JPS6381312A - Rear converter lens - Google Patents

Abstract

PURPOSE:To correct aberration such as curvature of image and correct variation of aberration when focusing by moving the whole rear converter lens by constituting the rear converter lens of a first lens group of positive refracting power and a second lens group of negative refracting power, and satisfying a specific conditional expression. CONSTITUTION:A rear converter lens R.C has two lens groups, i.e. the first lens group of positive refracting power and the second lens group of negative refracting power from the object side in order, and at least one lens surface of the first lens group has aspheric shape in which positive refracting power becomes stronger from the center toward peripheral part, and conditions of the inequality I are satisfied when back focus and F number of the main lens system L at the time of infinity state are Bf, F respectively, magnification when the rear converter lens R.C is mounted to the main lens system L is alpha, and height of incidence of luminous flux on the axis to the aspheric face is h. Thus, a rear converter lens of simple constitution for which variation of spherical aberration is corrected nicely can be obtained.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a rear converter lens, and in particular to a rear converter lens, which is detachably attached to the image field side of a main lens system to expand the focal length of the entire system and provide a focusing function. The present invention relates to a rear converter lens that is compact and suitable for high-performance photographic cameras, video cameras, and the like.

(Prior Art) Various rear converter lenses have been proposed that are attached to the image field side of a main lens system and expand the focal length of the entire system while holding the focal plane of the entire system at a constant position.

A rear converter lens has the advantage that the entire lens system can be made smaller and lighter than a front converter lens, which is attached to the main lens system. Further, when focusing is carried out by moving the rear converter lens along the optical axis, it has the advantage that it can be carried out quickly with simple operations because it is small and lightweight. For example, JP-A-58-1 uses a rear converter lens for focusing.
This method has been proposed in JP-A No. 84914, Japanese Patent Laid-Open No. 60-214327, and the like. This publication discloses an I-most shadow system in which focusing is achieved by moving all or part of the rear converter lens.

In general, when a rear converter lens is attached to the main lens system, the off-axis light beam does not cross the optical axis and pass through the rear converter lens system, so it is difficult to maintain aberrations of the entire system when it is attached to the main lens system. becomes very difficult. If you focus by moving the rear converter lens without sufficiently correcting the aberrations in the secondary lens, there will be the above-mentioned advantages, but on the other hand, aberrations such as spherical aberration will fluctuate a lot due to changes in object distance, which will affect the image formation. This will greatly reduce performance.

(Problems to be Solved by the Invention) The present invention satisfactorily corrects aberrations when the main lens system is mounted on the image plane side to expand the focal length of the entire system, and also changes object distance when focusing. The object of the present invention is to provide a rear converter lens having a simple configuration that satisfactorily corrects aberration fluctuations caused by spherical aberrations, especially spherical aberration fluctuations.

(Means for solving the problem) A rear converter lens is attached to the image plane side of the main lens system to expand the focal length of the entire system and has a focusing function, and the rear converter lens is attached to the object side. It has two lens groups, a first lens group having a positive refractive power and a second lens group having a negative refractive power, in order, and at least one lens surface of the first lens group has a shape extending from the center to the periphery. It has an aspherical surface with a shape that increases positive refractive power, and the back focus and F number when the main lens system is at infinity are Bf and F, respectively, and the rear converter lens is attached to the main lens system. When the magnification is α and the height of incidence of the axial light beam on the aspherical surface is □h, then 1, l〈α〈1.7 ・・・・・・・・・・・・・・・・・・
・・・・・・・・・(1) 0.7<12・Fh/Bfl<
1.0...(2) must be satisfied.

(Example) FIGS. 1 and 2 are lens sectional views when the rear converter lens of Numerical Example 1.2 according to the present invention is attached to the main lens system, respectively. In the figure, the main lens system is shown, and R and C are rear converter lenses.

In this embodiment, the rear converter lenses R and C are constructed with negative refractive power as a whole, and are attached to the main lens system to increase the focal length of the entire system. Focusing is achieved by moving the entire rear converter lens along the optical axis.

Generally, the main lens system performs photographing by itself, so aberrations are well corrected by the main lens system alone. Therefore, in order to obtain good overall optical performance when a rear converter lens is attached, it is necessary to achieve good aberration correction with the rear converter lens alone.

However, when a rear converter lens with a negative refractive power is attached to the image plane side of the main lens system and the focal length is changed to a long side, spherical aberration tends to be overcorrected in many cases, and the Petzval sum increases in the negative direction. However, the field curvature becomes large.

Furthermore, when focusing by moving the rear converter lens, the incident height of the axial light beam that enters the rear converter lens from the main lens system changes as the rear converter lens moves, resulting in a large amount of aberration variation, especially spherical aberration variation. It's coming.

Next, the relationship between the refractive power of the rear converter lens and the amount of movement during focusing will be explained.

The refractive power of the main lens system is Φ2, and the refractive power of the rear converter lens is Φ. , the distance between the principal points of the main lens system and the rear converter lens when they are focused on an object at infinity is e, and the magnification is α, and if α>■ and e<1/Φ2 are selected. In this case, if β is the combined photographic magnification of the main lens system and rear converter lens when focusing on an arbitrary short-distance object, then the possible range of the movement amount ΔC of the rear converter lens from infinity and the photographic magnification β is (0) In equation (A), when the magnification factor α and the principal point spacing e (<-) Φ- are increased, the negative refractive power of the rear converter lens as a whole becomes stronger. Furthermore, if the negative refractive power of the rear converter lens as a whole is weakened in formula (b), the amount of movement ΔC increases, and the possible range of photographic magnification becomes narrower from formula (8).

Generally speaking, in order to increase the effect of an aspherical surface on spherical aberration, the height of incidence of the axial light beam is relatively high and the height of incidence of the off-axis light beam is relatively low. It is advantageous to apply an aspherical surface to the lens surface, which occurs often.
The effect on off-axis aberrations is also relatively small.

Therefore, in this example, the rear converter lens is composed of two lens groups, the first lens group with positive refractive power and the second lens group with negative refractive power, so as to satisfy conditional expressions (1) and (2). By forming an aspheric surface of the above-described shape on at least one lens surface of the first lens group, preferably on the lens surface closest to the image plane, aberrations such as spherical aberration and field curvature can be corrected when attached to the main lens system. Also, aberration fluctuations when focusing by moving the entire rear converter lens are well corrected.

Conditional expression (1) relates to the magnification when the rear converter lens is attached. If the magnification becomes too large beyond the upper limit, the negative refractive power of the rear converter lens system as a whole becomes too strong, and aberration fluctuations, mainly spherical aberration fluctuations, increase.To correct this, it is necessary to use an aspherical shape. This is not a good idea because it requires a strong beam, which worsens off-axis aberrations, especially astigmatism. If the lower limit is exceeded, the amount of movement of the rear converter lens during focusing increases, the total length of the lens increases, and it becomes difficult to maintain a good balance between spherical aberration and off-axis aberration.

Conditional expression (2) is intended to fully exhibit the effect of the aspherical surface and to satisfactorily correct aberration fluctuations during focusing. If the lower limit is exceeded, the incident height of the axial light beam incident on the aspherical surface will be lowered, making it difficult to fully utilize the aspherical effect, and if the upper limit is exceeded, the rear converter lens will mechanically interfere with the main lens. It ends up.

In this example, when the aspherical shape is R, which is the paraxial radius of curvature of the aspherical surface, the following condition is satisfied between R-B>O (3) with the aspherical coefficient B, which will be described later. It is best to set it so that

Conditional expression (3) is a condition for this embodiment, and is a condition for making the positive refractive power stronger as it goes from the center to the periphery, and it reduces the amount of axial light flux incident on the aspherical surface as the object distance changes. This is to reduce the occurrence of off-axis aberrations when the incident height changes and to better correct fluctuations in spherical aberration. Conditional expression (
If 3) is exceeded, spherical aberration will be overcorrected when a rear converter lens is attached, which is not a good idea.

Next, numerical examples of a rear converter lens of the present invention and a main lens system to which the rear converter lens is attached will be shown. In the numerical example, R1゜ri is i-th in order from the object side.
The radius of curvature of the th lens surface, Di, and di are the thickness and air spacing of the ith lens, Ni, ni, and vi, in order from the object side.
, ri are the refractive index and Abbe number of the glass of the i-th lens in order from the object side.

For an aspherical surface, the displacement of the surface in the optical axis direction at a height H from the optical axis is expressed as
R is the paraxial radius of curvature, A, B, C.

When D, E, A', B', C', D', and E' are each aspherical coefficients, +EHIO+A'H3+B'H'+C'H'+D'H9
It is expressed by the formula.

Numerical example: Main lens system ΦM=0.02 FNo-1,82ω■46.8'
R1-37,554D I-3,10N 1-1.
80610 Vl-40,7R2-142,589D
2-0.29R3-20,991D 3-7.66
N 2-1.60342 V2-38. OR4-m
D 4-1.46 N :]-]7
．． 75520 V3-27.5R5-14,665D
5-5.50R6-Aperture D6・7.38 R7--14,33607-1,07N 4-1.72
825 V4-28.3R8-436,258D 8-
4.85 N 5-1.80610 V5-40.7
89--18.860 09-0.10 old 0- 274
．． 101 010-2.91 N 6-1.8
0610 V6-40.7R11--44,781 Rear converter lens numerical example 1 r! -133,505d 11.50 n 1-1
．． 88300 Si1-40.8r 2-22.860
d 2=5.0On 2=1.59270 C2-
35.3 "3- Aspherical surface d 3-1.95 r 4- -30.045 d 4-1.50
n 3=1.80610 shi3-40.9r
5 = 291.225 d 5 fable 0. IO
r 6- : ] 7.753 d 6-3.
00 n 4-1.54814 -v4=45.
8r 7- 66.238 Rear converter lens numerical example 2 r 1= 181.709 d l-3,86n 1
-1.59551 ν1=39.2r2- Aspherical surface d
2-1.37 r 3 = -33,277d 3-1.43 n 2
-1-81554 v2-44.3r 4-46.7
00 d 4=1.59r 5- 43.611
d 5-3.99 n 3-1.54814 υ3-
45.8r 6=-652,198 Numerical Example 1 Numerical Example 2 (Effects of the Invention) According to the present invention, by setting the lens configuration of the rear converter lens as described above, aberrations when attached to the main lens system are reduced. It is possible to achieve a rear converter lens having high optical performance with little variation and excellently correcting aberration variation when focusing by moving the rear converter lens in a wide range from close range to infinity.

[Brief explanation of the drawing]

Figures 1 and 2 are lens sectional views when the rear converter lens of Numerical Example 1.2 of the present invention is attached to the main lens system, and Figures 3 (A) and 3 (B) are Figure 1, respectively. Figures 4 (A) and (B) are aberration diagrams when the object distance is infinity and 0.7 m in the example shown in Figure 2, respectively. It is. In the figure, the main lens system, R and C are rear converter lenses, y is the image height, S is the sagittal image plane, and M is the meridional image plane. Patent Applicant Canon Co., Ltd. No. 1 Kuchi 2 Kuchi No. 3 M (A) Usagi 4 Figure (A) Ryo 4 Figure (B)

Claims (1)

[Claims] A rear converter lens that is attached to the image plane side of the main lens system, expands the focal length of the entire system, and has a focusing function, the rear converter lens having a positive polarity in order from the object side. It has two lens groups, a first lens group with a refractive power and a second lens group with a negative refractive power, and at least one lens surface of the first lens group has a positive refractive power as it goes from the center to the periphery. It has an aspherical surface with a shape that increases When α is the incident height of the axial light beam on the aspherical surface and h is the incident height of the axial light beam on the aspherical surface, the following conditions are satisfied: 1.1<α<1.7 0.7<|2・F・h/Bf|<1.0 A rear converter lens characterized by: