Fixed focal length optical lens architecture providing a customized depth of focus optical system
View Patent ↗Providing for a fixed focus optical system exhibiting extended depth of field is provided herein. By way of example, a compact and fast optical system that yields an asymmetric modulation transfer function (MTF) is disclosed. In some aspects, the asymmetric MTF results in extended depth of field for near field objects. Such a response can be particularly beneficial for small handheld cameras or camera modules having high resolution. According to some disclosed aspects, the resolution can be about 8 mega pixels. Additionally, the optical system can comprise four lenses in one aspect and five lenses in another, while remaining below about 5.3 mm total track length (TTL) for the respective systems. In at least one application, the disclosed optical systems can be employed for a high resolution compact camera, for instance in conjunction with an electronic computing device, communication device, display device, surveillance equipment, or the like.
1. An optical system comprising:
a plurality of optical elements arranged along a common optical axis for forming a real image of an object, said optical elements including:
a first lens having a positive refractive power, with both the surface facing the object side and the surface facing the image side having convex shapes;
a second lens having a negative refractive power and a meniscus shape, with the surface facing the object side having a concave shape near the optical axis and the surface facing the image side having a convex shape near the optical axis;
a third lens; and
a fourth lens having a meniscus shape near the optical axis, with the surface facing the object side having a convex shape-near the optical axis and the surface facing the image side having a concave shape near the optical axis; wherein
the total track length (TTL) of the optical system is less than about 5.5 mm, and
the ratio of the TTL of the optical system to a diameter of the real image is less than 0.95.
2. The optical system of claim 1 , wherein the optical system exhibits a modulation transfer function (MTF) that remains above a threshold value for a first range of positive focus shifts and a second range of negative focus shifts, wherein the second range is at least about fifty percent larger than the first range.
3. The optical system of claim 2 , wherein the MTF remains above a threshold value of 0.15 for positive focal shifts of less than 25 micro meters and negative focal shifts of less than 35 micro meters.
4. The optical system of claim 2 , wherein the MTF includes values ranging from 0 to 0.35.
5. The optical system of claim 1 , wherein a ratio of a focal length of the first lens (f — 1) to a focal length of the optical system (f) is less than about 0.7.
6. The optical system of claim 1 , wherein the third lens having a small positive refractive power, with both the surface facing the object side and the surface facing the image side of the third lens having convex shapes near the optical axis.
7. The optical system of claim 1 , wherein the F-number of the optical system is approximately 2.8.
8. The optical system of claim 1 , wherein at least one surface of the first lens and both surfaces of the second lens, the third lens and the fourth lens are aspheric.
9. The optical system of claim 1 , wherein lenses are made of plastic.
10. The optical system of claim 1 , further comprising an image sensor defining an imaging surface wherein at least one of the following:
the image sensor including approximately 8 million pixels and including a pixel having a length of about 1.4 micro meters; or
the image sensor including approximately 12 million pixels and including a pixel having a length of about 1.2 micro meters.
11. The optical system of claim 1 , wherein the refractive index of the lenses is within a range of 1.5 to 1.65.
12. The optical system of claim 1 , wherein the Abbe number for the first lens or the fourth lens is approximately between 50 and 60 and the Abbe number for the second lens or the third lens is approximately between 20 and 30.
13. The optical system of claim 1 , wherein the chromatic focal shift of the optical system is limited to below approximately 40 micro meters across the visible spectrum.
14. The optical system of claim 1 , wherein the lateral color range for the optical system is approximately between 3 and 4 micro meters.
15. The optical system of claim 1 , further comprising: one or more optical stops for mitigating stray light or for defining a pupil size for the optical system.
16. A fixed-focal length imaging system for a camera comprising:
a set of four lenses having a common optical axis for forming a real image of an object; and
an image sensor defining an imaging surface and having approximately 8 million pixels; wherein,
the total track length (TTL) of the imaging system is less than approximately 5.4 millimeters;
the ratio of the TTL to a real image circle diameter is less than approximately 0.95;
the image sensor includes a pixel having a length of approximately 1.4 micro meters;
the imaging system can exhibit an asymmetric modulation transfer function (MTF) that remains above 0.15 for a larger range of negative focal shifts than a range of positive focal shifts; and
the camera is implemented inside a portable electronics device.
17. The fixed-focal length imaging system of claim 16 , wherein the range of negative focal shifts is approximately between 1.5 and 2 times more than the range of positive focal shifts.
18. A method for a camera embedded in an electronic device comprising:
receiving optical information regarding an object by using at least four lenses having a common optical axis; and
forming a real image of the object by using an imaging surface; wherein
a modulation transfer function (MTF) of the real image corresponding to an object distance of 350 millimeters is asymmetric having an extended near field depth for the real image; further wherein,
the MTF can include values up to 0.35 and remains above the value of 0.15 for positive focal shifts of up to 25 micro meters and up to negative focal shifts associated with the object far field.