Claims
- 1. An optical system comprising, in the order from the object side:
- at least a first lens unit having a positive refractive power; and
- a second lens unit having a negative refractive power with an airspace between said first and second lens units, said second lens unit and said first lens unit disposed next to one another so that no other lens components are disposed between the first and second lens units;
- wherein said optical system varies focal length thereof by varying the airspace reserved between the first and the second lens unit; and
- wherein said first lens unit includes at least one graded refractive index lens element satisfying the following conditions (1) and (4):
- n(r)=n.sub.0 +.SIGMA.n.sub.1 r.sub.1.sup.21 ( 1)
- .PHI..sub.S .multidot..PHI..sub.M <0 (4)
- wherein the reference symbol n.sub.0 represents the refractive index of said graded refractive index lens element as measured on an optical axis thereof, the reference symbol r designates distance as measured in a radial direction from the optical axis, the reference symbol n(r) denotes the refractive index of said graded refractive index lens element as measured at a distance r from the optical axis, the reference symbols n.sub.1, n.sub.2, n.sub.3, . . . represent refractive index distribution coefficients of the second, fourth, sixth, . . . orders respectively, the reference symbol .PHI..sub.S designates the refractive power of a surface of said graded refractive index lens element, and the reference symbol .PHI..sub.M denotes the refractive power of a medium of said graded refractive index lens element and is expressed as .PHI..sub.M =-2n.sub.1 d.sub.G, where d.sub.G is the thickness of said graded refractive lens element as measured on the optical axis.
- 2. A vari-focal lens system having an optical system according to claim 1, wherein said vari-focal lens system further comprises, disposed after said second lens unit:
- a third lens unit having a positive refractive power; and
- a fourth lens unit having a positive refractive power;
- wherein said vari-focal lens system varies focal lengths thereof by moving said second lens unit and said third lens unit along the optical axis.
- 3. A vari-focal lens system according to claim 1 wherein said first lens unit is composed only of a single lens element.
- 4. A vari-focal lens system according to claim 3, wherein said graded refractive index lens element has a shape thicker at the portion thereof on the optical axis than at the marginal portion thereof.
- 5. A vari-focal lens system according to claim 1 satisfying the following conditions (5) and (6):
- 5.times.10.sup.-2 <d.sub.G /f.sub.W ( 5)
- 0<n.sub.1 .multidot.f.sub.W.sup.2 <5 (6)
- wherein the reference symbol d.sub.G represents thickness of said graded refractive index lens element as measured on the optical axis, the reference symbol f.sub.W designates the shortest focal length of said vari-focal lens system.
- 6. An optical system according to claim 1, further satisfying the following condition (3):
- .nu..sub.1d <.nu..sub.0d ( 3)
- wherein the reference symbol .nu..sub.0d represents an Abbe's number determined from refractive indices for the d-line, F-line and C-line as measured on the optical axis and the reference symbol .nu..sub.1d represents a value determined by the following formula when the coefficients of the second order for the d-line, F-line and C-line are represented by n.sub.1d, n.sub.1F and n.sub.1c respectively:
- .nu..sub.1d =n.sub.1d /(n.sub.1F -n.sub.1C).
- 7. An optical system according to claim 1, wherein said graded refractive index lens element has a positive refractive power of surface, a negative refractive power of medium and a positive refractive power as a whole.
Priority Claims (1)
Number |
Date |
Country |
Kind |
1-105940 |
Apr 1989 |
JPX |
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Parent Case Info
This is a continuation of application Ser. No. 07/514,546, filed on Apr. 26, 1990, now abandoned.
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Continuations (1)
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Number |
Date |
Country |
Parent |
514546 |
Apr 1990 |
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