Zoom lens having a long back focal length

In a retro-focus type lens system divided into three lens units with respect to the widest air spacing and the next widest air spacing between adjacent lens units, and improvement comprising:

• a first lens unit disposed on a long conjugate side and having negative refractive power, and including a negative lens and lens provided with an aspherical surface, wherein further said first lens unit has a meniscus-shaped negative lens having its convex surface facing said long conjugate side and an aspherical lens provided with an aspherical surface on at least one surface thereof;
• a second lens unit disposed subsequently to said first lens unit and provided with a positive lens block comprising a negative lens and a positive lens cemented together;
• and
• a third lens unit disposed subsequently to said second lens unit and having a positive refractive power, and said third lens unit further comprises a meniscus-shaped positive doublet lens comprising a negative lens and positive lens cemented together and having its convex surface facing a short conjugate side as a whole and a positive lens.

In a retro-focus type lens system divided into three lens units with respect to the widest air spacing and the next widest air spacing between adjacent lens units, and improvement comprising:

• a first lens unit disposed on a long conjugate side and having a negative refractive power, and said first lens further comprises an aspherical lens provided with an aspherical surface on at least one surface thereof and meniscus-shaped negative doublet lens comprising a positive lens and a negative lens cemented together and having its convex surface facing a long conjugate side as a whole;
• a second lens unit disposed subsequently to said first lens unit and provided with a positive lens block comprising a positive lens;
• and
• a third lens unit disposed subsequently to said second lens unit and having a positive refractive power, and comprising a negative lens whose both lens surfaces are concave surfaces and two positive lenses.

In a retro-focus type lens system divided into three lens units with respect to the widest air spacing and the next widest air spacing between adjacent lens units, the improvement comprising:

• a first lens unit disposed on a long conjugate side and having negative refractive power, and including a negative lens and an aspherical lens provided with aspherical surfaces or both lens surfaces thereof;
• and
• a second lens unit disposed subsequently to said first lens unit and provided with a positive lens block;
• and
• a third lens unit disposed subsequently to said second lens unit and having a positive refractive power, and including a negative lens and a positive lens;
• wherein if quartic aspherical coefficients of aspherical surfaces which are adjacent to a first conjugate point side and a second conjugate point side, respectively, are B1 and B2, then B1>B2, and the shape of the aspherical surfaces is expressed by the following equation:
• ##EQU5## where the x-axis is along the optical axis in the direction from the long-conjugate side, the H-axis is perpendicular to the x-axis and is taken through the vertex of the lens surface, and B, C, D and E are respective aspherical coefficients.

A projection lens, comprising:

• first lens unit including a meniscus-shaped negative lens with a convex surface facing a first conjugate point side and an aspherical lens provided with an aspherical surface on at least one surface thereof;
• and
• a second lens unit including a positive lens;
• and,
• a third lens unit including a negative lens with both lens surfaces thereof being concave surfaces and two positive lenses;
• whereby a projection lens comprising 6 or 7 lens elements is defined.