Wide-angle lens
A wide-angle lens includes, in order from the object side to the image side, a first lens element of negative refractive power, a second lens element of positive refractive power, a third lens element of positive refractive power and a cemented doublet element of positive refractive power consisting of a fourth lens element and a fifth lens element cemented together. The combined focal length of the second, third, fourth and fifth lens elements satisfies one specified condition, which allows the wide-angle lens to be effectively shortened and makes the construction thereof compact.
1. A wide-angle lens comprising, in order from an object side to an image side, a first lens element of negative refractive power, a second lens element of positive refractive power, a third lens element of positive refractive power and a cemented doublet element of positive refractive power consisting of a fourth lens element and a fifth lens element joined together, the first lens element having an object-side aspheric surface, the second lens element being of a meniscus lens and having an object-side concave surface and an image-side convex surface at least one of which is an aspheric surface, the third lens element being of a meniscus lens and having an object-side concave surface and an image-side convex surface at least one of which is an aspheric surface, wherein the second, third, fourth and fifth lens elements satisfy the following condition:
0.7< f 2345/ f <1.0
where f2345 represents a combined focal length of the second, third, fourth and fifth lens elements and f represents a total focal length of the wide-angle lens.
2. The wide-angle lens as claimed in claim 1 , wherein the object-side aspheric surface of the first lens element satisfies the following condition:
X−X′≧ 0
where X represents a horizontal distance between an arbitrary point sitting at an off-optical-axis position with respect to the object side aspheric surface of the first lens element and a corresponding on-optical-axis point sitting on the object-side surface of the first lens element, and X′ represents a horizontal distance between the arbitrary point sitting at the off-optical-axis position of the first lens element and a corresponding on-optical-axis point sitting on a reference spherical surface of the object-side aspheric surface of the first lens lens element.
3. The wide-angle lens as claimed in claim 1 , wherein the first lens element has a high refractive power and the object-side aspheric surface of the first lens element is convex toward the object side.
4. The wide-angle lens as claimed in claim 1 , wherein the second lens element has two aspheric surfaces.
5. The wide-angle lens as claimed in claim 1 , wherein the third lens elements has two aspheric surfaces.
6. The wide-angle lens as claimed in claim 1 further comprising an aperture disposed between the first lens element and the second element.
7. The wide-angle lens as claimed in claim 1 further comprising a glass cover disposed between the fifth lens element and the image side.
8. A wide-angle lens comprising, in order from an object side to an image side, a first lens element of negative refractive power, a second lens element of positive refractive power, a third lens element of positive refractive power and a cemented doublet element of positive refractive power consisting of a fourth lens element and a fifth lens element joined together, the first lens element having an object-side aspheric surface, the second and third lens elements both having at least one aspheric surface, wherein the second, third, fourth and fifth lens elements satisfy the following condition:
0.7< f 2345/ f <1.0
where f2345 represents a combined focal length of the second, third, fourth and fifth lens elements and f represents a total focal length of the wide-angle lens, and
wherein the object-side aspheric surface of the first lens element satisfies the following condition:
X−X′≧ 0
where X represents a horizontal distance between an arbitrary point sitting at an off-optical-axis position with respect to the object side aspheric surface of the first lens element and a corresponding on-optical-axis point sitting on the object-side surface of the first lens element, and X′ represents a horizontal distance between the arbitrary point sitting at the off-optical-axis position of the first lens element and a corresponding on-optical-axis point sitting on a reference spherical surface of the object-side aspheric surface of the first lens element.
9. The wide-angle lens as claimed in claim 8 , wherein the first, second, and third lens elements are aspheric lenses each having two aspheric surfaces.
10. The wide-angle lens as claimed in claim 8 further comprising an aperture disposed between the first lens element and the second element.
11. The wide-angle lens as claimed in claim 8 further comprising a glass cover disposed between the fifth lens element and the image side.