In recent years, cell-phones and mobile terminals each being equipped with an image pickup apparatus are in widespread use, which results from higher performance and downsizing of an image pickup apparatus employing a solid-state image pickup element such as a CCD (Charge Coupled Device) type image sensor or a CMOS (Complementary Metal Oxide Semiconductor) type image sensor. Further, for the image pickup lens mounted on these image pickup apparatuses, there are increasing demands for further higher performance and further downsizing. As an image pickup lens for this application, there has been suggested an image pickup lens of a four-element structure type because it has advantages that higher performance can be achieved, compared with a lens of a two-element type or of a three-element type.
As the image pickup lens of the four-element type, Japanese Patent Publication Open to Public Inspection (JP-A) No. 2004-341013 discloses a so-called inverted Ernostar type image pickup lens. The inverted Ernostar type image pickup lens is composed, in the order from the object side, of the first lens having a positive refractive power, the second lens having a negative refractive power, the third lens having a positive refractive power, and the fourth lens having a positive refractive power, to achieve higher performance. Further, each of JP-A Nos. 2002-365529, 2002-365530, 2002-365531, and 2005-292559 discloses an image pickup lens of a so-called telephoto type. The image pickup lens of the telephoto type is composed, in the order from the object side, of the first lens having a positive refractive power, the second lens having a negative refractive power, the third lens having a positive refractive power, and the fourth lens having a negative refractive power, to achieve a smaller size in terms of the total length of the image pickup lens which is a distance on the optical axis from a lens surface closest to the object in the image pickup lens to a focal position at the image side.
However, the image pickup lens described in JP-A No. 2004-341013 has a positive fourth lens due to the inverted Ernostar type and the image pickup lens provides a principal point position of the optical system closer to the image side and a longer back focus, compared with an image pickup lens of telephoto type which provides a negative fourth lens. Therefore, such type of image pickup lens as described in JP-A No. 2004-341013 is disadvantageous for downsizing. In addition, there is provided only one lens having negative refractive power among four lenses in the image pickup lens. Thus, it provides a difficulty in correction of Petzval's sum, and in securing excellent performance on the periphery portion of the image area.
The image pickup lens described in each of JP-A Nos. 2002-365529, 2002-365530, and 2002-365531 has a narrower imaging field angle and insufficiently corrects its aberrations. If the total length of the image pickup lens is further shortened, it becomes difficult to be used with an image pickup element with a large number of pixels due to deterioration of its performance.
Further, the image pickup lens described in JP-A No. 2005-292559 has the fourth lens in a meniscus shape whose convex surface faces the object. Due to this, a peripheral portion of the fourth lens is protruded toward the image. In order to prevent the fourth lens from interfering with a parallel plate such as an optical low-pass filter, infrared-ray cut filter, and a seal glass of a package of the solid-state image pickup element which are arranged between the fourth lens and the solid-state image pickup element, and a substrate of the solid-state image pickup element, it requires securing a long back focal length. The image pickup lens described in JP-A No. 2005-292559 actually has a long back focal length in spite of telephoto-type, and does not sufficiently achieve its downsizing. Additionally, the image pickup lens insufficiently corrects aberrations to be used with an image pickup element with a large number of pixels.