Digital Camera Patent AbstractA lens system (46) for a digital camera consecutively includes a
first lens element (20), a second lens element (30), and a third
lens element (40). The first lens element is biconvex and has a
first aspheric surface (22) and an opposite second aspheric surface
(24). The second lens element is concavo-convex and includes a third
aspheric surface (32) and an opposite fourth aspheric surface (34).
The third lens element convexo-concave and has a fifth aspheric
surface (42) and an opposite sixth aspheric surface (44). The first
lens element is made of glass, and the second lens element and the
third lens are made of optical plastic.
Digital Camera Patent ClaimsI claim:
1. A lens system comprising: a first lens element being biconvex,
and including a first aspheric surface and an opposite second aspheric
surface, the first lens element being made of glass, the first lens
element having a refractive index n and a dispersion coefficient
v which satisfy the following requirements: 1.65<n<1.75, 50<v<60;
a second lens element being concavo-convex, and including a third
aspheric surface and an opposite fourth aspheric surface, the third
aspheric surface facing the second aspheric surface, the second
lens element being made of plastic, the second lens element having
a refractive index n and a dispersion coefficient v which satisfy
the following requirements: 1.55<n<1.65, 25<v<35; and
a third lens element being convexo-concave, and including a fifth
aspheric surface and an opposite sixth aspheric surface, the fifth
aspheric surface facing the fourth aspheric surface, the third lens
element being made of plastic, the third lens element having a refractive
index n and a dispersion coefficient v which satisfy: 1.49<n<1.55,
55<v<60.
2. The lens system as claimed in claim 1, wherein each of the lens
elements is symmetrically disposed along an optical axis of the
lens system.
3. The lens system as claimed in claim 1, wherein the first lens
element is comprised of K-VC80, the refractive index n and the dispersion
coefficient v of the first lens element satisfy the following requirements:
n=1.69384, v=51.33.
4. The lens system as claimed in claim 1, wherein the second lens
element is comprised of OKP4, the refractive index n and the dispersion
coefficient v of the second lens element comprises satisfy the following
requirements: n=1.60726, v=26.64.
5. The lens system as claimed in claim 1, wherein the third lens
element is comprised of E48R, the refractive index n and the dispersion
coefficient v of the third lens element satisfy the following requirements:
n=1.53116, v=56.04.
6. The lens system as claimed in claim 1, further comprising an
aperture stop disposed in front of the first lens element.
7. The lens system a as claimed in claim 1, further comprising
an optical board located behind the third lens element.
8. The lens system as claimed in claim 7, wherein the optical board
is coated with an infrared-cut coating.
9. The lens system as claimed in claim 1, wherein at least one
surface of the first lens element, the second lens element and the
third element is coated with an infrared-cut coating.
10. A module for a digital camera, comprising: an aperture stop;
a lens system located behind the aperture stop, the lens system
comprising: a plurality of lens elements including first, second
and third elements, the first lens element being biconvex, and including
a first aspheric surface and a second aspheric surface; the second
lens element being concavo-convex, and including a third aspheric
surface and a fourth aspheric surface; the third lens element being
convexo-concave, and including a fifth aspheric surface and a sixth
aspheric surface. an optical board disposed behind the third lens,
and including a first plane and a second plane; and an image sensor,
the image sensor being disposed behind the optical board and including
an image plane; wherein the first lens element is made of glass,
the second lens element and the third lens element are both made
of optical plastic, the first lens element has a refractive index
n and a dispersion coefficent v which satisfy: 1.65<n<1.75,
50<v<60.
11. The module for a digital camera as claimed in claim 10, wherein
the second lens element has a refractive index n and a dispersion
coefficient v which satisfy: 1.55<n<1.65, 25<v<35; and
the third lens element has a refractive index n and a dispersion
coefficient v which satisfy: 1.49<n<1.55, 55<v<60.
12. The lens system for a digital camera as claimed in claim 10,
wherein the optical board is coated with an infrared-cut coating.
13. The lens system for a digital camera as claimed in claim 10,
wherein at least one surface of the first lens element, the second
lens element and the third element is coated with an infrared-cut
coating.
14. An image acquiring device comprising: an image sensor configured
for accepting light from an object of the image acquiring device
to generate desired image signals for the object; an aperture stop
disposed between the object and the image sensor so as to control
an amount of the light entering the image acquiring device; and
a lens system disposed between the aperture stop and the image sensor
for treating the entering light from the aperture stop before the
entering light reaches the image sensor, the lens system comprising
a plurality of lens elements including first, second and third aspheric
lens elements arranged in that order; wherein the first lens element
has a refractive index in a range from 1.65 to 1.75, and a dispersion
coefficient in a range from 50 to 60; the second lens element has
a refractive index in a range from 1.65 to 1.75, and a dispersion
coefficient in a range from 50 to 60; and the third element has
a refractive index in a range from 1.49 to 1.55, and a dispersion
coefficient in a range from 55 to 60.
15. The image acquiring device as claimed in claim 14, wherein
the first lens element is biconvex.
16. The image acquiring device as claimed in claim 14, wherein
the second lens element is concavo-convex.
17. The image acquiring device as claimed in claim 14, wherein
the third lens element is convexo-concave.
18. The image acquiring device as claimed in claim 14, wherein
the first lens element is made of glass, and the second lens element
and the third lens are made of optical plastic.
19. The claimed in claim 14, wherein at least one surface of the
first lens element, the second lens element and the third element
is coated with an infrared-cut coating.
20. The image acquiring device as claimed in claim 14, wherein
an effective focal length of the lens system is 4.817 mm, and a
field of view of the lens system is 61.75 degrees. Digital Camera Patent DescriptionBACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a lens systems for devices
such as digital cameras and, more particularly, to a lens system
that has optical elements with aspheric surfaces.
2. Discussion of the Related Art
Digital cameras utilizing high-resolution electronic imaging sensors
typically require high-resolution optical elements such as lenses.
In addition, the lenses generally must be very compact, so that
they can be incorporated into devices such as palm-sized computers,
cellular telephones, and the like.
Lenses for digital cameras generally have a plurality of individual
lens elements. The lens elements are typically spherical and so
usually create spherical aberration. Chromatic aberration, coma
aberration, distortion, and field curvature are also common optical
aberrations that occur in the imaging process of a typical lens.
A large number of lens elements are generally required in order
to balance these inherent optical aberrations. Lenses having a large
number of lens elements tend to be large, heavy, and expensive to
manufacture. This greater manufacturing cost is caused by the added
cost of assembling and mounting the lens elements into a lens barrel
as well as the materials used in their construction.
Furthermore, conventional lenses commonly use one or more aspheric
lens elements, each of which has one or two non-spherical surfaces.
The aspheric lens elements are usually made of plastic or glass.
Aspheric plastic lens elements may be produced by means of plastic
injection molding and are therefore relatively inexpensive. However,
the optical properties of most plastic lens elements are highly
sensitive to changes in temperature and humidity, such as when the
digital camera is used outdoors on very hot or cold days. On top
of this, the hardness of optical plastic material is lower than
that of an optical glass material, so the surfaces of such lens
elements are easily scraped or abraded, which can also affect image
precision. In comparison, glass aspheric lens elements have good
optical properties and scratch-resistant. However, glass aspheric
lenses cannot be easily produced by traditional glass grinding and
polishing techniques. In addition, glass lens elements are heavier
than plastic lens elements and thus defeats the goal of producing
more light-weight digital cameras.
Thus it can be seen that a typical lens system has both spherical
lens elements and aspheric lens elements. The lens system includes
a first spherical lens element, a second spherical lens element,
and a third lens element. The first lens element and the second
lens element are made of glass. The third lens element has two aspheric
surfaces and is made of plastic. Although the typical lens may satisfy
the requirements for imaging, the resolution of the lens is low
and may affect the image performance.
Accordingly, what is needed is a lens system for a digital camera
which is compact and which provides good imaging quality.
SUMMARY
A lens system for a digital camera of a preferred embodiment includes
a first lens element, a second lens element, and a third lens element.
The first lens element is biconvex and has a first aspheric surface
and a second opposite aspheric surface. The second lens element
is concavo-convex and includes a third aspheric surface and an opposite
fourth aspheric surface. The third lens element convexo-concave
and has a fifth aspheric surface and an opposite sixth aspheric
surface. The first lens element is made of glass, and the second
lens element and the third lens are made of optical plastic.
Other advantages and novel features of the present lens system
will become more apparent from the following detailed description
thereof when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Many aspects of lens system can be better understood with reference
to the following drawings. The components in the drawings are not
necessarily to scale, the emphasis instead being placed upon clearly
illustrating the principles of the present lens system. Moreover,
in the drawings, like reference numerals designate corresponding
parts throughout the several views.
FIG. 1 is a schematic, side-on cross-sectional view of a lens system
for a digital camera according to a preferred embodiment;
FIG. 2 is a graph showing the relationship between tangential (T)
and sagittal (S) field curvatures relative to image height in millimeters
in the lens system of FIG. 1;
FIG. 3 is a graph of optical distortion of the lens system of FIG.
1 relative to image height in millimeters;
FIG. 4 is a graph of Modulation Transfer Function (MTF) of the
lens system of FIG. 1 for a range of different spatial frequencies;
and
FIG. 5 is a graph showing relative illumination compared to image
height in millimeters of the lens system of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, an optical module 100 of a digital camera
of a preferred embodiment includes an aperture stop 10, a first
lens element 20, a second lens element 30, a third lens element
40, an optical board 50, and an imaging sensor 60, which are arranged
in that order from an object side designated as "Z.sub.obj"
to an image side designated as "Z.sub.img". The first
lens element 20, the second lens element 30, and the third lens
element 40 together may be considered, as a group, to constitute
a lens system 46 of the optical module 100. Line OO represents an
optical axis of the lens system.
The aperture stop 10 includes a stop plane 12, which faces the
first lens element 20. The aperture stop 10 is the first component
to receive light rays when the optical module 100 is used. Therefore,
the light rays can easily be controlled using the aperture stop
10.
The first lens element 20 is biconvex and aspheric. The first lens
element 20 includes a first aspheric surface 22 and an opposite
second aspheric surface 24. The second lens element 30 is concavo-convex
and includes a third aspheric surface 32 and an opposite fourth
aspheric surface 34. The third lens element 40 is convexo-concave
and includes a fifth aspheric surface 42 and an opposite sixth aspheric
surface 44. The first, second, third lens elements 20, 30, 40 of
the lens system 46 are symmetrically disposed in order along the
optical axis OO.
The first lens element 20 is advantageously made of optical glass.
A refractive index (designated as "n"), and a dispersion
coefficient (designated as "v") of the first lens element
20 need to satisfy the following requirements: 1.65<n<1.75,
50<v<60. The first lens element 20 is preferably made from
Glass Material K-VC80 obtainable from the Panasonic Electronic Devices
Co., Ltd in Japan. The refractive index of K-VC80 is 1.69384, and
its dispersion coefficient is 51.33.
The second lens element 30 is advantageously made of optical plastic
since optical plastic can be more readily shaped/machined into the
desired complex shape desired for the second lens element. A refractive
index and a dispersion coefficient of the second lens element 30
need to satisfy the following requirements (where refractive index
is designated as "n" and dispersion coefficient is designated
as "v"): 1.55<n<1.65, 25<v<36. The second
lens element 30 is preferably made from OKP4 obtainable from the
Osaka Gas Chemicals Co., Ltd in Japan. The refractive index of OKP4
is 1.60726, and its dispersion coefficient is 26.64.
The third lens element 40 is also advantageously made of optical
plastic. A refractive index and a dispersion coefficient of the
optical plastic need to satisfy the following requirements (where
refractive index is designated as "n", and a dispersion
coefficient is designated as "v"): 1.49<n<1.55,
25<v<60. The second lens element 30 is preferably made from
E48R produced by the Zeon Chemical Company in Japan. E48R is an
amorphous copolymer, and is a standard grade plastic used in molded
optical lenses and prisms for still cameras and video cameras. E48R
has low moisture absorption, low birefringence, and high transparency
The refractive index of E48R is 1.53116, and its dispersion coefficient
is 56.04.
The optical board 50 is made of glass, and includes a first plane
52 and a second plane 54. The optical board 50 is preferably made
from B270 obtainable from the Schott Company in Germany The refractive
index of B270 is 1.52308, and its dispersion coefficient is 58.57.
At least one surface of the first lens element 20, the second lens
element 30, the third lens element 40 and the optical board 50 is
coated with an infrared-cut (IR-cut) coating. The IR-cut coating
can filter infrared rays and hence improve image quality.
The image sensor 60 is located at an image side of the optical
board 50. The image sensor 60 includes an image plane 62. The optical
board 50 is used to protect the image plane 62 of the image sensor
60, so that dust or other contamination can not reach the image
plane 62. The image sensor 60 is usually a Charge Coupled Device
(CCD) or a Complementary Metal Oxide Semiconductor (CMOS). If the
image sensor 60 is used in a digital camera of a mobile phone, the
image sensor 60 is usually a CMOS for cost reasons. A pixel size
of the CMOS of the present embodiment is 2.8 .mu.m, and a resolution
of the CMOS is about 1640.times.1240 pixels. An effective area of
the CMOS is 4.592.times.3.472 mm, and a length of the diagonal is
5.76 mm.
Detailed structural parameters of the preferred embodiment of the
lens are shown in FIG. 1 and provided in Table 1. Surface radii
and axial distances are shown in millimeters (mm). The surfaces
are identified according to the corresponding drawing reference,
from the object side to the image side as shown.
TABLE-US-00001 TABLE 1 Surface Description Radius (r) Thickness
(d) Material Diameter Conic (k) 12 Stop plane .infin. 0.05190483
1.720458 0 22 First aspheric surface 2.238876 0.8006595 K-VC80 2.035909
0 24 Second aspheric surface 41.88976 0.6564028 2.179774 0 32 Third
aspheric surface 1.425641 0.6360871 OKP4 2.154307 0 34 Fourth aspheric
surface -2.161828 1.053919 2.521218 0 42 Fifth aspheric surface
4.75958 1.486146 E48R 4.865825 0 44 Sixth aspheric surface 3.945405
0.622858 5.451864 0 52 First plan .infin. 0.55 B270 5.613749 0 54
Second plane .infin. 0.1871944 5.7402 0 62 Image plane .infin. 5.806209
0
The aspheric surfaces are the surfaces 22, 24, 32, 34, 42 and 44,
and describe the following equation:
.times..times..times..times..times..times..times..times..times..times..tim-
es..times. ##EQU00001## Where: Z is the surface sag; C=1/r, where
r is the radius of the surface; K is the conic constant; r is the
distance from the optical axis; and a.sub.1, a.sub.2, a.sub.3, a.sub.4,
a.sub.5, and a.sub.6 are the aspheric coefficients. The aspheric
coefficients a.sub.1, a.sub.2, a.sub.3, a.sub.4, a.sub.5, and a.sub.6
are given by Table 2:
TABLE-US-00002 TABLE 2 Surface Description a.sub.1 a.sub.2 a.sub.3
a.sub.4 a.sub.5 22 first surface 0 -0.0100442617 -0.0042088922 0.00086816016
-0.0065897902- 24 second surface 0 -0.031802744 -0.0093614308 -0.0039816405
-0.0050110589- 32 third surface 0 0.072487613 0.056977611 -0.018391793
-0.0047098201 34 fourth surface 0 0.069181783 0.037452317 0.0021772689
-0.0036464379 42 fifth surface 0 -0.015033688 0.0025274925 -0.00021922009
-6.6899961e-006 44 sixth surface 0 -0.022922736 0.0016789464 -0.00013428379
2.527531e-006
The effective focal length of the lens is 4.817 mm in air, and
the maximum aperture is f/2.8. The field of view is 61.75 degrees.
The total length of the lens system 46 is 6.05 mm, and, as such,
the total length thereof is advantageously less than 8 mm. The lens
is well suited for use with state-of-the-art digital sensors having
a resolution of about 1640.times.1240 pixels.
The performance of the lens of the preferred embodiment is illustrated
in FIG. 2 through FIG. 5.
Referring to FIG. 2, field curvature represents the curved extents
of the image plane when visible light is focused through the lens.
Field curvature is very seldom totally eliminated. It is not absolutely
necessary to have the best correction. When cost is important, it
is often wise to select a more modestly priced configuration, rather
than have a high degree of correction. For the lens, it can be seen
that the tangential and sagittal field curvature is under -0.1 mm.
Referring to FIG. 3, distortion represents the inability of a lens
to create a rectilinear image of the subject. Distortion does not
modify the colors or the sharpness of the image, but rather the
shape of the image. The maximum geometric distortion of the lens
is typically higher than -1%, and lower than +1%. The lens can provide
crisp and sharp images with minimal field curvature, and is sufficient
for over 90 percent of photographic applications.
Referring to FIG. 4, Modulation Transfer Function (MTF) is the
scientific means of evaluating the fundamental spatial resolution
performance of an imaging system. When the MTF is measured, an imaging
height is divided into 1.0, 0.8, 0.6, and 0 fields. The MTF is measured
for each field. Each curved line represents the performance of the
lens system 46. The higher the modulation transfers, the better
the preservation of detail by the imaging system. The upper curved
lines designated as S1,T1 represent the performance of the lens
when the spatial frequency is 45 lp/mm. The middle curved lines
designated as S2, T2 represent the performance of the lens when
the spatial frequency is 90 lp/mm. The down curved lines designated
as S3, T3 present the performance of the lens when the spatial frequency
is 120 lp/mm. The higher the modulation transfers, the better the
preservation of detail by the imaging system. When the spatial frequency
is 120 lp/mm, the MTF is higher than 35%. This is considered satisfactory
for general imaging requirements.
Referring to FIG. 5, the lowest value of the relative illumination
is about 53%. Usually when the value of relative illumination is
higher than 50%, it is considered satisfactory for general requirements.
The optical module 100 may be used in a variety of digital camera
applications, including in personal digital cameras and other very
small electronic devices.
The lens system 46 may be used in a variety of digital camera applications,
including in personal digital cameras and other very small electronic
devices (e.g., web cams and cameras in mobile phones).
While certain specific relationships, materials and other parameters
have been detailed in the above description of preferred embodiments,
the described embodiments can be varied, where suitable, within
the principles of the present invention. It should be understood
that the preferred embodiments have been presented by way of example
only and not by way of limitation. Thus the breadth and scope of
the present invention should not be limited by the above-described
exemplary embodiments, but should be defined according to the following
claims and their equivalents.
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