1. Field of the Invention
This invention relates to zoom lenses suited to single-lens reflex cameras or video cameras and, more particularly, to zoom lenses of relatively wide angle, high range and large relative aperture of the type in which the front lens unit is negative in power, or the so-called "negative lead" type, having four or five lens units in total. Herein, the term "unit" is equivalent to "group".
2. Description of the Related Art
The negative lead type of zoom lens, because of the lens unit of negative refractive power coming first in arrangement, is easier to increase the maximum image angle and amenable to shorten the minimum possible object distance for close-up photography. With these merits on one hand, there are, on the other hand, drawbacks that the diameter of the stop is caused to increase and that the range of variation of the image magnification becomes difficult to increase.
To overcome these drawbacks, there have been made previous proposals for minimization of the bulk and size of the entirety of the lens system in combination with a great increase of the range, as in, for example, Japanese Patent Publications Nos. Sho 49-23912 and Sho 55-14403 and Japanese Laid-Open Patent Applications Nos. Sho 53-34539, Sho 57-163213, Sho 58-4113, Sho 63-241511, Hei 1-193709 and Hei 2-201310.
In each of these publications, the zoom lens is made to comprise four lens units of minus-plus-minus-plus power arrangement in this order from the object side as a whole, of which certain ones are moved in appropriate relation to vary the focal length.
Recently, for the single-lens reflex camera or video camera, the public is demanding a zoom lens which includes wide image angles and has a high zoom ratio as the standard equipment. For example, the market has already sold single-lens reflex cameras of 35 mm format having zoom lenses of as wide a maximum image angle as 35 mm or 28 mm in focal length with the telephoto end at 70 mm each as the standard equipment.
More recently, there is demanded a zoom lens having an extended range toward the telephoto end with the result of from 28 mm to 80 mm or to 85 mm in focal length and a large relative aperture.
As the field of view increases to so wide an angle and the range, too, extends to so long a focal length, however, it becomes necessary in general to admit an unduly large increase of the physical length of the zoom lens and moreover to make elaborate the relation in which the lens units move to effect zooming. This leads to a multi-sleeve construction of the lens mounting. Thus, a problem arises in that the lens barrel increases largely in bulk and size and its mounting mechanism comes to a very complicated structure.