1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses of compact form, which nevertheless get a high zoom ratio.
2. Description of the Related Art
Recent minimization of the bulk and size of leaf shutter type cameras or the like has led to a demand for shortening the total length of the compact zoom lens.
In Japanese Laid-Open Patent Application No. Sho 57-201213, there is proposed a compact zoom lens comprising two lens units of plus-minus refractive power from the object side with the air separation between these lens units being varied to effect zooming.
Also, the use of three lens units of minus-plus-minus refractive power in the zoom lens, which are movable for zooming, is proposed in, for example, Japanese Laid-Open Patent Applications No. Sho 63-271214, No. Sho 64-72114, etc. In these publications, the relation in which the first, second and third lens units move during zooming, and the construction and arrangement of the constituent parts of the three lens units are specified to thereby well correct any variation of aberrations with zooming. Thus, a relatively compact lens system having a high optical performance at a zoom ratio of 2 to 3 with 8 or 9 lens elements as a whole is achieved.
An example of adaptation of the zoom lens to the leaf shutter camera by extremely reducing the number of lens elements is proposed in Japanese Laid-Open Patent Application No. Sho 63-276013.
In this Japanese Laid-Open Patent Application No. Sho 63-276013, the front lens unit comprises negative and positive, refractive power or two, lenses, while the rear lens unit comprises positive and negative refractive power lenses. Thus, a zoom lens composed of four lens elements in total is achieved. However, because the zoom lens uses a refractive power distribution type lens (GRIN lens), the difficulty in manufacturing the zoom lens is extremely high, causing the amount of lenses produced through mass production to become low.
Further, Japanese Laid-Open Patent Application No. Sho 64-88512 (U.S. Pat. No. 4,756,609) discloses a zoom lens having four lens units of a minus-plus-plus-minus refractive power arrangement, of which the first and second lens units each comprise one lens element, the third lens unit is a cemented lens, and the fourth lens unit comprises three lens elements of plus-minus-minus refractive power.
Also, in Japanese Laid-Open Patent Application No. Sho 62-235916, there is disclosed a zoom lens having four lens units of a minus-plus-plus-minus refractive power arrangement, wherein the first lens unit is constructed with two elements of plus-minus refractive power, the second lens unit with three elements of plus-plus-minus refractive power, the third lens unit with one element, and the fourth lens unit with two elements of minus-plus refractive power.