Field of the Invention
The present invention relates to an imaging lens which forms an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device, and more particularly to an imaging lens built in an image pickup device mounted in an increasingly compact and low-profile smartphone or mobile phone, PDA (Personal Digital Assistant), game console or information terminal such as a PC, or a home appliance with a camera function.
Description of the Related Art
In recent years, there has been a general tendency that mobile terminals such as smartphones and home appliances have a camera function. Today, whereas high-end models with a high-resolution camera function comparable to a digital still camera have been introduced into the market, the need for inexpensive popular models with a certain level of camera performance is still high. An imaging lens composed of three elements (constituent lenses) may be suitable as an imaging lens to be built in an inexpensive popular model because it can deliver a certain level of imaging performance and can be applied to a low-profile device and supplied at low cost. However, with the recent trend toward smaller higher-pixel image sensors, the pixel size is becoming smaller and the pixel density is becoming higher. Nowadays, image sensors with a pixel pitch of less than 1.2 microns have been proposed. An imaging lens for use in such an image sensor is expected to not only feature the smallness of aberrations but also provide a high-brightness optical system with an F-value smaller than 2.8 (often seen in the past). As for mobile terminals in particular, the imaging lens is expected to be low-profile enough to be applicable to a low-profile product. Furthermore, the imaging lens is anticipated to be able to capture an image of an object over a wide field of view so as to flexibly cope with the various camera functions of the product.
Conventionally, as an example of an imaging lens composed of three constituent lenses, the imaging lens described in JP-A-2010-113306 (Patent Document 1) includes, in order from an object side, an aperture stop, a first lens with positive refractive power, a second lens with negative refractive power, and a third lens. The third lens has, on the both sides, aspheric surfaces which are convex-curved toward the object near an optical axis and concave-curved toward the object in the vicinity of the lens periphery so that the refractive power changes according to the distance from the optical axis. The thickness of the first lens on the optical axis, the air gap on the optical axis between the first and second lenses, the focal length of the third lens, and the relation in curvature radius between the object-side surface and the image-side surface of the first lens are determined so as to achieve a wide field of view.
The imaging lens described in JP-A-2007-127953 (Patent Document 2) is an optical system which corrects chromatic aberrations using a diffractive optical surface. The imaging lens described in Patent Document 2 includes at least three constituent lenses and at least one of the lens surfaces of these lenses has a diffractive surface. At least one of the lens surfaces of the lens located nearest to the image plane has negative optical power in its center and the optical power changes to positive power as the distance to the lens periphery decreases, making up a compact optical system with high telecentricity.
According to Patent Document 1, the imaging lens can capture an image of an object over a wide field of view from 76 to 78 degrees and corrects aberrations relatively properly. However, since the F-value is 2.8, there is difficulty in applying the imaging lens to a compact high-density image sensor as mentioned above. The imaging lens described in Patent Document 1 has a total track length of 4.6 to 5.5 mm and the ratio of the total track length to the diagonal length of the effective imaging plane of the image sensor (divide total track length by diagonal length of the effective imaging plane of the image sensor; hereinafter referred to as the ratio to diagonal) is about 1.0; however, in order to make the imaging lens more low-profile and offer brightness with an F-value of 2.8 or less, the problem related to correction of aberrations has to be addressed.
According to Patent Document 2, the imaging lens can capture an image of an object over a maximum field of view of 72 degrees and corrects aberrations relatively properly. However, the positive, positive and negative refractive power lenses are arranged in order from the object side and chromatic aberrations are corrected not by the combination of the first and second lenses but by the diffractive optical surface. When the dependence on the diffractive optical surface for correction of chromatic aberrations is high, the number of orbicular zones formed on the lens surface tends to increase, which means that flare is more likely to occur. If two diffractive optical surfaces are formed to address this problem, high precision is required in the lens forming and assembling processes to prevent misalignment between the diffractive optical surfaces, which implies a higher degree of manufacturing difficulty. In connection with low-profileness, the third lens, located nearest to the image plane, has negative refractive power and the back focus is relatively long, making it difficult to achieve low-profileness. In the imaging lens composed of three constituent lenses as described in Patent Document 2, the total track length is about 6.8 mm and the ratio to diagonal is more than 1.1. If the back focus is decreased to achieve low-profileness, the angle of incidence on the third lens must be increased and as a consequence, probably the effective diameter of the second lens must be larger. When the lens diameter is larger, spherical aberrations and coma aberrations increase. This will make it difficult to make the imaging lens more low-profile while ensuring high performance. In addition, since the F-value of the imaging lens disclosed in Patent Document 2 is 3.3, it is difficult to apply the imaging lens to the latest high-density image sensors.