JPS58215617A - Telephoto lens - Google Patents

Abstract

PURPOSE:To obtain a compact lens with a small telephoto ratio, by providing the 1st- the 4th positive, positive, negative, and positive single lenses and specifying conditions. CONSTITUTION:The telephoto lens system consists of the 1st positive, the 2nd positive, the 3rd negative, and the 4th positive single lenses all having convex surfaces on an object side. Then, this meets requirements shown by inequalities I -IV, where (f), (f12), and (f123) are the total focal length of the whole system, the composite focal length of the 1st and the 2nd lenses, and the composite focal length of the 1st - the 3rd lenses, respectively, and (d4) and (d6) are the air intervals between the 2nd and the 3rd, and the 3rd and the 4th lenses, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a four-element Ernostar type telephoto photographic lens with an angle of view of 18° and F/3.5.

Conventional Ernostar-type telephoto photographic lenses have insufficient correction of chromatic aberration due to the small number of constituent lenses, and even with a compact construction, chromatic aberration cannot be sufficiently corrected. In particular, in a compact lens system with a telephoto ratio of less than 0.9, the C-line and G-line image points do not match due to axial chromatic aberration and lateral chromatic aberration, and the amount of deviation from the F-line is large.

The present invention is a compact lens system with a small number of lenses (4 elements) and a small telephoto ratio, which allows the C-line and G-line image positions to be sufficiently close together, reduces the F-line deviation, and has sufficient chromatic aberration. This provides a well-corrected Ernostar-type telephoto lens.

The telephoto lens of the present invention includes a positive single lens ML lens, a positive single lens second lens, and a negative single lens third lens.
It is a lens system consisting of four lenses: a lens and a fourth lens, which is a positive single lens, and all of the lenses are arranged with their convex surfaces facing toward the object. Further, this lens system satisfies conditions (1) to (4) shown below.

(], + 0.34 f < f, 2 < 0.
4Of(2) 1,7 f < f, '23 <
2.3 f (810,05f < d4 < 0
,09 f(4) 0,23 f < d4+
d6<0.3Of, where f is the focal length of the entire system, f1
2 is the ml lens, the composite focal length of the second lens, fI23
is the composite focal length of the ml lens, the second lens, and the third lens, d4 is the air distance between the second lens and the third lens, and d6
is the air spacing between the third and fourth lenses.

Among the above conditions, condition (]) and condition (2) are for making the lens system more compact. In order to make the lens system more compact, it is necessary to shorten the combined focal length f12 of the ml lens and the second lens.

If f12 exceeds the upper limit of condition (1), the refractive power of the ml lens and the second lens will be insufficient, making it impossible to reduce the telephoto ratio to 0.9 or less. On the other hand, if glass with a large Abbe's number is used for the first lens in order to properly correct chromatic aberration, the refractive index of existing glasses will be low. As a result, the curvature of the ml lens has to be strengthened, and as a result, spherical aberration becomes insufficiently corrected, and other aberrations also occur, which cannot be corrected completely by other lenses. For the above reasons, if f,2 is less than the lower limit of condition (1), it is desirable to make the lens system more compact, but spherical aberration will be insufficiently corrected, and chromatic aberrations will occur, which cannot be corrected with other lenses. It disappears.

Condition (2) is the combined focal length f1 of the first lens and the second lens.
2 within the scope of condition (1), and further,
This defines the combined focal length f3,3 of the second lens and the third lens. In other words, by setting f and 23 within the range of condition (2), the refractive power of the third lens is balanced with the refractive index of the second lens and the second lens, thereby reducing each aberration generated in the second lens and the second lens. This is a correction. If the upper limit of this condition is exceeded, the negative refractive power of the third lens will become stronger, and the curvature of the image-side surface of the third lens will inevitably become stronger. As a result, the spherical aberration of each color becomes over-corrected, and in particular, the g-line becomes over-corrected and cannot be corrected by other lenses. Furthermore, the power of the first lens, second lens, and third lens is insufficient, making it difficult to make the lens system compact. Further, if the lower limit of condition (2) is exceeded, the negative refractive power of the third lens becomes weak, and the third lens cannot correct the undercorrected spherical aberration caused by the first lens and the second lens. As mentioned above, if the negative refractive power of the third lens is weak, it is difficult to correct the color shift (the light that passes through the ML lens and the second lens is shifted depending on the wavelength) that occurs in the second lens and the second lens. This will worsen the chromatic aberration of the lens system.

As mentioned in AiJ, a color shift occurs in the first lens and the second lens, but it is necessary to correct the shift between the g-line and the C-line by letting the light enter the third lens, which is a concave lens, before this color shift becomes large. .

Therefore, under condition (1,), C2), it is necessary to make d4 small, and d, satisfies condition (3).
If the upper limit is exceeded, the color shift when the light enters the concave lens (third lens) becomes large, and since the height of the light is low, the correction of the color shift becomes insufficient and chromatic aberration worsens. Also, condition (3
), under conditions (1) and (2), the effect of the concave lens becomes too strong and the correction for the g-line is too strong and cannot be compensated for by the fourth lens iE.

Condition (4) is used to determine the placement position of the fourth lens, and is used to balance axial chromatic aberration and lateral chromatic aberration. d4. When t and ci6 become less than the lower limit of condition (4), the ray height of the marginal ray increases, and the refractive effect of the fourth lens on this marginal ray increases, but the ray height of the off-axis principal ray decreases, resulting in chromatic aberration of magnification. It becomes difficult to correct. If it is larger than the upper limit, the color shift of the g-line and c-line of the light rays incident on the fourth lens will become relatively large, and the diameter of the lens will become large, which is against the need for compactness.

As explained above, a lens system that satisfies conditions (1) to (4) with the above lens configuration is a compact telephoto photographic lens in which chromatic aberration is well corrected.

However, when further considering distortion, etc., it is desirable to satisfy the following conditions (5) to (7).

t5) 5f < r, < (1) (6)
(+, 8 f < f4 < 1, 1 f(7)
ν1. ν〉60. ν3. 〈30〉 is the radius of curvature of the object-side surface of the third lens, f is the focal length of the fourth lens, C2,1 C3. 4 is each lens (first lens, second lens, third lens).

4th lens).

The object-side surface of the third lens is where the marginal ray enters at a large angle with respect to the optical axis. radius of curvature r of this surface
, exceeds the lower limit of condition (5), the occurrence of higher-order spherical aberrations decreases, and it becomes impossible to correct the undercorrected spherical aberrations occurring in the first lens and the second lens. When the upper limit is exceeded and r5 becomes a negative value, this surface is no longer concentric with respect to the aperture, and the astigmatism difference becomes large. In addition, spherical aberration is also overcorrected.

Condition (6) is a condition that defines the refractive power of the lens placed after the diaphragm, and is a condition for correcting positive distortion that occurs before the diaphragm. Under this condition, the positive distortion cannot be corrected beyond the eye. Also, if the lower limit is exceeded, the condition (1
, ), (2), the g-line is undercorrected due to axial chromatic aberration, making it difficult to match the focal positions of the g-line and the C-line.

Conditions (If C1 and C2 are made smaller than 60 at 7th lens, the color shift caused by the second lens and the second lens will become larger,
It cannot be corrected with the third or fourth lens.

C3. If shi4 is made larger than 30, the color shift that occurs in the first lens and the second lens cannot be completely corrected by the third lens and the fourth lens. Therefore, deviating from either condition will worsen the chromatic aberration of the lens system.

Next, each example of the photographic lens of the present invention described above will be shown.

Example 1 f=1,0, F3,5, 2ω
-18°r, = (1,: 226 d, =0.0466 n, =1.48749
v, =70.15r,,---35,5607 d,, =0.0(+38 r3-=0.2683 d3=0.0409 n,, =1.56873
shi2=63.16r, =0.5984 d4 =0.0722 r5==7.1866 d5=0.0522 n3=1.72151
z = 29.24r6 = 0.1819 d6 = 0.1,849. .

r7 =0.6019 d7 =0.0229 n4 =1,84666
ν4 = 23.88r8 = 2.1347 f = 0.368 f, f, 23==
], 96] f2 f =0,983f, telephoto ratio T =0.88
8 Aperture position is 0.0561 behind surface r6 Example 2 f=1. O, F3,5, 2 ω = 18゜r
, =0.3172 d, =0.0466 n, =1.48749
ν, =70. +5r2 =~75,8846 d2=0.0023 r, =0,2746 d3=0.0409 n2=1.56873
v2=63.16r,,=0.6199 d,=0.0723 r,=2(in 8409 d,=0.0527 nMarch, 72151 1/3=
29.2/1r6-0.1842 d6=0.1812 r7-0.6314 d7=0.0229 n4=1.84666
ν4 = 23.88r8 = 2.8223 f4 = 0.956 f, telephoto ratio T = 0.89
The 2nd aperture position is 0.0561 behind surface r6, however,
, r2..., 4r8 is the radius of curvature of each lens surface, d,
, d2. ...ld7 is the wall thickness and air spacing of each lens, 11. , n2. +11. n4 is the refractive index of each lens, 1ν3. C2. C3. C4 is the Atsube number of each lens.

[Brief explanation of drawings]

FIG. 1 is a sectional view of the lens system of the present invention, and FIGS. 2 and 3 are aberration curve diagrams of Examples 1 and 2 of the invention, respectively. Applicant Olympus Optical Industry Co., Ltd. Agent Hiroshi Mukai −

Claims (1)

[Claims] (1) A positive single lens lens, a positive single lens second lens, a negative single lens third lens, and a positive single lens, all of which are arranged with their convex surfaces facing the object side. Ernostar type telephoto X, which is the fourth lens of
[/7 S. (],) 0.34 f < f, 2 <
0.40 f+2) 1,7f < f
<2.3f23 +a+ 0. (15f<d4<
0,09 f (410,23 f < d4+
d6<0.3Of, where f is the focal length of the entire system, 1 f. is the composite focal length of the first lens and the second lens, f
123 is the combined focal length of the first lens, the second lens, and the third lens, d4 is the air distance between the second lens and the third lens, and d6 is the air distance between the third lens and the fourth lens. (2) The Ernostar telephoto lens according to claim (1), which satisfies the following conditions (5 to 7): (5) 5f < r5 (■ t6) 0.8 f < f4 < 1. 1
f(7) C1. C2〉60 C3. C4〈30 However, r5 is the radius of curvature of the object side surface of the third lens, f4
is the focal length of the fourth lens, and ``11ν2.shi3.shi4'' are the Abbe numbers of each lens, respectively.