1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a solid-state image sensor such as a CCD sensor or C-MOS sensor used in a compact image pickup device and more particularly to imaging lenses which are built in image pickup devices mounted in mobile terminals such as smart phones and mobile phones, which are becoming increasingly compact and thin, and PDAs (Personal Digital Assistants), game consoles and information terminals such as PCs.
2. Description of the Related Art
In recent years, the market of tablet multifunctional terminals as typified by smart phones has been rapidly expanding. Today, mobile terminals which incorporate high-performance, high-quality cameras comparable to digital still cameras are becoming popular and high-pixel density cameras of over 8 megapixels are becoming mainstream. Since the trend towards high-pixel density cameras is expected to be accelerated, there is a growing demand for high-performance imaging lenses built in image pickup devices which cope with this trend. Furthermore, since portability and convenience are important for mobile terminals, there is a strong demand for thinner models. This accelerates the need for thinner image pickup device models and thinner imaging lenses built therein.
At the same time, the imaging lenses are expected to ensure high brightness to cope with the increase in the number of pixels and provide a wide angle of view to capture an image of an object in a wider perspective.
In addition to many types of imaging lens composed of four lenses which have been proposed so far, imaging lenses composed of five or six lenses are proposed in order to meet the trend toward more compactness and thinness and higher performance.
For example, JP-A-2007-264180 (Patent Document 1) discloses an imaging lens which includes, in order from the object side, a positive first lens having a convex object-side surface, a negative meniscus second lens having a concave image-side surface, a positive meniscus third lens having a convex image-side surface, a negative fourth lens as a double-sided aspheric lens having a concave image-side surface near the optical axis, and a positive or negative fifth lens as a double-sided aspheric lens.
Also, JP-A-2011-085733 (Patent Document 2) discloses an imaging lens system which includes, in order from the object side, a first lens group including a first lens with a convex object-side surface, a second lens group including a second lens with a concave image-side surface, a third lens group including a meniscus third lens with a concave object-side surface, a fourth lens group including a meniscus fourth lens with a concave object-side surface, and a fifth lens group including a meniscus fifth lens with an aspheric object-side surface having an inflexion point, and an imaging lens composed of six lenses including a positive lens having a slightly convex surface on the object side of the above lens system and a slightly concave surface on its image side.
The imaging lens described in Patent Document 1, composed of five lenses, offers a high-performance imaging lens system which can correct axial chromatic aberration and chromatic aberration of magnification and copes with the increase in the number of pixels, by optimizing the lens material and lens surface shapes. However, its total track length is about 8 mm and there is difficulty in applying it to an increasingly thin image pickup device. Also, the F-number is about 2.8 and the angle of view is about 32 degrees, which is not sufficient to provide high brightness and a wide angle of view as expected in recent years.
Patent Document 2 discloses a high-resolution imaging lens which is relatively low-profile, in which the total track length of the five-lens type is about 6 mm and that of the six-lens type is about 6.6 mm. However, in this technique, the F-number is about 2.8 and the angle of view is about 32 degrees, so it is difficult to provide all the following features as expected in recent years: high resolution, low-profile design, high brightness, and a wide angle of view.