In recent years, camera modules for taking photos have begun to be incorporated in mobile terminals such as mobile phones and lap-top computers. Downsizing the camera modules is a prerequisite for enhancing the portability of these apparatuses. The camera module operates with an image pickup device such as a CCD (Charged Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor). Recently, a pixel having the size of approximately a few micrometers has become commercially feasible, and an image pickup device with high resolution and a compact size can now be commercialized. This is accelerating the demand for downsizing of image pick-up lens systems so that they are able to be suitably used with miniaturized image pick-up devices. It is also increasing expectations of cost reductions in image pick-up lens systems, commensurate with the lower costs enjoyed by modem image pickup devices. All in all, an image pick-up lens system needs to satisfy the oft-conflicting requirements of compactness, low cost, and excellent optical performance.
Compactness means in particular that a length from a lens edge of the lens system to an image pick-up surface should be as short as possible.
Low cost means in particular that the lens system should include as few lenses as possible; and that the lenses should be able to be formed from a resin or a plastic and be easily assembled.
Excellent optical performance can be classified into the following five main requirements:
First, a high brightness requirement, which means that the lens system should have a small F number (FNo.) Generally, the FNo. should be 2.8 or less.
Second, a wide angle requirement, which means that half of the field of view of the lens system should be 30° or more.
Third, a small incident angle of a chief ray on an image pick-up surface is required, because too wide an incident angle of the chief ray seriously affects illumination of edges of the field of view.
Fourth, a uniform illumination on the image surface requirement, which means that the lens system has few eclipses and/or narrows down an angle of incidence onto an image pick-up device.
Fifth, a high resolution requirement, which means that the lens system should appropriately correct fundamental aberrations such as spherical aberration, coma aberration, curvature of field, astigmatism, distortion, and chromatic aberration.
In a lens system which satisfies the low cost requirement, a single lens made from a resin or a plastic is desired. Typical such lens systems can be found in U.S. Pat. No. 6,297,915B1 and EP Pat. No. 1271215A2. However, even if the lens has two aspheric surfaces, it is difficult to achieve excellent optical performance, especially if a wide angle such as 70° is desired. Thus, the single lens system can generally only be used in a low-resolution image pick-up device such as a CMOS. In addition, a thick lens is generally used for correcting aberrations. Thus, a ratio of a total length of the lens system to a focal length of the lens (L/f) is about 2. In other words, it is difficult to make the lens system compact.
In a lens system which satisfies the excellent optical performance requirement, three or even more lenses are desired. A typical three-lens system can be found in U.S. Pat. No. 5,940,219. A typical four-lens system can be found in U.S. Pat. Application Publication No. 2004/0012861. However, the addition of extra lenses increases costs proportionately.
In order to satisfy all the requirements of compactness, low cost and excellent optical performance, it is commonly believed that a two-lens system is desirable.
A well-known two-lens system is the retro-focus type lens system. A typical such lens system can be found in U.S. Pat. No. 6,449,105B1. The lens system comprises, from an object side to an image side, a first meniscus lens having negative refracting power and a convex surface on the object side, a stop, and a second meniscus lens having positive refracting power and a convex surface on the image side. The lens system helps correct wide angle aberrations. However, a shutter is positioned between the second lens and the image side, which adds to the distance between the second lens and the image side. Thus, the compactness of the lens system is limited.
In order to overcome the above described problems, an image pick-up lens system generally employs two lenses which are made from plastic or resin and which have four aspheric surfaces. A typical configuration of such system can be found in U.S. Pat. Application Publication No. 2004/0036983 and EP Pat. No. 1357414A1. The system can satisfy the compactness and low cost requirements. In addition, the system can provide a resolution of 300,000 pixels.
However, a ratio of a total length of such system to a total focal length of the lenses (L/f) is generally about 2. The smallest ratio obtainable is 1.7, which still constitutes a limitation on the compactness of the lens system. In addition, it is difficult to improve resolution due to inherent properties of the materials of which the lenses are made. In one aspect, for cost reduction, the two lenses are made from a same plastic or resin material. This makes it difficult to correct lateral chromatic aberrations. In another aspect, because there are only a few varieties of plastic and resin materials which can be suitably used to make lenses, even if the two lenses are made from a different plastic or resin material, the range of variation of optical properties of the two lenses is limited. For example, one lens may be made from polymethyl methacrylate (PMMA) having a low refractive index and low color dispersion, and the other lens may be made from polycarbonate (PC) having a high refractive index and high color dispersion. In such case, the range of variation of the refract indexes and the Abbe constants is narrow. Typically, the refractive indexes vary in the range from 1.49 to 1.59, and the Abbe constants vary in the range from 30 to 55. This makes it difficult to effectively correct lateral chromatic aberration. Furthermore, compared with optical glass, the optical properties of a plastic or resin material are inferior. Such optical properties include optical transmission, wavelength range for light transmittance, temperature resistance, and humidity resistance. Thus, it is difficult to apply the image pick-up lens system with lenses made from plastic or resin materials to products requiring a resolution of 1,000,000 pixels or more. On the other hand, if the two lenses are made of optical glass, it is unavoidable that the total cost of the system is increased.
Therefore, a low-cost image pick-up lens system which has a compact configuration and excellent optical performance and which can be used in products having over 1,000,000 pixels resolution is desired.