Keplerian zoom finder optical system

A Keplerian zoom finder optical system comprising:

• an objective lens group having a positive refracting power on the whole and constructed of, in order from an object side, a first lens unit having a negative refracting power, a second lens unit having a positive refracting power, a third lens unit having a negative refracting power and a fourth lens unit having a positive refracting power;
• a regulating unit, disposed in the vicinity of a focal position of said objective lens group, which regulates a visual field;
• and
• an eyepiece unit having a positive refracting power,
• wherein said Keplerian zoom finder optical system is a finder for varying a magnification by moving at least said second lens unit along an optical axis and satisfies the following conditions:
• -<
• f.sub.3 /f.sub.w <-3
• 0.1<D.sub.4 /f.sub.e <0.3
• where
• f.sub.w :
• the synthetic focal length of said first and second lens units in a minimum magnification state,
• f.sub.3 :
• the focal length of said third lens unit,
• f.sub.e :
• the focal length of said eyepiece unit, and
• D.sub.4 :
• the distance on the optical axis from the eye-point-side surface of said fourth lens unit to said regulating unit.

A Keplerian zoom finder optical system comprising:

• an objective lens group having a positive refracting power on the whole and constructed of, in order from an object side, a first lens unit having a negative refracting power, a second lens unit having a positive refracting power, a third lens unit having a negative refracting power and a fourth lens unit having a positive refracting power;
• a reticle plate constructed of a parallel plane flat plate disposed in the vicinity of a focal position of said objective lens group;
• an eyepiece unit having a positive refracting power;
• a first mirror disposed in an air spacing between said third lens unit and said fourth lens unit;
• a prism having two reflecting surfaces and disposed between said reticle plate and said eyepiece unit or disposed in said eyepiece unit;
• and
• a second mirror disposed in an air spacing between said prism and said reticle plate,
• wherein said first mirror, said second mirror, and said two reflecting surfaces of said prism are arranged so that a finder image of said Keplerian zoom finder optical system is erected by said first and second mirrors in combination with the two reflecting surfaces, and said Keplerian zoom finder optical system is constructed to vary a magnification when at least said second lens unit is moved along an optical axis.

A Keplerian zoom finder optical system comprising:

• an objective lens group having a positive refracting power on the whole and constructed of, in order from an object side, a first lens unit having a negative refracting power, a second lens unit having a positive refracting power, a third lens unit having a negative refracting power and a fourth lens unit having a positive refracting power;
• a regulating unit, disposed in the vicinity of a focal position of said objective lens group, which regulates a visual field;
• and
• an eyepiece unit having a positive refracting power,
• wherein said Keplerian zoom finder optical system is a finder for varying a magnification by moving at least said second lens unit along an optical axis and satisfies the following condition:
• 1. 0<.beta..sub.34 <1.3
• where
• .beta..sub.34 :
• the synthetic magnification of said third and fourth lens units, and
• wherein at least said second lens unit is moved so that when varying the magnification from low to high, said finder reduces a distance between said first lens unit and said second lens unit and increases a distance between said second lens unit and said third lens unit, and wherein said fourth lens unit is fixed in varying the magnification.

A Keplerian zoom finder optical system comprising:

• an objective lens group having a positive refracting power on the whole, said lens group being constructed of, in order from an object side, a first lens unit having a negative refracting power, a second lens unit having a positive refracting power, a third lens unit having a negative refracting power and a fourth lens unit having a positive refracting power;
• a regulating unit, disposed in the vicinity of a focal position of said objective lens group, which regulates a visual field;
• and
• an eyepiece unit having a positive refracting power,
• wherein said Keplerian zoom finder optical system is a finder for varying a magnification by moving at least said second lens unit along an optical axis and by varying the distances between said second lens unit and lens units at opposite sides of said second lens unit, and at least a first surface of said second lens unit and a first surface of said fourth lens unit have aspherical surfaces each exhibiting a refracting power which becomes weaker as it approaches the periphery thereof,
• and wherein at least said second lens unit is moved so that when varying the magnification from low to high, said finder reduces a distance between said first lens unit and said second lens unit and increases a distance between said second lens unit and said third lens unit, and wherein said fourth lens unit is fixed in varying the magnification.

A Keplerian zoom finder optical system comprising:

• an objective lens group having a positive refracting power on the whole and constructed of, in order from an object side, a first lens unit of a biconcave negative lens having an aspherical surface on the object side, a second lens unit of a biconvex positive lens having an aspherical surface on the object side, a third lens unit having a negative refracting power and a fourth lens unit of a biconvex positive lens,
• a regulating unit, disposed in the vicinity of a focal position of said objective lens group, which regulates a visual field;
• and
• an eyepiece unit having a positive refracting power,
• wherein a magnification of said Keplerian zoom finder optical system varies when at least said second lens unit moves along an optical axis, and said finder satisfies the following conditions:
• -<
• r.sub.12 /r.sub.11 <-1.1
• -1.5<r.sub.13 /r.sub.14 <-1
• herein, the aspherical surface is generally expressed by:
• S(y)=(y.sup.2 /R)/{1+(1-k.times.y.sup.2 /R.sup.2).sup.1/2 }+C.sub.2 .times.y.sup.2 +C.sub.4 .times.y.sup.4 +C.sub.6 .times.y.sup.6 +C.sub.8 .times.y.sup.8 +C.sub.10 .times.y.sup.10 +
• where y is the height in a direction perpendicular to the optical axis, S(y) is the quantity of shift in the optical-axis direction at the height y, r is the fiducial radius of curvature, k is the conical coefficient, and C.sub.n is the nth-order aspherical surface coefficient,
• and, when the paraxial curvature radius r of this surface is defined such as:
• r=1/(2.times.C.sub.2 +1/R)
• the following is given:
• r.sub.11 :
• the paraxial curvature radius of an object-side surface of said first lens unit,
• r.sub.12 :
• the curvature radius of an eye-point-side surface of said first lens unit,
• r.sub.13 :
• the paraxial curvature radius of an object-side surface of said second lens unit, and
• r.sub.14 :
• the curvature radius of an eye-point-side surface of said second lens unit.

A Keplerian zoom finder optical system comprising:

• an objective lens group having a positive refracting power on the whole and constructed of, in order from an object side, a first lens unit having a negative refracting power, a second lens unit having a positive refracting power, a third lens unit having a negative refracting power and a fourth lens unit having a positive refracting power;
• a reticle disposed in the vicinity of a focal position of said objective lens group;
• and
• an eyepiece unit having a positive refracting power,
• wherein said Keplerian zoom finder optical system is a finder for varying a magnification by moving at least said second lens unit along an optical axis and satisfies the following condition:
• 0.1<D.sub.4 /f.sub.e <0.3
• where
• f.sub.e :
• the focal length of said eyepiece unit, and
• D.sub.4 :
• the distance on the optical axis from the eye-point side surface of said fourth lens unit to the object-side surface of said reticle;
• wherein at least said second lens unit is moved so that when varying the magnification from low to high, said finder reduces a distance between said first lens unit and said second lens unit and increases a distance between said second lens unit and said third lens unit, and wherein said fourth lens unit is fixed in varying the magnification.

A Keplerian zoom finder optical system comprising:

• an objective lens group having a positive refracting power on the whole and constructed of, in order from an object side, a first lens unit having a negative refracting power, a second lens unit having a positive refracting power, a third lens unit having a negative refracting power and a fourth lens unit having a positive refracting power;
• a field frame disposed in the vicinity of a focal position of said objective lens group;
• and
• an eyepiece unit having a positive refracting power,
• wherein said Keplerian zoom finder optical system is a finder for varying a magnification by moving at least said second lens unit along an optical axis and satisfies the following condition:
• 0.1<D.sub.4 /f.sub.e <0.3
• where
• f.sub.e :
• the focal length of said eyepiece unit, and
• D.sub.4 :
• the distance on the optical axis from the eye-point-side surface of said fourth lens unit to said field frame;
• wherein at least said second lens unit is moved so that when varying the magnification from low to high, said finder reduces a distance between said first lens unit and said second lens unit and increases a distance between said second lens unit and said third lens unit, and wherein said fourth lens unit is fixed in varying the magnification.

A Keplerian zoom finder optical system comprising:

• an objective lens group having a positive refracting power on the whole and constructed of, in order from an object side, a first lens unit having a negative refracting power, a second lens unit having a positive refracting power, a third lens unit having a negative refracting power and a fourth lens unit having a positive refracting power;
• a regulating unit, disposed in the vicinity of a focal position of said objective lens group, which regulates a visual field;
• and
• an eyepiece unit having a positive refracting power,
• wherein said Keplerian zoom finder optical system is a finder for varying a magnification by moving at least said second lens unit along an optical axis and by varying the distances between said second lens unit and lens units at opposite sides of said second lens unit, and at least one surface of said second lens unit and one surface of said fourth lens unit are aspherical surfaces each having a curvature radius which increases as it approaches the periphery thereof, wherein at least said second lens unit is moved so that when varying the magnification from low to high, said finder reduces a distance between said first lens unit and said second lens unit and increases a distance between said second lens unit and said third lens unit, and wherein said fourth lens unit is fixed in varying the magnification.