JPS6159306A - Lens with variable focal length - Google Patents

Abstract

PURPOSE:To improve the performance of relatively short distance photography by allowing a variable-focal-length lens which varies its focal length by varying the distance between a front group with negative refracting power and a rear group with positive refracting power to meet specific requirements. CONSTITUTION:This lens system with variable focal length consists of the front group G1 having negative refracting power and the rear group G2 having positive refracting power and varies its focal length by varying the gap between both groups; the front group consists of the 1st negative meniscus lens L1 having the convex surface in the front, the 2nd negative lens L2, and the 3rd positive lens L3 having a large-curvature surface in the front, and the rear group consists of the 4th positive lens L4, the 5th positive meniscus lens L5 having the convex surface in the front, the 6th negative lens L6, and the 7th positive lens L7. Then, the lens system meets the respective requirements shown by inequalities I-V, thereby improving the performance of the variable-focal-length lens for finite distance which has about 1/20-1/5 photographic power.

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a variable focal length lens whose focal length can be changed continuously, and in particular to a variable focal length lens for finite distances.

(Background of the Invention) Conventionally, various variable focal length lenses are known that are composed of two groups: a front group with negative refractive power and a rear group with positive refractive power. However, these were designed mainly for objects located far away, and it was difficult to maintain sufficient performance when used at imaging magnifications of about 1/20 to 115.

(Object of the invention) The object of the present invention is to set a photographing magnification of about 1/20 to 115,
It has good performance, with an effective F number of 3 on the short focal length side.
．． 5. To provide a variable focal length lens with a long focal length of about 5.2.

(Summary of the Invention) A variable focal length lens according to the present invention has a front group G with negative refractive power.
1 and a rear group G2 with positive refractive power, and the focal length is changed by changing the distance between the two groups.As shown in Fig. 1, the front group has a convex surface facing forward. It is composed of a negative meniscus lens 1.1, a negative second lens L2, and a positive third lens L with a surface with a stronger curvature facing toward the front, and the rear group is a positive fourth lens. 1.4. The fifth lens 1, 5 is a positive meniscus lens with its convex surface facing forward, and the sixth lens 1 is a negative lens.
J 2.7 < -1 < 5.5 (]) 1 f,
1 nI, n2 >1.6
(3) n It n s < 1.65
(4) 1.55 < n 7 < 1
．． 75 (5) However, r7 and r9 are respectively the front lens surface of the fourth lens L in the rear group and the fifth lens], the radius of curvature of the front lens surface of 5,
dl and d3 are the center thicknesses of the first lens 1.1 and the second lens in the front group, respectively, and d2 is the center thickness of the second lens l, 1.
and the second lenses 1 and 2, fl is the air distance between the front group G and the second lenses 1 and 2.
, represents the composite focal length of nI + n Z+n i+
n 5 + n 7 are the first, second, fourth, and
It represents the refractive index of the fifth and seventh lenses with respect to the d-line (λ-587.6 nm).

Each of the above conditions according to the present invention will be explained below.

The condition in equation (1) is to maintain good spherical aberration over the entire zoom range; if this lower limit is exceeded, the spherical aberration will largely change to the positive side as you go from short focal length to long focal length. Resulting in.

Moreover, if the upper limit of this condition is exceeded, the opposite tendency becomes significant and it becomes difficult to maintain good performance.

(2) If the upper limit of the second condition is exceeded, the entire lens system becomes long and large. Furthermore, if an attempt is made to correct aberrations in this state, the distance between the front group and the rear group becomes smaller on the long focal length side, so the refractive power of the front group must be strengthened, which is not preferable in terms of aberration correction. If the lower limit is exceeded, the refractive power of the air lens formed between the first lens and the second lens must be weakened in order to secure the amount of peripheral light, and the refractive power of the second lens and the second lens must be weakened. must be borne by the front lens surface r of the second lens and the rear surface r4 of the second lens, making it difficult to correct aberrations.

Both the conditions of equations (3) and (4) are conditions for correcting the Petzval sum favorably, and if these conditions are violated, the Petzval sum becomes excessive in the negative direction, making it difficult to correct it. To overcome this, 1. The ratio of the refractive power between the front group and the rear group must be changed, but this increases the range of variation in the distance between the front group and the rear group for zooming, and as a result, the lens This is not preferable because the overall system configuration becomes large.

When the upper limit of the condition of equation (5) is exceeded, it becomes difficult to correct the Petzval sum, as in the case where the conditions of equations (3) and (4) are exceeded. On the other hand, if the lower limit is exceeded, it becomes difficult to maintain good spherical aberration.

In the present invention as described above, furthermore, regarding the refractive index n3 of the third lens as a positive lens in the front group, n3<1.6
It is desirable that conditions 5 be satisfied.

According to this, it becomes possible to correct the Petzval sum more favorably. Further, when the distance between the second lens and the third lens is d4, it is also desirable to satisfy the following conditions: 0.4<-□-<2.0 d, +d2+c+3. If the lower limit of this condition is exceeded, it becomes difficult to correct spherical aberration on the long focal length side, and if the upper limit is exceeded, the entire lens system becomes undesirably large.

(Example) Examples according to the present invention will be described below.

In all the embodiments according to the present invention, the object-image distance is 1888.2.
It is a so-called zoom lens that can change the magnification at a constant value of mm, and as shown in the first zoom lens, it has three elements in the front group,
It consists of a total of seven lenses, four in the rear group.

The table below shows the specifications of the first to third embodiments. In each table,
fe is the effective F number, f is the composite focal length of the entire system, ω
represents the half angle of view, β is the photographing magnification, and d.

Bf represents the distance from the object surface to the forefront lens surface vertex, Bf represents the distance from the final lens surface vertex to the image surface, and the numbers at the left end of the table represent sequential numbers from the front.

1st example focal length f-100~]84.5 fe=3.6~5
．． 42ω = 22° ~ 30° f, = -170,69
Second embodiment Focal length f = 100 ~ 199.6 fe = 3.6 ~
5.22ω-22°~30° f, −-164,
368 Third Example Focal Length f-100~197.6 fe=3.6-5
．． 22ω-22°~30° f I=-16(1,
(157 Regarding the first to third examples above, various aberration diagrams when the object-image distance is 1888.2 mm are shown in Figures 2 to 4, respectively. (A) in each aberration diagram is a short (B) shows an intermediate focal length state, and (C) shows a long focal length state.

It is clear from each aberration diagram that all the examples have excellent imaging performance over the entire zoom range. In the above embodiments, the lens group with negative refractive power is arranged in the front, but it goes without saying that it is also possible to use the lens group with the front and back reversed. The photographing magnification will be changed to the reciprocal value of the photographing magnification β shown in the table above.

(Effect of the invention) As described above, according to the present invention, the imaging magnification is 1/20 to 11
A variable focal length lens for finite distances with good performance at about 5 times has been achieved. For example, if this lens is used in the reverse direction, it can be used as an excellent variable focal length lens for enlargement with a magnification of 5 to 20 times. It becomes possible to realize things.

[Brief explanation of the drawing]

FIG. 1 is a lens configuration diagram of a variable focal length lens according to the present invention, and FIGS. 2, 3, and 4 are various aberration diagrams of the first, second, and third embodiments of the present invention, respectively. In the aberration diagrams, (A) shows a short focal length state, (B) shows an intermediate focal length state, and (C) shows a long focal length state. [Explanation of symbols of main parts] 1.1... Negative meniscus lens lens 1.2...
・Negative second lens

Claims (1)

[Claims] A variable focal length lens that is composed of a front group with a negative refractive power and a rear group with a positive refractive power, and which converts the focal length by changing the distance between the two groups, with the front group being placed at the front side. The rear group is composed of a first negative meniscus lens with a convex surface facing toward the front, a second negative lens with a positive third lens having a surface with a stronger curvature facing toward the front, and a fourth lens with a positive lens as the rear group. A variable focal length lens comprising: a fifth lens that is a positive meniscus lens with a forward convex surface, a sixth lens that is a negative lens, and a seventh lens that is a positive lens, and which satisfies the following conditions. 2.7<r_7/r_9<5.5(1) 0.06<(d_1+d_2+d_3)/|f_1|<
0.17(2) n_1, n_2>1.6(3) n_4, n_5<1.65(4) 1.55<n_7<1.75(5) However, r_7 and r_9 are respectively in the rear group. The radii of curvature, d_1 and d_3, of the front lens surface of the fourth lens and the front lens surface of the fifth lens are respectively
The center thickness of the lens and the second lens, d_2 is the air distance between the first lens and the second lens, f_1 is the composite focal length of the front group, n_1, n_2, n_4, n_5, n
_7 represents the refractive index of the first, second, fourth, fifth, and seventh lenses with respect to the d-line (λ=587.6 nm), respectively.