Zoom lens

1. A zoom lens comprising:

a first lens unit fixed when a magnification of the zoom lens is changed; and

at least two positive lens units arranged on an image side of the first lens unit,

relative spacings between individual lens units being varied when the magnification is changed,

wherein the first lens unit has a prism containing a reflecting surface at a most object side position, an entrance surface of the prism is configured as a concave surface directed toward an object side, and the concave surface is an aspherical surface that divergence is impaired progressively in separating from an optical axis.

2. A zoom lens according to claim 1 , wherein the first lens unit has only a positive lens on the image side of the prism.

3. A zoom lens according to claim 1 , wherein, except for a normally fixed lens unit, a most image-side lens unit is moved and thereby focusing is performed.

4. A zoom lens comprising:

a first lens unit fixed when a magnification of the zoom lens is changed;

an aperture stop located on an image side of the first lens unit; and

a lens unit interposed between the first lens unit and the aperture stop,

wherein the first lens unit has a prism containing a reflecting surface at a most object side position, an entrance surface of the prism is configured as a concave surface directed toward an object side, the concave surface is an aspherical surface that divergence is impaired progressively in separating from an optical axis, and the lens unit is moved in only one direction when the magnification is changed, varying from a wide-angle position to a telephoto position, in focusing of an infinite object point.

5. A zoom lens according to claim 4 , wherein the lens unit interposed between the first lens unit and the aperture stop comprises, in order from the object side, a negative lens and a positive lens.

6. A zoom lens according to claim 4 , wherein the aperture stop is substantially fixed with respect to an image plane.

7. A zoom lens comprising, in order from the object side:

a first lens unit with positive refracting power;

a second lens unit with negative refracting power;

an aperture stop substantially fixed with respect to an imaging plane;

a lens unit with positive refracting power; and

a last lens unit with positive refracting power,

wherein the first lens unit comprises a prism having a path entrance surface, a reflecting surface for bending a path, and an exit surface, and a rear-side lens subunit, and

wherein when a magnification of the zoom lens is changed, the first lens unit remains fixed and the second lens unit and the lens unit with positive refracting power are moved, and the lens unit with positive refracting power is moved in only one direction when the magnification is changed, varying from a wide-angle position to a telephoto position, in focusing of an infinite object point.

8. A zoom lens according to claim 7 , wherein the lens unit with positive refracting power has at least one cemented lens component made up of a positive lens and a negative lens.

9. A zoom lens according to claim 7 , wherein the lens unit with positive refracting power is first placed on an image side of the aperture stop.

10. A zoom lens according to claim 7 , wherein the lens unit with positive refracting power is a third lens unit from an object side and includes, in order from the object side, a cemented lens component made up of a positive lens and a negative lens and a single positive lens.

11. A zoom lens according to claim 7 , wherein the lens unit with positive refracting power is a third lens unit from an object side and includes, in order from the object side, a single positive lens and a cemented lens component made up of a positive lens and a negative lens.

12. A zoom lens according to claim 7 , wherein the lens unit with positive refracting power is a fourth lens unit from an object side and includes only a cemented lens component made up of a positive lens and a negative lens.

13. A zoom lens according to claim 7 , wherein the second lens unit comprises, in order from the object side, a negative lens and a positive lens.

14. A zoom lens according to claim 7 , wherein a total number of positive lens units included in the zoom lens is three.

15. A zoom lens according to claim 7 , wherein a total number of positive lens units included in the zoom lens is four, the lens unit with positive refracting power is a fourth lens unit from an object side, and each of the lens units with positive refracting power and the last lens unit is constructed of a cemented lens component made up of a positive lens and a negative lens.

16. An electronic imaging device comprising:

a zoom lens; and

an image sensor located at a position of an image plane of the zoom lens,

the zoom lens comprising:

a first lens unit fixed when a magnification of the zoom lens is changed; and

at least two positive lens units arranged on an image side of the first lens unit,

relative spacings between individual lens units being varied when the magnification is changed,

wherein the first lens unit has a prism containing a reflecting surface at a most object-side position, an entrance surface of the prism is configured as a concave surface directed toward an object side, and the concave surface is an aspherical surface that divergence is impaired progressively in separating from an optical axis.

17. A zoom lens comprising, in order from the object side:

a first lens unit with positive refracting power;

a second lens unit with negative refracting power;

an aperture stop substantially fixed with respect to an imaging plane;

a lens unit with positive refracting power; and

a last lens unit with positive refracting power,

wherein when a magnification of the zoom lens is changed, the first lens unit remains fixed and the second lens unit and the lens unit with positive refracting power are moved, and the lens unit with positive refracting power is moved in only one direction when the magnification is changed, varying from a wide-angle position to a telephoto position, in focusing of an infinite object point,

wherein the lens unit with positive refracting power is a third lens unit from an object side and includes, in order from the object side, a cemented lens component made up of a positive lens and a negative lens and a single positive lens,

wherein the first lens unit has a prism having a reflecting surface for bending a path of rays, and a positive lens arranged on an image side of the prism, and

wherein only the positive lens of the first lens unit is arranged between the prism and the second lens unit.

18. A zoom lens comprising, in order from the object side:

a first lens unit with positive refracting power;

a second lens unit with negative refracting power;

an aperture stop substantially fixed with respect to an imaging plane;

a lens unit with positive refracting power; and

a last lens unit with positive refracting power,

wherein when a magnification of the zoom lens is changed, the first lens unit remains fixed and the second lens unit and the lens unit with positive refracting power are moved, and the lens unit with positive refracting power is moved in only one direction when the magnification is changed, varying from a wide-angle position to a telephoto position, in focusing of an infinite object point, and

wherein the lens unit with positive refracting power is a fourth lens unit from an object side and includes only a cemented lens component made up of a positive lens and a negative lens.