JPH11190817A - Photographic lens and camera using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photographic lens constituted of two-group two lenses having a wide photographing viewing angle without causing the deterioration of various kinds of aberration and the reduction of field illuminance. SOLUTION: The photographic lens 30 is constituted of a 1st meniscus lens 31 having a convex surface on an object side and a 2nd lens 32 whose refractive power is positive and having a high-curvature convex surface on an image side. As for the 1st lens 31 and the 2nd lens 32, at least one surface of each lens is formed to an aspherical shape. Thus, the increase of spherical aberration is suppressed and the curvature of a field is reduced. Besides, provided that the synthesized focal distance of the whole photographic lens 30 is expressed by (f), the focal distance of the 1st lens 31 is expressed by (f1), the following condition is satisfied; -1<=f/f1<=0.45. Then, the various kinds of aberration is well-balanced while widening the photographic viewing angle, and also, the reduction of the field illuminance is suppressed. Besides, by curving the object image forming surface in a longitudinal direction so that it may have a concave surface on the object side and adjusting the radius of curvature in the diagonal direction of the curved image forming surface and the diagonal half viewing angle of the photographic lens 30, the curvature of the field is reduced, then the more satisfactory image is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-group two-lens photographing lens suitable for a low-priced camera and a camera using the same.

[0002]

2. Description of the Related Art At present, a great number of types of cameras for photographing are sold according to their uses and functions. Recently, a film unit with a lens has been manufactured and sold by the present applicant so that photographing can be easily enjoyed. The film unit with lens is
An unexposed photographic film is built in the unit body that incorporates a photographic mechanism, such as a photographic lens and shutter device. It is widely used because of its convenience.

The film unit with a lens has an advantage that it can be provided at a low price, and has a very simple configuration because it needs to be manufactured at the lowest possible cost. For example, the photographing lens is positioned and held on a lens holder having a fixed diameter aperture opening, and is focused so that standard photographing can be performed in more photographing opportunities. 3 for film unit with lens
A photographic film having a width of 5 mm is built in, and the photographic lens is configured to have a focal length of 32 mm, a half-diagonal angle of view of 34 °, and an F-number of about 9.5.

A photographing lens mounted on a lens-fitted film unit or a low-priced camera generally comprises one or two lenses, and recently has a two-group, two-element structure because it is advantageous for aberration correction. Mainly used. As a photographic lens having two groups and two lenses, one in which two meniscus lenses are arranged with the concave surfaces of the lenses facing each other,
In many cases, a positive meniscus lens is disposed on the object side and a biconvex lens is disposed on the image side.

[0005] In order to further enhance the appeal and enjoyment of printed photographs, in addition to the conventional standard prints, panoramic prints in which the length in the horizontal direction are doubled, and hi-vision that approximates the aspect ratio of high-definition television broadcasting Various print photographs having different aspect ratios, such as prints, are provided. The way to get these printed photos is
A method that limits the exposure range within the exposure screen specified on the photo film and exposes only within the range specified in a similar shape to the desired print photo. There is a method in which size data is recorded on a photographic film, and only a range corresponding to the print size data in the exposure screen is printed.
In either method, a print photograph of each size can be obtained by changing the screen size on the photograph film.

In a 35 mm-wide photographic film incorporated in the above-described film unit with a lens, the exposure screen is defined to have a standard size of 24 mm × 36 mm (aspect ratio: 1.5). When the upper and lower portions of the exposure screen are slightly trimmed, the size becomes a high definition size (aspect ratio: 1.8), and when the upper and lower portions of the exposure screen are further trimmed, the size becomes a panorama size (aspect ratio: 2.8).

Recently, a photographic film cartridge in which a photographic film having a width of 24 mm is stored in a cartridge body has been sold. In the case of a photographic film having a width of 24 mm, the exposure screen is defined to have a size of 16.7 mm × 30.2 mm of a high vision (aspect ratio: 1.8). When the left and right portions of the exposure screen are trimmed, the standard size (aspect ratio: 1.4) is obtained, and when the upper and lower portions of the exposure screen are trimmed, the panorama size (aspect ratio: 3.2) is obtained. When a 24 mm wide photo film is built in a film unit with a lens, the photographing lens has a half-angle diagonal angle of view of 34 ° and a focal length so that the angle of view is the same as that of a photo film with a 35 mm width. Is set to 25.53 mm.

[0008]

However, since a 24 mm wide photographic film and a 35 mm wide photographic film have different aspect ratios of a standard size where printing is most frequently performed, a film with a lens having a built-in 24 mm wide photographic film. With the unit, it is not possible to obtain a printed photograph of an image exactly the same as that containing a photographic film having a width of 35 mm.

In order to solve the above-mentioned problem, in the case of a photographic film having a width of 24 mm, the left and right portions of the exposure screen are trimmed to a standard size. It is conceivable that the configuration is the same as that for 35 mm width. However, when the angle of view of the image is widened, the spherical aberration increases with this, and not only the image at the periphery of the image surface deteriorates, but also the illuminance of the image surface decreases.

The present invention has been made in view of the above circumstances, and does not cause deterioration of various aberrations or decrease of image plane illuminance.
It is an object of the present invention to provide a two-group two-lens photographing lens having a wide photographing angle of view.

[0011]

In order to achieve the above object, a photographic lens according to the present invention has, in order from the object side, a first meniscus lens having a convex surface facing the object side and a positive lens. And a second lens having a convex surface having a strong curvature directed to the image side. At least one surface of each of the first lens and the second lens is formed in an aspherical shape, and a combined focal point of the entire photographing lens is formed. Assuming that the distance is f and the focal length of the first lens is f1, the condition of -1 ≦ f / f1 ≦ 0.45 is satisfied.

A camera equipped with a photographic lens having the above-mentioned structure is characterized in that a film surface on which a subject image is formed by the photographic lens is curved so that its longitudinal direction is concave toward the object side, and the curved film is formed. The radius of curvature in the diagonal direction of the surface is R _F , and the diagonal half angle of view of the taking lens is ω
In this case, the condition of 8.0 ≦ (R _F tanω) ^1/2 ≦ 12.0 is satisfied.

[0013]

According to the present invention, at least one surface of each of the two lenses constituting the taking lens is formed in an aspherical shape, thereby suppressing an increase in spherical aberration and reducing field curvature. . By satisfying the condition of −1 ≦ f / f1 ≦ 0.45, the angle of view can be widened,
The balance of various aberrations can be maintained in a good state, and a decrease in image plane illuminance can be suppressed. If the lower limit of the above conditional expression is exceeded,
The overall length of the photographic lens becomes longer, which leads to an increase in the size of the camera on which it is mounted. If the overall length of the lens is to be reduced, the thickness of the first lens is increased, which makes it difficult to ensure the lens performance and increases the manufacturing cost because the molding material must be increased. On the other hand, when the value exceeds the upper limit of the conditional expression, the first lens has a positive refractive power.
Although the overall length of the lens can be reduced, the angle of incidence of the off-axis light beam on the second lens becomes acute, the spherical aberration increases, the image at the periphery of the image surface deteriorates, and the image surface illuminance decreases. .

In the camera equipped with the photographing lens, a film surface on which a subject image is formed is curved so that its longitudinal direction is concave toward the object side, and conditional expression 8.0 ≦ (R _F · By configuring so as to satisfy (tan ω) ^1/2 ≦ 12.0, the curvature of field can be reduced and a better image can be obtained. If the lower limit of the conditional expression is exceeded, the film surface is greatly curved, which is advantageous for alleviating the curvature of field. However, on the other hand, the distortion is excessively corrected and the image is deteriorated. When the value exceeds the upper limit of the conditional expression, not only the field curvature cannot be sufficiently corrected, but also the effect of correcting distortion decreases.

[0015]

FIG. 17 shows the structure of a unit body of a film unit with a lens incorporating a photographic lens according to the present invention. The unit main body 10 includes a main body base 11, and a front cover 12 and a rear cover 13 that cover the main body base 11 from front and rear. The cartridge body 1 is provided on both sides of the body base 11.
And a photo film 1 rolled out from the cartridge body 15 in which the cartridge 4 is stored.
A film roll chamber 17 for housing the film 6 is provided, and an aperture 18 for defining an exposure range of the photographic film 16 is formed between them. On the front surface of the aperture 18, an exposure unit 19, which is a unit formed by integrally assembling a photographing lens 30, a finder optical system, a film frame stopping mechanism, a shutter mechanism, and the like, is mounted. The exposure unit 19 is sandwiched between the front cover 12 and the rear cover 13. Is done.

A film support surface 21 is formed on the rear cover 13 at a portion facing the aperture 18. A gap between the film support surface 21 and the main body base 11 allows the cartridge chamber 15 and the film roll chamber 17 to be separated from each other. A communicating film traveling path 22 is formed. Then, the photo film 16 is moved from the film roll chamber 17 side to the cartridge chamber 15 side every time a photograph is taken,
The exposed portion is wound into the cartridge body 14 and the unexposed portion is positioned on the back side of the aperture 18. The width of the photo film 16 is 24 mm
The size of the exposure screen defined by the aperture 18 is 16.7 × 30.2 mm.

Aperture 18 and film support surface 2
1 is curved so that the running direction of the photographic film 16 becomes concave toward the object side. As a result, the film surface of the photographic film 16 positioned on the back side of the aperture 18 also curves concavely toward the object side. When the radius of curvature of the curved film surface of the photographic film 16 in the diagonal direction is R _F and the half angle of view of the photographic lens 30 is ω, “8.0 ≦ (R _F tan ω) ^{1 / 2} ≦ 12.
0 ”.

As shown in FIG. 1, the taking lens 30 includes a first meniscus lens 31 having a convex surface facing the object side,
Second with positive refractive power, convex surface with strong curvature on the image side
The lens 32 is positioned on the object side of the aperture opening 33. Object-side lens surface 31 of first lens 31
a, and the image-side lens surface 32b of the second lens 32,
It is formed in an aspherical shape. On the image side of the aperture 33,
A light shielding plate 35 for regulating the optical path of the off-axis light beam is provided. In the drawing, reference numeral 31b denotes an image-side surface of the first lens 31, and reference numeral 32a denotes an object-side surface of the second lens 32. The taking lens 30 satisfies the condition of “−1 ≦ f / f1 ≦ 0.45”, where f is the combined focal length of the entire optical system and f1 is the focal length of the first lens 31.

"First Embodiment" The specifications of the taking lens of the first embodiment are as follows. f = 24mm f1 = -1168.07mm L = 23.60mm ω = 34.9 ° R F = -105mm F no = 8.5

In the above data, f is the combined focal length of the entire photographing lens 30, f1 is the focal length of the first lens 31, L
Denotes the full length of the lens, ω denotes a diagonal half angle of view, R _F denotes a radius of curvature of a curved film surface of the photographic film 16 serving as an image surface in a diagonal direction, and F _no denotes an F number.

The characteristic value "f / f1" in the present invention.
And each value of "(R _F · tanω) ^1/2" is, f / f1 = 24 / ( - 1168.07) ≒ -0.02 (R F · tanω) 1/2 = (- 105 · tan34. 9) a ^1/2 ≒ 8.56, meets -1 ≦ f / f1 ≦ 0.45 8.0 ≦ (R F · tanω) the conditional expressions ^1/2 ≦ 12.0.

The lens data of the first embodiment is shown in Table 1 below.
Shown in The surface number i is a number assigned to the lens surface of each lens in order from the object side, and the surface interval D represents the lens thickness or the air interval between the next surface (unit: mm).

[0023]

[Table 1]

The aspheric surface is represented by the following conditional expression: Z = ch ² / [1+ {1− (1 + K) c ² h ² }] +
It is formed to satisfy Ah ⁴ + Bh ⁶ + Ch ⁸ + Dh ¹⁰ . In the equation, c represents the reciprocal of the radius of curvature (= 1 / R), and h represents the height of the light beam from the optical axis. Table 2 shows the aspheric coefficient.

[0025]

[Table 2]

FIG. 2 shows an aberration diagram of the taking lens of the first embodiment. In the figure, (A) represents spherical aberration, (B) represents astigmatism, and (C) represents distortion.
Symbols e and g in the spherical aberration diagram of (A) are e-line (546.1 nm) and g-line (435.8 nm), respectively.
And S and T in the astigmatism diagram of FIG. 3B represent aberrations with respect to the spherical and meridional image planes, respectively. Further, the curvature of the radius of curvature R _F with respect to the spherical image surface was measured, and the result was also indicated as a symbol F in the astigmatism diagram of FIG.

[Second Embodiment] FIG. 3 shows a second embodiment of the configuration of the taking lens, which has the same lens configuration as the first embodiment. The reference numerals in the figure are used in common with the first embodiment shown in FIG. The specifications of the taking lens of the second embodiment are as follows.

F = 24.0129908 mm f1 = −135.66 mm L = 24.02 mm ω = 35.4 ° R _F = −105 mm F _no = 8.5

In the second embodiment, the characteristic value "f /
f1 ", each value of" (R _F · tanω) ^1/2 "is, f / f1 = 24.0129083 / ( - 135.66) ≒ -0.177 (R F · tanω) 1/2 = (- 105 · tan35.4) is ^1/2 ≒ 8.64, satisfies -1 ≦ f / f1 ≦ 0.45 8.0 ≦ (R F · tanω) the conditional expressions ^1/2 ≦ 12.0 I have.

Tables 3 and 4 show lens data and aspherical coefficients of the second embodiment. The definition of the aspheric surface is
All the embodiments described below are the same as the first embodiment.

[0031]

[Table 3]

[0032]

[Table 4]

FIG. 4 shows aberration diagrams of the taking lens of the second embodiment. In the figures, (A), (B), and (C) represent spherical aberration, astigmatism, and distortion in all the embodiments described below, respectively, as in the first embodiment.
The reference numerals in each aberration diagram are used in common with the first embodiment shown in FIG.

[Third Embodiment] FIG. 5 shows a third embodiment of the configuration of the taking lens, in which the light shielding plate 35 is not arranged in the optical path of the taking lens 30. The specifications of the taking lens of the third embodiment are as follows.

F = 24.0000887 mm f1 = −100.08 mm L = 24.10 mm ω = 35.5 ° R _F = −105 mm F _no = 8.5

In the third embodiment, the characteristic value "f /
f1 ", each value of" (R _F · tanω) ^1/2 "is, f / f1 = 24.0008987 / ( - 100.08) ≒ -0.24 (R F · tanω) 1/2 = (- 105 · tan35.5) is ^1/2 ≒ 8.65, satisfies -1 ≦ f / f1 ≦ 0.45 8.0 ≦ (R F · tanω) the conditional expressions ^1/2 ≦ 12.0 I have.

Tables 5 and 6 show lens data and aspherical coefficients of the third embodiment.

[0038]

[Table 5]

[0039]

[Table 6]

FIG. 6 shows an aberration diagram of the taking lens of the third embodiment.

[Fourth Embodiment] FIG. 7 shows a fourth embodiment of the configuration of the taking lens, which has the same configuration as the third embodiment. The specifications of the taking lens of the fourth embodiment are as follows.

F = 22.99997891 mm f1 = 91.85 mm L = 22.80 mm ω = 35.8 ° R _F = −105 mm F _no = 8.5

The characteristic value "f /
f1 ", the value of" (R _F · tanω) ^1/2 "is, f / f1 = 22.9997891 / 91.85 ≒ 0.25 (R F · tanω) 1/2 = (- 105 · tan35. 8) ^1/2 ≒ 8.72, and satisfies the following conditional expressions: −1 ≦ f / f1 ≦ 0.45 8.0 ≦ (R _F tanω) ^1/2 ≦ 12.0.

Tables 7 and 8 show lens data and aspherical coefficients of the fourth embodiment.

[0045]

[Table 7]

[0046]

[Table 8]

FIG. 8 shows an aberration diagram of the taking lens of the fourth embodiment.

[Fifth Embodiment] FIG. 9 shows a fifth embodiment of the configuration of the taking lens, which is another embodiment in which the configuration is the same as that of the third embodiment. The specifications of the taking lens of the fifth embodiment are as follows.

F = 25 mm f1 = 3045.01 mm L = 25.00 mm ω = 34.1 ° R _F = −105 mm F _no = 8.5

The characteristic value “f /
f1 ”and“ (R _F tan ω) ^1/2 ”are f / f 1 = 25 / 3045.01 ≒ 0.008 (R _F tan ω) ^1/2 = (− 105 tan 34.1) ^1/2 ≒ is 8.43, which satisfies the conditional expressions -1 ≦ f / f1 ≦ 0.45 8.0 ≦ (R F · tanω) 1/2 ≦ 12.0.

Tables 9 and 10 show lens data and aspherical coefficients of the fifth embodiment.

[0052]

[Table 9]

[0053]

[Table 10]

FIG. 10 shows aberration diagrams of the taking lens of the fifth embodiment.
Shown in

[Sixth Embodiment] FIG. 11 shows a sixth embodiment of the construction of the taking lens, which is another embodiment in which the construction is the same as that of the first embodiment. The specifications of the taking lens of the sixth embodiment are as follows.

F = 24 mm f1 = −50.95 mm L = 24.50 mm ω = 36.1 ° R _F = −105 mm F _no = 8.5

In the sixth embodiment, the characteristic value "f /
f1 ", the value of" (R _F · tanω) ^1/2 "is, f / f1 = 24 / ( - 50.95) ≒ -0.47 (R F · tanω) 1/2 = (- 105 · Tan36.1) is ^1/2 ≒ 8.75, meets -1 ≦ f / f1 ≦ 0.45 8.0 ≦ (R F · tanω) the conditional expressions ^1/2 ≦ 12.0.

Tables 11 and 12 show lens data and aspherical coefficients of the sixth embodiment.

[0059]

[Table 11]

[0060]

[Table 12]

FIG. 12 shows aberration diagrams of the taking lens of the sixth embodiment.
Shown in

[Seventh Embodiment] FIG. 13 shows a seventh embodiment of the configuration of the taking lens, which is still another embodiment in which the configuration is the same as that of the first embodiment. The specifications of the taking lens of the seventh embodiment are as follows.

F = 23 mm f1 = −48.19 mm L = 23.90 mm ω = 37.9 ° R _F = −150 mm F _no = 8.5

The characteristic value “f /
f1 ", each value of" (R _F · tanω) ^1/2 "is, f / f1 = 23 / ( - 48.19) ≒ -0.48 (R F · tanω) 1/2 = (- 150 · Tan37.9) is ^1/2 ≒ 10.08, satisfies -1 ≦ f / f1 ≦ 0.45 8.0 ≦ (R F · tanω) the conditional expressions ^1/2 ≦ 12.0.

Tables 13 and 14 show lens data and aspheric coefficients of the seventh embodiment.

[0066]

[Table 13]

[0067]

[Table 14]

FIG. 14 shows aberration diagrams of the taking lens of the seventh embodiment.
Shown in

[Eighth Embodiment] FIG. 15 shows an eighth embodiment of the configuration of the taking lens, which is still another embodiment in which the configuration is the same as that of the first embodiment. The specifications of the taking lens of the eighth embodiment are as follows.

F = 23 mm f1 = −40.84 mm L = 24.60 mm ω = 38.5 ° R _F = −150 mm F _no = 8.0

In the eighth embodiment, the characteristic value “f /
f1 ", each value of" (R _F · tanω) ^1/2 "is, f / f1 = 23 / ( - 40.84) ≒ -0.56 (R F · tanω) 1/2 = (- 150 · tan38.5) ^1/2 ≒ 10.92, and satisfies the following conditional expressions: −1 ≦ f / f1 ≦ 0.45 8.0 ≦ (R _F tanω) ^1/2 ≦ 12.0.

Tables 15 and 16 show lens data and aspherical coefficients of the eighth embodiment.

[0073]

[Table 15]

[0074]

[Table 16]

FIG. 16 shows aberration diagrams of the taking lens of the eighth embodiment.
Shown in

In all of the above embodiments, the stop aperture is arranged on the image side of the second lens.
It is also possible to arrange a diaphragm aperture between the lens and the lens.
In this case, it is preferable to dispose a light shielding plate for restricting the optical path of the off-axis light beam on the image side of the second lens.

The photographic lens of the present invention is not limited to the case where it is mounted on a film unit with a lens, but can be used for any fixed focus type camera. In this case, also in this case, the longitudinal direction of the film surface is curved so as to be concave toward the object side,
So as to satisfy _{"8.0 ≦ (R F · tanω)} 1/2 ≦ 12.0 " following condition, to set a diagonal half angle of the curvature radius and the imaging lens in the diagonal direction of the film plane preferable.

[0078]

As described above, according to the photographic lens of the present invention, at least one surface of each of the two lenses constituting the photographic lens is formed to be aspherical, thereby increasing spherical aberration. And field curvature can be reduced. In addition, by adjusting the combined focal length of the entire photographing lens and the focal length of the first lens located on the object side, the balance of various aberrations can be maintained in a good state while the photographing angle of view is widened. A decrease in surface illuminance can be suppressed.

In a camera equipped with the taking lens of the present invention, a film surface on which a subject image is formed is curved so that its longitudinal direction is concave toward the object side, and a diagonal of the curved film surface is used. By adjusting the radius of curvature in the direction and the diagonal half angle of view of the taking lens, the field curvature is reduced,
A better image can be obtained.

[Brief description of the drawings]

FIG. 1 is an optical path diagram showing a first embodiment of a taking lens of the present invention.

FIG. 2 is an aberration diagram of the photographing lens shown in FIG. 1;
(A) represents spherical aberration, (B) represents astigmatism, and (C) represents distortion.

FIG. 3 is an optical path diagram showing a second embodiment of the taking lens of the present invention.

FIG. 4 is an aberration diagram of the taking lens shown in FIG. 3;
(A) represents spherical aberration, (B) represents astigmatism, and (C) represents distortion.

FIG. 5 is an optical path diagram showing a third embodiment of the taking lens of the present invention.

FIG. 6 is an aberration diagram of the taking lens shown in FIG. 5;
(A) represents spherical aberration, (B) represents astigmatism, and (C) represents distortion.

FIG. 7 is an optical path diagram showing a fourth embodiment of the taking lens of the present invention.

8 is an aberration diagram of the photographing lens shown in FIG. 7,
(A) represents spherical aberration, (B) represents astigmatism, and (C) represents distortion.

FIG. 9 is an optical path diagram showing a fifth embodiment of the taking lens of the present invention.

FIG. 10 is an aberration diagram of the taking lens shown in FIG. 9;
(A) represents spherical aberration, (B) represents astigmatism, and (C) represents distortion.

FIG. 11 is an optical path diagram showing a sixth embodiment of the taking lens of the present invention.

12 is an aberration diagram of the photographing lens shown in FIG.
(A) represents spherical aberration, (B) represents astigmatism, and (C) represents distortion.

FIG. 13 is an optical path diagram showing a seventh embodiment of the taking lens of the present invention.

14 is an aberration diagram of the photographing lens shown in FIG.
(A) represents spherical aberration, (B) represents astigmatism, and (C) represents distortion.

FIG. 15 is an optical path diagram showing an eighth embodiment of the taking lens of the present invention.

16 is an aberration diagram of the taking lens shown in FIG.
(A) represents spherical aberration, (B) represents astigmatism, and (C) represents distortion.

FIG. 17 is a schematic view showing a configuration of a unit main body of a film unit with a lens using the photographing lens of the present invention.

[Explanation of symbols]

 Reference Signs List 30 shooting lens 31 first lens 32 second lens 33 aperture opening 35 light shielding plate

Claims (2)

[Claims] 1. A lens comprising, in order from the object side, a first meniscus lens having a convex surface facing the object side, and a second lens having a positive refractive power and a convex surface having a strong curvature facing the image side. ,
At least one surface of each of the first lens and the second lens is formed in an aspherical shape, and the combined focal length of the entire optical system is f, and the focal length of the first lens is f1.
Wherein -1 ≦ f / f1 ≦ 0.45 is satisfied. 2. A fixed focus camera in which a photographing lens is positioned and held in a state of being focused on a fixed distance, wherein the photographing lens has a meniscus shape having a convex surface facing the object side in order from the object side. And a second lens having a positive refractive power and a convex surface having a strong curvature directed to the image side. At least one surface of each of the first lens and the second lens is an aspheric surface. When the combined focal length of the entire photographing lens is f and the focal length of the first lens is f1, a lens satisfying a condition of -1 ≦ f / f1 ≦ 0.45 is used. A film surface on which a subject image is formed by a lens is curved such that its longitudinal direction is concave toward the object side, and the radius of curvature in the diagonal direction of the curved film surface is R _F ,
A camera characterized by satisfying a condition of 8.0 ≦ (R _F tanω) ^1/2 ≦ 12.0, where ω is a diagonal half angle of view of the taking lens.