surface on the object side and a 5th negative lens with a concave surface on the object side and has negative refracting power on the whole. Then zooming is carried out by varying the gap between the 1st and 2nd groups, at least three lenses are made of plastic, and at least one aspherical surface is arranged properly in each group to obtain the lens system which is superior in power variation ratio and telephoto ratio at the wide-angle end has various excellent aberrations.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a compact, lightweight, and low-cost zoom lens suitable for cameras, particularly compact cameras using silver halide film.

[Conventional technology] In recent years, the use of zoom lenses in silver-halide compact cameras has progressed, and it is expected that cameras using zoom lenses will become mainstream in the future due to the advantage of increasing the variety of shooting options. Ru.

An example of a compact zoom lens having a zoom ratio of approximately 2 times is disclosed in Japanese Patent Application Laid-Open No. 62-264019. This zoom lens consists of a first group with positive refractive power and a second group with negative refractive power in order from the object side, and magnification is changed by changing the distance between the two groups. It is a compact lens system. However, the number of lens components is as large as 8, which is high cost, and the overall length is not sufficiently short as the telephoto ratio at the wide-angle end is about 1.3.

Therefore, in current compact cameras with zoom lenses, the zoom lens occupies a large proportion of the main body in terms of size, weight, and cost.
Light weight and cost reduction are extremely important issues.

Making lenses from plastic is an extremely effective means of reducing the weight and cost of lens systems. However, the types of plastics that can currently be used for optical purposes are extremely limited, and those that can be used have a low refractive index of about 15 to 1.6.

Therefore, it is not easy to use plastic lenses in lens design. In other words, if a plastic lens is simply replaced with a conventional lens system, the performance will deteriorate significantly.

For example, as a compact zoom lens with a small number of lenses, there is a lens system described in Japanese Patent Application Laid-Open No. 57-201213. This conventional zoom lens consists of a positive first group and a negative second group in order from the object side, and the first group has three lenses, positive, negative, and positive, and the second group has two lenses, positive and negative. It is a lens system with a small number of lenses, consisting of a total of five lenses. This conventional lens configuration is very advantageous for downsizing the lens system. However, this conventional zoom lens has a variable power ratio of 1.
5 and small. Furthermore, the refractive index of each lens is around 1.7, and as mentioned above, it is not possible to replace the plastic lenses with low refractive indexes as they are.

With the same lens configuration as the conventional example above, if you try to set the zoom ratio to 2 and the telephoto ratio at the wide-angle end to about 1.2, the powers of the first and second groups will become extremely large, resulting in spherical aberration and Distortion aberration, astigmatism, and coma aberration all become large, and imaging performance deteriorates significantly.

[Problems to be Solved by the Invention] The present invention is a zoom lens for a compact camera with a zoom ratio of about 2, which simultaneously achieves small size, light weight, and low cost.
The object of the present invention is to provide a material having good optical performance.

[Means for Solving the Problems] In order to achieve the above-mentioned object, the zoom lens of the present invention includes a first positive lens having a convex surface facing the object side in order from the object side.
The first group has a positive refractive power as a whole, which is composed of a second lens, which is a negative lens with a concave surface facing the object side, and a third lens, which is a biconvex lens, and a positive meniscus lens with a concave surface facing the object side. A second lens with negative refractive power as a whole, consisting of a fourth lens and a fifth lens, which is a negative lens with a concave surface facing the object side.
This is a lens system that performs zooming by changing the distance between the first group and the second group. The lens is characterized in that at least three of the five lenses mentioned above are plastic lenses, and each group is provided with at least one aspherical surface.

The zoom lens of the present invention has a configuration of five lenses in total, including three positive, negative, and positive lenses in the first group and two positive and negative lenses in the second group, as in the conventional example described above, and has at least three plastic lenses. Despite using a lens, by appropriately arranging at least one aspherical surface in each of the first and second groups, the lens is superior to conventional models in terms of variable power ratio and telephoto ratio at the wide-angle end. Furthermore, a lens system with good aberrations can be achieved.

Among these aspherical surfaces, the aspherical surface used in the first group is effective mainly for correcting spherical aberration, and in particular, when used on a surface close to the aperture, only spherical aberration can be controlled. The shape of this aspherical surface must be such that it corrects spherical aberration, that is, the positive refractive index weakens as it moves away from the optical axis.

Further, the aspheric surface used in the second group is effective mainly for correcting off-axis aberrations such as astigmatism and distortion. The shape of the aspheric surface used here needs to be such that it corrects negative distortion, that is, the negative refractive power becomes weaker as it moves away from the optical axis.

It is desirable that these aspheric surfaces satisfy the following conditions (1) and (2).

fl) 5X 10-'<(ΣΔx+l/fw<
1 x 1O-2(, Y = Va, 7) (2) 5X 10-3<(ΣΔxi)/h<l
x 10-' (y2NEC) However, Ei 1 is the amount of deviation of the aspherical surface used for the first group from the reference spherical surface, Δx■ is the amount of deviation from the reference spherical surface of the aspherical surface used for the second group, and fw is the wide-angle end. The focal length of the entire lens system is
h is the maximum image height, y is the height from the optical axis, 'IS, 7 is the marginal ray height of F/3.7 at the wide-angle end on the aspheric surface of the first group, yl:. is the height of the chief ray on the maximum side of the aspherical surface of the second group at the wide-angle end. Further, regarding the sign of 8Xl and 8g, for Ei 1, the direction in which the positive refractive power is weakened is taken as positive, and for 8X[, the direction in which the negative refractive power is weakened is taken as positive.

If the lower limit of the above condition (1) is exceeded, the spherical aberration will be under-corrected, and if the upper limit is exceeded, the spherical aberration will be over-corrected.

If the lower limit of condition (2) is exceeded, negative distortion will be excessive on the wide-angle side and astigmatism will worsen; if the upper limit is exceeded, negative distortion will be overcorrected on the wide-angle side, and astigmatism and coma will be worsened. Aberrations worsen.

Next, the zoom lens of the present invention uses many plastic lenses. Currently, plastic materials that can be used for optical purposes such as plastic lenses include those with low refractive index and low dispersion (refractive index of about 15, Atsbe's number 60) represented by acryl, and medium-sized materials such as polycarbonate. Refractive index, high dispersion (refractive index approximately 1.6. Atsbe number 30)
Two things can be mentioned.

When applying a plastic lens to the zoom lens of the present invention, in consideration of correction of chromatic aberration, it is suitable for the first lens, third lens, fourth lens, and fifth lens to have low refractive index and low dispersion; A medium refractive index and high dispersion lens is suitable for the second lens.

If a lens with high dispersion is used in addition to the second lens, chromatic aberration will be excessive, and if a lens with low dispersion is used as the second lens, chromatic aberration will be insufficiently corrected.

From the above, it is desirable to satisfy the following condition (3).

(3) ν2ku45 However, ν2 is the Abbe number of the second lens.

If this condition (3) is not met, chromatic aberration will be insufficiently corrected.

As described above, when plastic lenses are used in the lens system of the present invention, each positive lens of the first lens, third lens, and fourth lens has a low refractive index and low dispersion. In this case, since the positive lens has a low refractive index, the Petzval sum tends to be a large positive value, making it difficult to maintain good image plane properties. This is difficult to correct even if an aspherical surface is used.

In order to correct the above Petzval sum and maintain image plane properties, it is necessary to use a material with a low refractive index for the fifth lens, which is a negative lens. Therefore, it is preferable that the refractive index n of the fifth lens satisfies the following condition (4).

(4) n5<1.6 If the range of condition (4) is exceeded and plastic lenses are frequently used, the Petzval sum becomes a large positive value and image plane properties deteriorate.

In the lens system of the present invention, in order to better correct various aberrations, it is desirable to satisfy the following conditions (5) and +61.

+5] -0,3< fl, /f4<mouth (6)
6 <fw/r'+ O < 3 However, f4.
fs is the focal length of the fourth lens and the fifth lens, r'+
o is the radius of curvature of the object-side surface of the fifth lens.

Condition (5) defines the ratio of the focal lengths of the fourth lens and the fifth lens constituting the second group.

In order to obtain a zoom lens with a zoom ratio of about 2 and a telephoto ratio of about 1.2 at the wide-angle end, which is the object of the present invention, the second group needs to have a large negative power.

If this power is calculated using paraxial theory, it will be around -1/fw, and this power will be shared between the fourth and fifth lenses. Here, as the positive power of the fourth lens becomes stronger, the negative power of the fifth lens becomes stronger, and higher-order aberrations are excessively generated. Conversely, when the positive power of the fourth lens becomes weak, the ability to correct the aberrations generated by the fifth lens is lost. Therefore, the power distribution between the fourth lens and the fifth lens is important, and preferably satisfies condition (5).

When the lower limit of condition (5) is exceeded, astigmatism in particular worsens, and both the fourth and fifth lenses tend to become decentered.
If the upper limit is exceeded, astigmatism and coma aberration worsen, both of which are undesirable.

Condition (6) is a condition that defines the radius of curvature of the object-side surface of the fifth lens.

In correcting aberrations, the zoom lens of the present invention has the following features:
The problem is how well to balance off-axis aberrations on the wide-angle side and spherical aberrations and comatic aberrations on the telephoto side, and condition (6) is provided to balance this.

Exceeding the lower limit of condition (6) worsens astigmatism, especially at the wide-angle end, and exceeding the upper limit worsens spherical aberration and coma aberration at the telephoto end, which is undesirable.

In the zoom lens of the present invention, all five lenses can be made of plastic by having the configuration as described above. If all five lenses were made of plastic, it would be extremely low cost and lightweight, but on the other hand, it would be easily affected by temperature and humidity. In other words, changes in temperature and humidity change the refractive index and shape of the lens, resulting in a shift in focus. This is not desirable in a compact camera that focuses by driving the lens to a preset point because it will not be able to focus. However, this is not fatal because it can be corrected in combination with an autofocus mechanism as shown in, for example, Japanese Unexamined Patent Publication No. 62-111223.

In the lens system of the present invention, when using 1.2 glass lenses, it is desirable to satisfy the following conditions (71, (8)).

(7) n+<1.75 (8) ν1>45 where nl and ν are the refractive index and Abbe number of the first lens, respectively.

If the range of condition (7) is exceeded, the Petzval sum will take on a large negative value, which is undesirable because the image surface properties will deteriorate. Moreover, if the range of condition (8) is exceeded, chromatic aberration becomes excessive, which is not preferable.

In the lens system of the present invention, when 1.2 out of the 5 lenses are replaced with glass lenses, the effects of temperature and humidity can be greatly reduced by appropriately selecting the lenses. However, if three or more lenses are made of glass lenses, the cost will be lower.
The benefits of weight reduction are reduced.

As described above, the zoom lens of the present invention is based on changing the distance between the first group and the second group to change the magnification. However, aberrations can be corrected even better by minutely changing the lens spacing in each group during zooming. This is because the degree of freedom in correcting aberrations increases, and in particular, astigmatism and the like can be corrected well.

Focusing in the zoom lens of the present invention is normally carried out by extending the entire barrel 1, but it can also be carried out by moving the second group.

[Example] Next, embodiments of the zoom lens of the present invention will be shown.

Example 1 f = 36.05-67.9, F/3.64-F75
．． 2 Maximum image height 21.6, wide-angle end telephoto ratio 1.28
r = 20.9531 d, "3.0000 n," 1.49216
v, =57.50r2==69.4838 d2= 2.13000 r3=: -21,4411 da= 4.0129 112= 1.58362
v2= 30.37r4=145°2381 d4= 3.0820 r5=21.2739 ds" 4.2000 ns= 1.49216
ν3 = 57.50r6: -18,008
7 (Aspherical surface) d6 = 1.0000 r7 = (Capital) (Aperture) d7 = D, (Variable) r8 = 30.5685 (Aspherical surface) d8 = 2.6
000 n4”1.49216 v4=57.
50r9=-22,8150 d925.2513 r+o=-9,7569 d,. =2.0000 n5”1.49216
v, =57.5Or,, =-72,16
31 Aspheric coefficient (6th surface) P = 0.8842, A2 = 0, A4 = 0.6746
8 xlO-'Aa= -0,51333x 10-8
, A, = 0.19994 x 1O-8 (8th surface) P = 1.9017, A2 = 0, A, = 0.6659
4 x 10-'x 10-6 A, = Q, [1
4631x 10-650 67.9 6.112 2.200 1, OXl0-3 1.3 Xl0-2 141, fw/r+.

A, = 0.20332 f 36.05 [], 11.854 (ΣΔxI)71w (ΣΔXnl/h f5/f4= -Q Example 2 f = 36.05 to 67.9, maximum image height 21.6 ++ =21.1119 d, = 3.0000 Q, = 1.65844
r2= 39.3452 d2= 2.0000 17.7024 da”3.3041 r4= 211.2865 d4=D, (variable) r5= 21.9567 d5= 3.4000 1”6= 15.1G14 (Aspheric ) n3=1.
49216 3.69 F/3.64 to F76, 9 Wide-angle end telephoto ratio 1.22 1.58362 ν, =50.86 shi2 =30.37 57.50 d6= 1.000 (1 r7=oo(aperture ) d, = 02 (variable) 26.6032 (aspherical surface) d, = 2.6000 114 = 1.49216
v, = 57.5Or, = 20.771
4 d, = 4.9909 rho = -9,5334 d,, = 2.0000 n5 = 1.49216
v5 =57.5Or,, =-42,7519 Aspheric coefficient (6th surface) P=D, 4066 A2=0, A4=0.7564
6xlO-'A, = -0,17438x 10-6,
A, = 0.26160 x 1O-8 (8th surface) P = 1.4686, A2 = O, A4 = 0.85086
x 10-'Ag=-0,36793x 1(1-
7, Aa = 0.916B2 x 1O-8f 3
6.05 50 67.9D, 1.862
2.504 2.2228212.960 6.
143 2.000r8: (Σ△x+)71w = 1.7 X 1O-3(
ΣΔxn)/h= 1.3 x 1O-2f5/f4
= -0,152, fw/rho = -3,78 Example 3 f = 36.05-67.9, F/3.64-F75
．． 2 Maximum image height 21.6, wide-angle end telephoto ratio 1.
2°r+ = 15.1724 d+ = 3. [) ODD nl = 1.49
216 v1= 57.50r2= 66,775
2 d, = 1.7(1(1(1 r3= 27.2601 d-”2.943L ・n2=1.71736
ν2=29.51r< = 68.5097 d4= 3.4436 rs=27.8714 ds" 3.4000 Q3= 1.51742
1) 2 = 52.411”6 = 48.1090
(Aspherical surface) d6 = 1.0000 r7 = ■ (Aperture) dy = D + (Variable) r, = -30,13,86 da” 3.0000 n4 = 1.49216
ν4 = 57.50r9= -17,580
5 (aspherical surface) d9=D2 (variable) rho:=-9,2526 d,. ”2.0000 n5=1.49216
v5=57.5Ora+=189.5
LO3 Aspheric coefficient (6th surface) P = -3,2018, A2=O A4= -0,14123x 10-'A6=O,1B
344 x l[l-6,A, =[1,21423x
1O-B (9th surface) P = 2.2519, A, =O, A, =-[1,67
556x 10-'A6= 0.67061 x 10
-6, A, = -0,20258x 10-'f
36.05 50 67.9D+ 10.87
1 5.372 2.200D, 4°803
4.457 3.900(ΣΔXI)71w =3.
Q Xl[+-4(Σ 8xn)/h=+1. s
X 10-2 Q f5/L= -0,250, iw/rho =
3.90 Example 4 f=36.05-67.9, F/3.64-F/7.
0 Maximum image height 21.6, wide-angle end telephoto ratio 1.2
3r, :19.2603 d, = 3.0[l[)C1ro, = 1.49216
ν, =57.50r2= 53.0583 d2=D1 (variable) r3= 19.0163 d3= 4.0043 n2:l: 1.5836
2 1/2 = 30.37r4 = 169.808
2 d4=D, (variable) r5= 22.1427 d5= 3.4000 fi3= 1.49216
v3= 57.50i”6= 16.1589
(Aspherical surface) d6 = 1.0000 r, = oo (aperture) d, = 03 (variable) r, = 33.3918 (Aspherical surface) da = 2.6
000 n,=1.49216 v4=57.
50ra = 22.8566 d, = 04 (variable) rho = 9.7414 d,, = 2.0OOO rz = -80,5917 Aspheric coefficient (6th surface) P = 0.5168, A6 = -0,25576 (8th side) P = 0.8762, A6. = -0,9640[1 (11th surface) P = 11.2777 A, -0,12558 A6 = -0,35885 f 36.05 D, 2.096 D21.906 D312.472 ns = 1.49216 C5 = 57.50 (aspherical surface) A2 = 0, A, = 0.72823 xlO-
'X 10-6, A, = 0.40774 x 1
O-8A2=G, A4=0.71911x
10-'l/X 10-6, Ae= (1,83133X 1O-
11A, = 0.18755 x 10-967.9 1.851 2.629 2.200 A2 = 0, 1O-6 x 10-7, 1.700 2.916 6.209 IL 4.900 4.811 4 ．．
946 (ΣΔx+l/fw = 1.5 x 1O
-3(Σ 8 x n)/b= 1.4 x 10-
"f5/f, = -0,167, f'w/r+o =
3.70 In the above data, r+, r2.
... is the radius of curvature of each lens surface, dl+d2. ... is the wall thickness and air spacing of each lens, n + + n 21
... is the refractive index of each lens, ν1, ν2. ... is the Atsube number of each lens.

Example 1 has a lens configuration shown in FIG. 1, in which only the distance between the first group and the second group is changed during zooming. All lenses are made of plastic, with only the second lens made of polycarbonate and all others made of acrylic. The aspherical surfaces are used for the image-side surface of the third lens and the object-side surface of the fourth lens.

The polycarbonate used for the second lens has stronger anomalous dispersion than ordinary glass (Helperger's equation, which calculates the refractive index of each wavelength from the d-line refractive index and Atsube's number, does not hold. For reference, the refractive indices of the four main wavelengths are shown below.

na=1.58362 nc =1, 57809 nr"'1.59731 n,=1.60888 Next, the influence of temperature and humidity when using a plastic lens will be described.

Plastic lenses change their refractive index and shape due to changes in temperature and humidity, which causes them to become out of focus. This defocus is caused more by a change in refractive index than by a change in shape. However, the resulting focus shift can be sufficiently corrected by an appropriate combination of a positive lens and a negative lens.

The refractive index of plastic decreases by approximately -10 due to temperature changes.
-'/°C change. For example, when the temperature changes by ±30°C, the refractive index changes by approximately 0003°. In the lens system of Example 1, the amount of focus shift when the temperature changes by ±30'C and the refractive index changes by 0.003 is ±0.
．． 3mm (wide-angle end) ~ ±0.8mm (telephoto end)
It is. This value is a value that poses no practical problem even when the influence of humidity is included.

The aberration conditions at the wide-angle end, intermediate focal length, and telephoto end for an object at infinity in Example 1 are shown in FIG. 5, respectively.

As shown in FIGS. 6 and 7. Furthermore, the aberrations at the wide-angle end, intermediate focal length, and telephoto end when the second group is moved to focus on an object 2 m from the front of the lens are shown in Figures 8, 9, and 10, respectively. That's right.

Embodiment 2 is a lens system having the lens configuration shown in FIG.
The distance between the second lens and the third lens in the group is also slightly changed. In this embodiment, only the first lens is a glass lens, and all the other lenses are plastic lenses. The aspherical surfaces are used for the image-side surface of the third lens and the object-side surface of the fourth lens.

If the influence of temperature and humidity in this example is calculated in the same way as in Example 1, the amount of focus movement will be ±0.2 mm (wide-angle end) to ±0.
4mm (telephoto end). This is about half of Example 1,
This is an effect achieved by using a glass lens as the first lens.

The aberration conditions at the wide-angle end, intermediate focal length, and telephoto end for an object at infinity in this embodiment are as shown in FIGS. 11, 12, and 13, respectively.

Embodiment 3 is a lens system having the lens configuration shown in FIG. 3, in which the distance between the first and second groups is changed and the distance between the fourth and fifth lenses is slightly changed during zooming. . In this embodiment, the second lens and the third lens are glass lenses, and the remaining lenses are plastic lenses. Further, aspherical surfaces are used for the image-side surface of the third lens and the image-side surface of the fourth lens.

Calculating the effects of temperature and humidity in this example using the same method, the amount of focus movement is from ±0.2 mm (wide-angle end) to ±0.4 mm.
mm (telephoto end), which is about the same as in Example 2, and the amount of focus movement is small. This is also due to the use of glass lenses.

Further, in this example, the total length of the lens system is very short, and is 43.26 mm at the wide-angle end, which is 1.20 when converted into a telephoto ratio.

The aberration conditions at the wide-angle end, intermediate focal length, and telephoto end for an object at infinity in this embodiment are as shown in FIGS. 14, 15, and 16, respectively.

Embodiment 4 is a lens system having the lens configuration shown in FIG. 4, in which the distance between the first group and the second group is changed during zooming, and all the gaps between the other lenses are also changed by minute amounts. It is. In this embodiment, all lenses are plastic lenses. The aspherical surface is the image-side surface of the third lens, the fourth lens
It is used for the object side surface of the lens and the image side surface of the fifth lens.

The influence of temperature and humidity in this example is similar to that in Example 1.

The aberration conditions at the wide-angle end, intermediate focal length, and telephoto end for an object at infinity in this embodiment are as shown in FIGS. 17, 18, and 19, respectively.

The shape of the aspheric surface used in each of the above embodiments is expressed by the following equation, where the origin is the intersection with the optical axis, the X axis is in the optical axis direction, and the y axis is in the direction perpendicular to the optical axis.

be.

[Effects of the Invention] The zoom lens of the present invention has a zoom ratio of about 2, the number of lenses is extremely small at 5, and it is a small, lightweight, and low-cost lens system that uses many plastic lenses, and has extremely good optical performance. It is.

[Brief explanation of the drawing]

1 to 4 are cross-sectional views of Examples 1 to 4 of the present invention, FIGS. 5 to 10 are aberration curve diagrams of Example 1, and FIGS. 11 to 13 are cross-sectional views of Example 2. 14 to 16 are aberration curve diagrams of the third embodiment, and FIGS. 17 to 19 are aberration curve diagrams of the fourth embodiment. Applicant Olympus Optical Industry Co., Ltd.

Claims (1)

[Claims] The first lens is composed of, in order from the object side, a first lens which is a positive lens with a convex surface facing the object side, a second lens which is a negative lens with a concave surface facing the object side, and a third lens which is a biconvex lens and has a positive refractive power as a whole. a fourth lens group, which is a positive meniscus lens with a concave surface facing the object side, and a fifth lens group, which is a negative lens with a concave surface facing the object side, and has a negative refractive power as a whole; A zoom lens system that performs zooming by changing the distance between a group and a second group, characterized in that at least three lenses are made of plastic, and each group has at least one aspherical surface.