U.S. patent number RE42,642 [Application Number 12/397,270] was granted by the patent office on 2011-08-23 for imaging lens.
This patent grant is currently assigned to Fujinon Corporation. Invention is credited to Kenichi Sato, Minoru Taniyama.
United States Patent |
RE42,642 |
Sato , et al. |
August 23, 2011 |
Imaging lens
Abstract
An imaging lens comprises, in order from an object side: a first
lens having a positive refractive power; a second lens having a
negative refractive power whose concave surface faces to the
object; and a third lens having a positive refractive power and
having a meniscus form that has a convex surface, facing to the
object, in a portion at and around an optical axis of the imaging
lens; wherein the first, second and third lenses have at least one
aspheric surface, and wherein the imaging lens satisfies
conditional expressions given below: 0.7<f1/f<1.3 (1)
0.3<D2/f<0.5 (2) 1.0<|f2/f|<3.0 (3) 1.2<f3/f<4.0
(4) where f: a focal length of an overall system, f1: a focal
length of the first lens, f2: a focal length of the second lens,
f3: a focal length of the third lens, and D2: a spacing, on the
optical axis, between the first lens and the second lens.
Inventors: |
Sato; Kenichi (Saitama,
JP), Taniyama; Minoru (Saitama, JP) |
Assignee: |
Fujinon Corporation
(Saitama-Shi, JP)
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Family
ID: |
38293941 |
Appl.
No.: |
12/397,270 |
Filed: |
March 3, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
11727606 |
Mar 27, 2007 |
7342729 |
Mar 11, 2008 |
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Foreign Application Priority Data
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Mar 28, 2006 [JP] |
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2006-087410 |
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Current U.S.
Class: |
359/753;
359/716 |
Current CPC
Class: |
G02B
13/0035 (20130101); G02B 9/16 (20130101); G02B
13/22 (20130101); G02B 13/18 (20130101) |
Current International
Class: |
G02B
13/04 (20060101); G02B 13/18 (20060101) |
Field of
Search: |
;359/716,753 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1 348 990 |
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Oct 2003 |
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EP |
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2004-226487 |
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Aug 2004 |
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JP |
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2004-302058 |
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Oct 2004 |
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JP |
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2005-173319 |
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Jun 2005 |
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JP |
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2005-227755 |
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Aug 2005 |
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JP |
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2005-292235 |
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Oct 2005 |
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JP |
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Primary Examiner: Sugarman; Scott J
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch, LLP
Claims
What is claimed is:
1. An imaging lens comprising, in order from an object side: a
first lens having a positive refractive power; a second lens having
a negative refractive power whose concave surface faces to the
object; and a third lens having a positive refractive power and
having a meniscus form that has a convex surface, facing to the
object, in a portion at and around an optical axis of the imaging
lens; wherein the first, second and third lenses have at least one
aspheric surface, and wherein the imaging lens satisfies
conditional expressions given below: 0.7<f1/f<1.3 (1)
.[.0.3<D2/f<0.5 (2).]. .Iadd.0.21.ltoreq.D2/f<0.5 (2')
.Iaddend. 1.0<|f2/f|<3.0 (3) 1.2<f3/f<4.0 (4) where f:
a focal length of an overall system, f1: a focal length of the
first lens, f2: a focal length of the second lens, f3: a focal
length of the third lens, and D2: a spacing, on the optical axis,
between the first lens and the second lens.
2. An imaging lens according to claim 1, wherein a conditional
expression given below is further satisfied: 20<.nu.1-.nu.2 (5)
where .nu.1: Abbe number of the first lens, and .nu.2: Abbe number
of the second lens.
3. An imaging lens according to claim 1, wherein the first lens
has: an object-side surface having a convex form in a portion at
and around the optical axis; and an image-side surface having all
aspheric form and having a positive refractive power increasing as
a peripheral portion thereof is neared, and the second lens has a
meniscus form in a portion at and around the optical axis.
4. An imaging lens according to claim 2, wherein the first lens
has: an object-side surface having a convex form in a portion at
and around the optical axis; and an image-side surface having an
aspheric form and having a positive refractive power increasing as
a peripheral portion thereof is neared, and the second lens has a
meniscus form in a portion at and around the optical axis.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an imaging lens to be mounted on
an imaging apparatus, e.g. a digital still camera, a
camera-equipped cellular phone or a personal digital assistant
(PDA), that uses an imaging device, such as a CCD (charge coupled
device) or a CMOS (complementary metal oxide semiconductor).
2. Description of the Related Art
Recently, size reduction and pixel-density increase have advanced
conspicuously for the imaging devices, such as of CCDs and CMOSs.
This in turn requires an imager proper and a lens for mount
thereon, that are smaller in size but higher in performance. For
size reduction, there is a need to reduce the overall length and
the height (diametrical reduction orthogonal to the optical axis).
Meanwhile, besides size reduction, the imaging optical system
generally is required to have a telecentricity, i.e. characteristic
to place the main rays of light incident upon the imaging device at
an angle nearly parallel with the optical axis (incident angle upon
the image plane nearly zero relative to the normal line to the
image plane). In order to secure a telecentricity, it is
advantageous to arrange an optical aperture stop in a position as
close as possible to the object. JP-A-2005-292235 describes an
imaging lens having totally three lenses wherein its optical
aperture stop is arranged in a position closest to the object.
Meanwhile, JP-A-2004-302058 discloses an imaging lens having
totally three lenses wherein its optical aperture stop is arranged
at between the first lens and the second lens.
In the meanwhile, as pixel-density increases for the imager, there
is a trend required for the imager for taking a still image to
provide a mechanical shutter in order to reduce signal noises at
the imager. Where providing a shutter, it is advantageous to
arrange it in a position close to the optical aperture stop in
order to reduce the unevenness of light amount. Meanwhile, for an
imaging lens arranged with three lenses, the optical aperture stop
is advantageously arranged in a position as close as possible to
the object, e.g. in a position in front or back of the first lens,
in order to secure a telecentricity as noted above. However, it is
unadvantageous to arrange the shutter mechanism in front of the
first lens or closest to the object, in respect of size reduction.
For this reason, it can be contemplated to provide a shutter
mechanism in a position inside the lens system, i.e. at between the
first and second lenses. For this purpose, there is a desire to
develop a lens having a high aberrational performance compatible
with pixel-density increase while securing sufficiently an
air-spacing at between the first and second lenses, in order to
arrange a shutter mechanism in an imaging lens arranged with three
lenses. JP-A-2005-292235 includes example 3 that a spacing is
secured comparatively broad at between the first and second lenses,
thus providing a lens arrangement advantageous in arranging a
shutter mechanism. However, there is a desire for developing a lens
which is more advantageous in shutter mechanism arrangement and
higher in aberrational performance than that structure.
SUMMARY OF THE INVENTION
The present invention has been made in view of the foregoing
problem, and it is an object thereof to provide a small-sized
high-performance imaging lens that an interspacing is secured
sufficiently to arrange a shutter mechanism therein while
maintaining a high aberrational performance compatible with
pixel-density increase.
An imaging lens comprises, in order from an object side: a first
lens having a positive refractive power; a second lens having a
negative refractive power whose concave surface faces to the
object; and a third lens having a positive refractive power and
having a meniscus form that has a convex surface, facing to the
object, in a portion at and around an optical axis of the imaging
lens; wherein the first, second and third lenses have at least one
aspheric surface, and wherein the imaging lens satisfies
conditional expressions given below: 0.7<f1/f<1.3 (1)
0.3<D2/f<0.5 (2) 1.0<|F2/f|<0.3 (3) 1.2<f3/f<4.0
(4) where
f: a focal length of an overall system,
f1: a focal length of the first lens,
f2: a focal length of the second lens,
f3: a focal length of the third lens, and
D2: a spacing, on the optical axis, between the first lens and the
second lens.
The imaging lens of the invention can be reduced in size by
providing the lenses with a proper form and refractive power
through use of a lens arrangement having totally three lenses.
Meanwhile, by satisfying the conditional expression (2), a spacing
can be secured broad at between the first lens and the second lens.
This is advantageous in arranging a shutter mechanism therein.
Meanwhile, by satisfying the conditional expressions (1), (3) and
(4), power distribution is optimized over the lenses, thus
maintaining a high aberrational performance compatible with
pixel-density increase.
In the imaging lens of the invention, preferably a conditional
expression given below is further satisfied: 20<.nu.1-.nu.2 (5)
where
.nu.1: Abbe number of the first lens, and
.nu.2: Abbe number of the second lens.
Meanwhile, in the imaging lens of the invention, preferably the
first lens has an object-side surface made in a convex form in a
portion at and around the optical surface and an image-side surface
made in an aspheric form having a positive refractive power
increasing as a peripheral portion thereof is neared, the second
lens being in a meniscus form in a portion at and around the
optical axis. This makes it easy to satisfy the conditional
expressions, thus easily improving the performance.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a lens sectional view corresponding to an imaging lens
according to example 1 of the invention;
FIG. 2 is a lens sectional view corresponding to an imaging lens
according to example 2 of the invention;
FIG. 3 is a lens sectional view corresponding to an imaging lens
according to example 3 of the invention;
FIG. 4 is a lens sectional view corresponding to an imaging lens
according to example 4 of the invention;
FIGS. 5A and 5B are figures showing the lens data of an imaging
lens according to example 1 of the invention, wherein FIG. 5A shows
the basic lens data and FIG. 5B shows the lens data concerning an
aspheric surface;
FIGS. 6A and 6B are figures showing the lens data of an imaging
lens according to example 2 of the invention, wherein FIG. 6A shows
the basic lens data and FIG. 6B shows the lens data concerning an
aspheric surface;
FIGS. 7A and 7B are figures showing the lens data of an imaging
lens according to example 3 of the invention, wherein FIG. 7A shows
the basic lens data and FIG. 7B shows the lens data concerning an
aspheric surface;
FIGS. 8A and 8B are figures showing the lens data of an imaging
lens according to example 4 of the invention, wherein FIG. 8A shows
the basic lens data and FIG. 8B shows the lens data concerning an
aspheric surface;
FIG. 9 shows a figure showing, collectively on each example, the
values related to the conditional expressions
FIGS. 10A to 10C are aberration diagrams showing the aberrations of
the imaging lens according to example 1 of the invention, wherein
FIG. 10A shows a spherical aberration, FIG. 10B shows an
astigmatism and FIG. 10C shows a distortion;
FIGS. 11A to 11C are aberration diagrams showing the aberrations of
the imaging lens according to example 2 of the invention, wherein
FIG. 11A shows a spherical aberration, FIG. 11B shows an
astigmatism and FIG. 11C shows a distortion;
FIGS. 12A to 12C are aberration diagrams showing the aberrations of
the imaging lens according to example 3 of the invention, wherein
FIG. 12A shows a spherical aberration, FIG. 12B shows an
astigmatism and FIG. 12C shows a distortion; and
FIGS. 13A to 13C are aberration diagrams showing the aberrations of
the imaging lens according to example 4 of the invention, wherein
FIG. 13A shows a spherical aberration, FIG. 13B shows an
astigmatism and FIG. 13C shows a distortion.
DETAILED DESCRIPTION OF THE INVENTION
With reference to the drawings an embodiment of the present
invention will now be explained in detail.
FIG. 1 shows a first arrangement example of an imaging lens
according to an embodiment of the invention. This arrangement
example corresponds to the lens arrangement in a first numerical
example (FIGS. 5A and 5B), referred later. FIG. 2 shows a second
arrangement example wherein this arrangement example corresponds to
the lens arrangement of a second numerical example (FIGS. 6A, 6B),
referred later. FIG. 3 shows a third arrangement example wherein
this arrangement example corresponds to the lens arrangement of a
third numerical example (FIGS. 7A, 7B), referred later. FIG. 4
shows a fourth arrangement example wherein this arrangement example
corresponds to the lens arrangement of a fourth numerical example
(FIGS. 8A, 8B), referred later. In FIGS. 1 to 4, reference numeral
Ri represents a radius-of-curvature of an i-th surface whose
subscript is provided to increase as the image is neared provided
that the constituent element closest to the object has a surface
taken as the first. Reference numeral Di represents a
surface-to-surface spacing between the i-th and (i+1)-th surfaces
on an optical axis Z1. Note that the explanation below is basically
on the first arrangement example shown in FIG. 1 because the
arrangements are similar one to another in respect of the basic
structure.
The imaging lens is suitable for use in various imaging appliances
using imaging devices such as of CCD and CMOS, e.g. digital still
cameras, camera-equipped cellular phone and personal digital
assistants. The imaging lens includes a first lens G1, a second
lens G2 and a third lens G3, in the closer order to the object on
the optical axis Z1. At least one of the surfaces of the first,
second and third lenses G1, G2, G3 is made aspheric.
An optical aperture stop St is preferably arranged as close as
possible to the object, in order to secure a telecentricity. In the
arrangement examples of FIGS. 1, 2 and 4, an aperture stop St is
arranged in back of the first lens Gl. In the FIG. 3 arrangement
example, an aperture stop St is arranged in front of the first lens
G1, i.e. in a position of the lens system closest to the object. A
shutter 10 is arranged between the first lens G1 and the second
lens G2.
An imaging device, such as a CCD, is arranged on an image surface
Simg of the imaging lens. Between the third lens G3 and the imaging
device, various optical members GC are arranged in accordance with
the camera structure the relevant lens is to be mounted. For
example, plate-like optical members are arranged which include an
image-surface-protecting cover glass and an infrared-blocking
filter.
The first lens G1 has a positive refractive power. The first lens
G1 has a surface, close to the object, made convex in an area
nearby the optical axis. In the arrangement examples of FIGS. 1 and
2, the first lens G1 is made convex in form at both surfaces in
areas nearby the optical axis, wherein the image-side surface is
made in an aspheric form where positive refractive power increases
as the periphery is neared. In the arrangement examples of FIGS. 3
and 4, the first lens G1 is made as a positive meniscus form whose
convex surface faces, in its area nearby the optical surface, to
the object.
The second lens G2 has a concave surface facing to the object, thus
having a negative refractive power. The second lens G2 is
preferably in a meniscus form in an area nearby the optical axis.
This makes it easy to satisfy the numerical range of conditional
expression (3), referred later.
The third lens G3 is made in a positive meniscus form having a
convex surface facing to the object, in an area nearby the optical
axis. The third lens G3 is a lens arranged closest to the image
surface. Accordingly, in the third lens G3, luminous flux is
separated at each angle-of-view as compared to the first and second
lenses G1, G2. By properly using the aspheric surface in the third
lens G3, aberration can be easily corrected at each angle-of-view,
thus making it easy to correct for curvature-of-field and
distortion. In addition, a telecentricity can be easily secured.
For this reason, the image-side surface of the third lens G3 is
preferably made concave in an area nearby the optical axis and
convex at the peripheral area thereof.
The imaging lens satisfies the following conditions, wherein f is a
focal length of the system overall, f1 is a focal length of the
first lens G1, f2 is a focal length of the second lens G2, f3 is a
focal length of the third lens G3, and D2 is a spacing between the
first lens G1 and the second lens G2 on the optical axis Z1.
0.7<f1/f<1.3 (1) 0.3<D2/f<0.5 (2) 1.0<|f2/f|<3.0
(3) 1.2<f3/f<4.0 (4)
The imaging lens preferably satisfies further the following
conditional expression. In the expression, .nu.1 is an Abbe number
of the first lens G1 and .nu.2 is an Abbe number of the second lens
G2. 20<.nu.1-.nu.2 (5)
Description is now made on the operation and effect of the imaging
lens constructed as above.
The imaging lens can provide a lens system that is advantageous in
reducing the overall length and securing a telecentricity by virtue
of the aperture stop St arranged in front or back of the first lens
through use of the arrangement of as less as three lenses in total.
Furthermore, by satisfying the conditional expressions,
optimization is achieved for the material of and the power
distribution over the lenses, thus sufficiently securing an
interspacing for arranging a shutter mechanism therein while
maintaining a high aberrational performance compatible with
pixel-density increase. Meanwhile, in the imaging lens, aberration
can be corrected further effectively by optimizing the aspheric
surfaces. In order to cope with an imaging device having high pixel
density, there is a need to provide a telecentricity, i.e.
characteristic to place the main rays of light incident upon the
imaging device at an angle nearly parallel with the optical axis
(incident angle upon the image plane nearly zero relative to the
normal line to the image plane). In the imaging lens, the
image-side surface of the third lens G3 which is the final lens
surface closest to the imaging device for example is made concave
toward the image in an area nearby the optical axis and convex
toward the image in a peripheral area thereof, aberration can be
properly corrected at each angle-of-view thus regulating the angle
of an incident luminous flux upon the imaging device. This makes it
possible to relieve the unevenness of light amount over the entire
image surface, and provides an advantage in correcting for
curvature-of-field and distortion.
The conditional expression (1) concerns the focal length f1 of the
first lens G1. In case exceeding the numerical range of the same,
the power of the first lens G1 is excessively small thus making it
difficult to reduce the overall length. Meanwhile, in case going
below the same, corrections are difficult for curvature-of-field
and astigmatism and further the angle-at-exit-pupil is provided
unpreferably excessively great.
The conditional expression (2) concerns the spacing D2 between the
first lens G1 and the second lens G2 and the overall focal length
f. In case exceeding the numerical range of the same, it is
difficult to reduce the overall length. Meanwhile, in case going
below the same, the spacing D2 cannot be secured sufficiently
between the first lens G1 and the second lens G2, thus unpreferably
making it difficult to arrange a shutter mechanism.
The conditional expression (3) concerns the focal length f2 of the
second lens G2. In case the numerical range is exceeded, the power
of the second lens G2 is excessively small, thus making it
difficult to reduce the overall length. Meanwhile, in case going
below the same, corrections are unpreferably difficult for
curvature-of-field, astigmatism and so on. The conditional
expression (4) concerns the focal length f3 of the third lens G3.
In case going out any of the upper and lower limits of the
numerical range, power is out of balance with the second lens G2
thus unpreferably snaking it difficult to correct for aberrations
while maintaining the overall length short. The conditional
expression (5) concerns the Abbe number of the first and second
lenses G1, G2. In case going below the numerical range of the same,
chromatic aberration cannot be unpreferably corrected to a
sufficient extent.
As explained so far, according to the imaging lens of the
embodiment, predetermined conditional expressions are satisfied by
means of the arrangement having lenses as less as three in total,
thus optimizing the form, refractive power and arrangement of the
lenses. This can realize a lens system small in size but high in
performance wherein interspacing is sufficiently secured to arrange
a shutter mechanism therein while maintaining a high aberrational
performance compatible with pixel-density increase.
EXAMPLES
Now explanation is made on concrete numerical examples of the
imaging lens in the present embodiment. First to fourth numerical
examples are explained collectively in the following.
FIGS. 5A and 5B show, as example 1, the concrete lens data
corresponding to the FIG. 1 imaging lens arrangement. Particularly,
FIG. 5A shows the basic lens data while FIG. 5B shows the data
concerning aspheric surfaces. In the column of the surface number
Si of the lens data shown in FIG. 5A, the surface of the
constituent element closest to the object is taken as the first, to
show the number of the i-th (i=1-8) surface given with a symbol
gradually increasing as the image end is neared. In the column of
the radius-of-curvature Ri, shown is a value of radius-of-curvature
(mm) of the i-th surface as counted in the closer order to the
object correspondingly to the symbol Ri attached in FIG. 1. In the
column of the surface-to-surface spacing Di, shown is a spacing
(mm) on the optical axis between the i-th surface Si and the
(i+1)-th surface (Si+1) as similarly counted in the closer order to
the object. Ndj represents a value of refractive index at d-line
(wavelength: 587.6 nm) of the optical element as counted in the
closer order to the object. In the column of .nu.dj, shown is an
Abbe number at d-line of the optical element located j-th in the
closer order to the object. FIG. 5A also shows the values of
overall-system paraxial focal length f (mm) and F-number (FNO.), as
various ones of data.
The imaging lens in example 1 is made all aspheric in form at both
surfaces of the first, second and third lenses G1, G2, G3. In the
basic lens data in FIG. 5A, the aspheric surfaces are shown with
numerals of radius-of-curvature in an area at or around the optical
axis. In the numeral shown as aspheric data in FIG. 5B, the symbol
"E" represents that the following numeral is a "power exponent"
having a bottom of 10 wherein the numeral represented by an
exponential function, having such a bottom 10, is to be multiplied
on the numeral preceding "E". For example, "1.0E-02" if given
represents "1.0.times.10.sup.-2".
The aspheric data is described with the values of coefficients
A.sub.n, K that are given in an aspheric-form expression
represented by the following expression (A). Specifically, Z
represents the length (mm) of a vertical line drawn from a point,
on the aspheric surface and located at a height h with respect to
the optical axis Z1, onto a tangential plane to the apex of the
aspheric surface (a plane vertical to the optical axis Z1). The
imaging lens in example 1 is expressed by effectively using
third-to-tenth order coefficients A.sub.3-A.sub.10 on the
assumption the spherical surfaces are of respective aspheric
coefficients A.sub.n.
Z=Ch.sup.2/{1+(1-KC.sup.2h.sup.2).sup.1/2}+.SIGMA.A.sub.nh.sup.n
(A)
(n: integer equal to or greater than 3)
where
Z: depth of the aspheric surface (mm)
h: distance (height) of from the optical surface to the lens
surface (mm)
K: eccentricity (second-order aspheric coefficient)
C: paraxial curvature=1/R (R: paraxial radius-of-curvature)
A.sub.n: n-th aspheric coefficient
Similarly to the imaging lens of example 1, FIGS. 6A and 6B show,
as example 2, the concrete lens data corresponding to the imaging
lens arrangement shown in FIG. 2. Likewise, FIGS. 7A and 7B show,
as example 3, the concrete lens data corresponding to the imaging
lens arrangement shown in FIG. 3. Likewise, FIGS. 8A and 8B show,
as example 4, the concrete lens data corresponding to the imaging
lens arrangement shown in FIG. 4. Incidentally, concerning any one
of the imaging lenses of examples 2 to 4, the first, second and
third lenses G1, G2, G3 are made aspheric in form at both surfaces
thereof similarly to example 1.
FIG. 9 shows collectively the values related to the foregoing
conditional expressions. As seen from FIG. 9, the examples have
values fallen within the numeral ranges of the respective
conditional expressions.
FIGS. 10A to 10C respectively show a spherical aberration, an
astigmatism and a distortion, of the imaging lens of example 1. In
the aberration diagrams, aberrations are shown at d-line as a
reference wavelength. In the spherical aberration diagram, also
shown are aberrations at g-line (wavelength: 435.8 nm) and at
C-line (wavelength: 656.3 nm). In the astigmatism diagram, the
solid line represents an aberration in a sagittal direction while
the broken line an aberration in a tangential direction, wherein
FNO. represents an F value while .omega. represents a half
angle-of-view.
Likewise, FIGS. 11A-11C show the aberrations as to the imaging lens
of example 2, FIGS. 12A-12C show the aberrations as to the imaging
lens of example 3, and FIGS. 13A-13C show the aberrations as to the
imaging lens of example 4.
As can be seen from the numerical-value data and the aberration
diagrams, optimization is achieved for the lens material, the lens
surface form and the power distribution over lenses in each example
by the totally three-lens arrangement, thus realizing an imaging
lens system smaller in size but higher in performance wherein
interspacing is sufficiently secured to arrange a shutter mechanism
therein.
The invention is not limited to the embodiment and examples but can
be modified in various ways. For example, the radius-of-curvature,
the surface-to-surface spacing, the refractive index, etc. of the
lenses are not limited to the values but can take other values.
According to the imaging lens of the invention, by satisfying
predetermined conditional expressions through use of an arrangement
of as less as totally three lenses, optimization is achieved for
the form, refractive power and arrangement of the lenses. Thus, a
small-sized high-performance lens system can be realized secured
sufficiently with an interspacing to arrange a shutter mechanism
therein while maintaining a high aberrational performance
compatible with pixel-density increase.
The entire disclosure of each and every foreign patent application
from which the benefit of foreign priority has been claimed in the
present application is incorporated herein by reference, as if
fully set forth.
* * * * *