Level having an autofocus system with controllable focusing lens group

A surveying apparatus, such as a level, includes an autofocus system. An object to be observed through a telescope of the surveying apparatus, is brought into focus through the autofocus system. The autofocus system includes a pair of image forming lenses, each forming an image of the object, and a pair of line sensors, each having a plurality of light receiving elements defining a light receiving area. The object images are respectively formed on the light receiving areas through the pair of image forming lenses. A focusing lens group is provided in the telescope. A mechanism moves the focusing lens group along an optical axis of the telescope in accordance with data output from the pair of line sensors. A device for detecting a position of the focusing lens group and a mechanism for selecting a number of the plurality of light receiving elements of each of the pair of line sensors to be used in a focusing operation are provided. The number of light receiving elements is selected depending on the position of the focusing lens group detected by the detecting device. A device is provided for controlling the moving mechanism in accordance with data output from the selected number of the plurality of light receiving elements.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a level including a telescope and a 
leveling device that is widely used in the construction fields and more 
specifically, to a level of the type having an autofocus system provided 
for the focusing optical system of the telescope through which a reference 
position of a subject is automatically brought into focus. 
2. Description of the Related Art 
A level is a piece of equipment generally used at a construction site by an 
engineer or surveyor for leveling, or for measuring bearings, horizontal 
angles, vertical angles, etc. The level is generally used with a tripod, 
the level being attached thereon. 
One type of widely known level is the automatic level which is equipped 
with an automatic leveling instrument or device for automatically 
establishing a horizontal plane of sight. 
The automatic level basically comprises a surveyor's telescope and a 
horizontal plane establishing optical system (horizontal plane 
compensating optical system) which functions as an automatic leveling 
device. The automatic level will now be explained. 
When a reference position (sighting point that is set at a distance from 
the level), is sighted through the telescope, the horizontal plane 
establishing optical system ensures that a horizontal fine line of a 
reticle of the telescope lies on a real horizontal, even if the optical 
axis of the telescope is not correctly positioned in a horizontal plane. 
When another sighting point is sighted after the telescope has been 
rotated about the vertical axis, perpendicular to the optical axis 
thereof, the sighting point is located in the same horizontal plane as the 
reference position. 
The optical system of the telescope of such an automatic level comprises an 
objective lens group, a focusing lens group and an eyepiece, arranged in 
this order from the object side. Due to the focusing lens group, a clear 
image of a sighted reference object (reference point) can be observed, 
regardless of the object distance. The position of the focusing lens group 
is adjusted depending on the object distance, so as to form a sharp object 
image on the reticle provided on the focal plane. The object image formed 
on the reticle can be viewed through the eyepiece. 
Assuming that the observable object distance range of the surveyor's 
telescope is for example, 0.2 meters to .infin. (infinity), and that the 
focusing lens group is comprised of a concave lens, the movement range of 
the focusing lens group is approximately 30 mm. The focusing lens group is 
usually moved along the optical axis by rotating a rotatable focusing knob 
provided on the telescope. If the range of movement of the focusing lens 
group is small relative to the amount of rotation of the focusing knob, it 
is sometimes necessary to rotate the knob by a large amount to move the 
focusing lens group to a position at which an in-focus condition is 
obtained. That, it sometimes takes a long time to obtain an in-focus 
condition, although the image remains on the reticle on the focal plane 
for a long period of time. 
Conversely, if the range of movement of the focusing lens group is large 
relative to the rotation of the focusing knob, achieves proper focusing. 
However the focusing knob need not be rotated by a large amount the time 
in which the image remain on the reticle on the focal plane is too short 
with respect to the amount of rotation of the focusing knob. That is, the 
focusing lens group moves by a large amount even when the focusing knob is 
rotated by a small amount. It is therefore difficult to obtain an in-focus 
condition quickly since the focusing knob must be rotated step by step, 
resulting in a time consuming operation. 
Furthermore, it is sometimes the case in the conventional automatic level 
that an in-focus condition is obtained by a slight rotation of the 
focusing knob when the object to be sighted is located far away, whereas a 
large amount of rotation of the focusing knob is required to obtain an 
in-focus condition when the object is located at a close distance. Still 
furthermore, since it is impossible for the naked eye to check whether the 
object to be sighted is in a front or rear focus state, the focusing knob 
is often firstly mistakenly rotated in a wrong direction, i.e., opposite 
to the direction needed for focusing. In any event, in the conventional 
automatic level, the focusing operation is troublesome and requires an 
extended period of time. 
SUMMARY OF THE INVENTION 
To eliminate the drawbacks of the conventional automatic level as mentioned 
above, it is an object of the present invention to provide an improved 
level having an autofocus system with which the time required to focus is 
shorter than the prior art and precise focusing can be achieved. 
To achieve the objects mentioned above, according to the present invention, 
there is provided a level having an autofocus system through which an 
object to be observed through a telescope of the level is brought into 
focus. The autofocus system includes a pair of image forming lenses, each 
of which forms an image of the object, and a pair of line sensors, each 
having a plurality of light receiving elements defining a light receiving 
area, with the object images being respectively formed on the light 
receiving areas through the pair of image forming lenses. A focusing lens 
group is provided in the telescope, and a device moves the focusing lens 
group along an optical axis of the telescope in accordance with data 
output from the pair of line sensors. A device detects a position of the 
focusing lens group. A mechanism selects a number of the plurality of 
light receiving elements of each of the pair of line sensors to be used in 
a focusing operations. The number of light receiving elements selected 
depends on the position of the focusing lens group detected by the 
detecting device. A mechanism controls the moving device in accordance 
with data output from the selected number of the plurality of light 
receiving elements. 
With this structure, not only can the time required to focus the telescope 
on an object be shortened by the autofocus system, but precise focusing is 
also achieved since the number of the plurality of light receiving 
elements of each of the pair of line sensors used in a focusing operation 
is selected depending on the position of the focusing lens group detected 
by the detecting device. If the number of the light receiving elements of 
each of the pair of line sensors in a focusing operation is fixed (i.e., 
if the size of a focus measuring area, determined by the light receiving 
area to be used on each line sensor, is always the same) the percentage of 
the area of the object image to be focused with respect to the focus 
measuring area decreases as the object distance increases. The smaller the 
percentage of the area of the object image to be focused with respect to 
the focus measuring area is, the higher the chances are that a focusing 
error will occur due to a disturbance of light and the like. However, 
according to the present invention, the influence of disturbance of light 
on the pair of line sensors can be effectively reduced at any object 
distance due to the structure of the present invention. The number of the 
plurality of light receiving elements of each of the pair of line sensors 
used in a focusing operation is thus selected depending on the position of 
the focusing lens group detected by the detecting device, whereby precise 
focusing is achieved. 
Preferably, the number of light receiving elements selected by the 
selecting mechanism decreases as a distance of the object to be observed 
increases, and the object distance is detected from the position of the 
focusing lens group. 
Preferably, the telescope includes an objective lens group, the focusing 
lens group, a beam splitter, a reticle and an eyepiece in this order from 
the side of the object to be observed. A part of the light passing through 
the objective lens group and the focusing lens group is reflected by the 
beam splitter towards the pair of image forming lenses and is subsequently 
split into two split images. The two split images are respectively formed 
on the pair of line sensors through the pair of image forming lenses. 
Preferably, the autofocus system further includes a condenser lens 
positioned between the beam splitter and the pair of image forming lenses. 
Preferably, the autofocus system further includes a device for storing a 
plurality of sets of effective area defining data where each set defines a 
different number of light receiving elements to be used. Thus depending 
upon the position of the focusing lens group as detected by the detecting 
device, one set of the plurality sets of effective area defining data is 
input to the selecting mechanism. The selecting mechanism selects the 
number of the plurality of light receiving elements in accordance with the 
selected set of effective area defining data. 
Preferably, the telescope further includes a horizontal plane establishing 
optical system positioned between the focusing lens group and the beam 
splitter for automatically establishing a horizontal plane of sight. 
Preferably, the telescope further includes an AF frame formed on a 
transparent plate positioned in an optical path of the telescope. The AF 
frame indicates a focus measuring area corresponding to each of the light 
receiving areas. 
Preferably, the transparent plate is an LCD plate indicating the AF frame. 
The AF frame varies in size, depending on the number of light receiving 
elements selected by the selecting mechanism. 
Preferably, the AF frame varies in size to correspond to the size of each 
of the light receiving areas such that the size of the AF frame decreases 
as a distance of the object to be observed increases, the object distance 
being detected from the position of the focusing lens group. 
Preferably, each of the pair of line sensors is a multi-segment CCD sensor 
having a plurality of photodiodes serving as the plurality of light 
receiving elements. 
The selecting mechanism and the controlling mechanism may be provided in a 
single CPU. 
According to another aspect of the present invention, there is provided a 
level having an autofocus system through which an object to be observed 
through a telescope of the level is brought into focus. The telescope 
includes (in order from the object side) an objective lens group, a 
focusing lens group, a reticle and an eyepiece. The focusing lens group 
moves along an optical axis to form an object image of the object on the 
reticle. The object image formed on the reticle is observed through the 
eyepiece. A beam splitter is provided in an optical path between the 
focusing lens group and the reticle. A pair of image forming lenses each 
form an object image. The pair of image forming lenses each receiving 
light emitted from the beam splitter; A pair of line sensors, each having 
an array of light receiving elements, define a light receiving area. The 
object images are being respectively formed on the light receiving areas 
through the pair of image forming lenses. A device moves the focusing lens 
group along the optical axis in accordance with data output from the pair 
of line sensors. A device detects a position of the focusing lens group. A 
mechanism selects, from the array on each of the pair of line sensors, at 
least some of the light receiving elements to be used in a focusing 
operation in which the moving device moves the focusing lens group so as 
to form the object image of the object on the reticle, depending on the 
position of the focusing lens group detected by the detecting device. A 
mechanism controls the moving device in accordance with data output from 
the selected light receiving elements. 
The present disclosure relates to subject matter contained in Japanese 
Patent Application No. 7-84230 (filed on Apr. 10, 1995) which is expressly 
incorporated herein by reference in its entirety.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 9 and 10 show an embodiment of an automatic level 10 to which the 
present invention is applied. The automatic level 10 is provided with a 
surveyor's telescope 8 including an objective lens group 11 of positive 
power, a focusing lens 12 of negative power, a horizontal plane 
establishing optical system 13, a beam splitter (semitransparent mirror) 
18, a reticle plate 14, and an eyepiece lens 15 of positive power, in this 
order from the object side (i.e., left to right in FIGS. 1, 9 or 10). For 
purposes of illustration the objective lens group 11 is drawn as a single 
lens in FIG. 1, although the objective lens group 11 is actually comprised 
of a plurality of lenses as shown in FIG. 10. On the reticle plate 14, a 
reticle is visibly formed thereon. The reticle consists of a fine 
horizontal line h and a fine vertical line v (see FIG. 8) intersecting 
perpendicular to each other. 
The horizontal plane establishing optical system 13, per se known, is 
provided, as shown in FIG. 11, with a first compensating prism 13a, a 
compensating mirror 13b and a second compensating prism 13c, and has a 
symmetrical shape with respect to the center of the compensating mirror 
13b. The horizontal plane establishing optical system 13 hangs from a 
string 13e attached to a shaft 13d. 
The telescope 8 of the automatic level 10 is supported on a supporting 
frame 19 fixed on a rotatable table 17. The rotatable table 17 is 
rotatable about a vertical axis 17X, which is perpendicular to the optical 
axis O of the telescope. Objects located at different distances from the 
automatic level 10, but located on a common horizontal plane, can be 
observed through the telescope 8. 
The magnification of the telescope 8 (having the above-mentioned optical 
elements) of the automatic level 10, is set, e.g., at twenty-four times 
(.times.24). The rotatable table 17 is detachably attached to a tripod 
(not shown) when the automatic level 10 is used. A reference pole B (see 
FIGS. 3 to 7) on which a scale is printed, is often used as an object to 
be sighted by the automatic level 10. The bottom end of the reference pole 
B is placed on a reference point on the ground, while the top end is 
generally held by a person. 
The automatic level 10 is provided with a focusing lens drive mechanism 9 
for moving the focusing lens 12 along the optical axis O for focusing. The 
focusing lens drive mechanism 9 (FIG. 1) includes a lens drive motor 42, a 
clutch-incorporated speed reduction mechanism 41, a nut 44 and an encoder 
40. The lens drive motor 42 may be a stepping motor. The 
clutch-incorporated speed reduction mechanism 41 transmits the rotation of 
the lens drive motor 42 to a screw shaft 43. The nut 44 is fixed to the 
focusing lens 12 and is engaged with the screw shaft 43. Therefore, when 
the screw shaft 43 rotates the focusing lens 12 moves along the optical 
axis O. The encoder 40 outputs lens positional information in the form of 
pulse signals, the number of which corresponds to the amount of rotation 
of the screw shaft 43. 
A part of the light emitted from the horizontal plane establishing optical 
system 13 is reflected by the beam splitter 18 at a right angle towards a 
focus detecting sensor 21 provided near the beam splitter 18. Between the 
beam splitter 18 and the focus detecting sensor 21, an imaginary 
equivalent surface 14C is formed and located at a position optically 
equivalent to the position at which the reticle plate 14 is placed. The 
focus detecting sensor 21 receives light reflected by the beam splitter 18 
and outputs corresponding signals to a focus-condition detecting portion 
48. 
The automatic level 10 is provided with an AF controller 49 which includes 
a CPU 47, a motor driver 45, an encoder pulse detecting portion 46, the 
focus-condition detecting portion 48 (mentioned above), a RAM 50 and a ROM 
51. The focus condition on the equivalent surface 14C is detected by the 
focus-condition detecting portion 48 in accordance with the signals 
received from the focus detecting sensor 21. The principle of the focus 
detecting sensor 21 will now be discussed with reference to FIG. 2. 
The focus detecting sensor 21 consists of a condenser lens 21a, a pair of 
separating lenses 21b and a pair of line sensors 21c. The pair of line 
sensors 21c are located in the vicinity of the equivalent surface 14C for 
receiving light emitted from the beam splitter 18. Each line sensor 21c is 
a multi-segment CCD sensor having an array of photodiodes (i.e., a 
plurality of light receiving elements). The pair of line sensors 21c are 
aligned horizontally, i.e., along a horizontal plane perpendicular to the 
reference pole B. 
A common object image (e.g., the object image such as shown in a focus 
measuring area Z shown in FIG. 3) is incident on each of the pair of line 
sensors 21c. Relative locations of the two object images on the pair of 
line sensors 21c change depending upon the position of the focusing lens 
12, that is, the location of the focal point relative to the equivalent 
surface 14C. Namely, relative locations of the two object images on the 
pair of line sensors 21c change in the following three cases: (a) when the 
focal point of the object image is located precisely on the equivalent 
plane 14C (i.e., in-focus condition), (b) when the focal point of the 
object image is located between the beam splitter 18 and the equivalent 
surface 14C (i.e., front focus condition), and (c) when the focal point of 
the object image is located between the equivalent surface 14C and the 
focus detecting sensor 21 (i.e., rear focus condition). Through the 
focusing detecting sensor 21 the location of the focal point, i.e., 
in-focus, front focus or rear focus can be detected. 
The defocus amount can also be detected through the pair of line sensors 
21c by detecting the position on each line sensor 21c at which the 
corresponding object image is formed through the condenser lens 21a and 
the corresponding separating lens 21b. When the focus-condition detecting 
portion 48 receives sensor signals output from each line sensor 21c, the 
focus-condition detecting portion 48 amplifies the output sensor signals 
through an amplifier (not shown) provided in the focus-condition detecting 
portion 48. Subsequently, it inputs the amplified output sensor signals to 
an operation circuit (not shown) provided in the focus-condition detecting 
portion 48 so as to detect an in-focus, a front focus or a rear focus 
condition, and the amount of defocus. The focus-condition detecting 
portion 48 outputs the detected condition and defocus amount to the CPU 
47. 
When the reference pole B is placed at a location spaced from the automatic 
level 10 by five meters (considered to be near to the automatic level 10), 
the reference pole B observed through the telescope 8 of the automatic 
level 10 will appear in the telescope view F as shown in FIG. 3. In this 
state, since the image of the reference pole B occupies almost the whole 
of the telescope view F with respect to the background, the chance of a 
focusing error, due to a disturbance of light and the like, is minimal. 
In FIG. 3, "a" represents the width of the image of the reference pole B on 
the equivalent surface 14C. In this particular embodiment of the present 
invention, the actual width of the reference pole B is 70 mm, and the 
width "a" of the image thereof on the equivalent surface 14C is 3.3 mm. 
"b" represents the width of the focus measuring area Z corresponding to 
the light receiving area of each line sensor 21c. The width "b" is 4 mm in 
this embodiment. The focus measuring area Z is indicated in the telescope 
view F by an AF frame zm (see FIG. 8) consisting of pairs of brackets n, 
k, j or i. These pairs of brackets n, k, j and i may be formed as ON/OFF 
segments on an LCD plate provided in the optical path of the telescope 8 
(and in the vicinity of the reticle plate 14), and one of the pairs of 
brackets is activated (i.e., turned ON) to be visible depending upon the 
detected distance of an object to be observed. "h" and "v" respectively 
designate the fine horizontal line and the fine vertical line of the 
reticle formed on the reticle plate 14. Each fine line has a thickness of 
0.003 mm. 
The farther the reference pole B is located from the automatic level 10, 
the smaller the image of the reference pole B, observed in the telescope 
view F, becomes. More specifically, when the reference pole B is spaced 
from the automatic level 10 by 10 m, 20 m, 30 m and 50 m, the widths of 
the images of the reference pole B on the equivalent surface 14C become 
"c", "d", "e" and "g" respectively, as shown in FIGS. 4, 5, 6 and 7. The 
widths "c", "d", "e" and "g" are approximately 1.6 mm, 0.8 mm, 0.6 mm and 
0.3 mm, respectively, in this embodiment. 
If the width of the focus measuring area Z is constant, as the reference 
pole B moves away from the automatic level 10, the ratio of the area 
occupied by the image of the reference pole B to the size of the focus 
measuring area Z gradually decreases, whereas the ratio of the background 
to the focus measuring area Z increases. Concrete data showing these 
variations are shown in the following table. 
The data in the following table shows the case where the magnification of 
the telescope 8 of the automatic level 10 is twenty-four times 
(.times.24), the diameter of the telescope view F on the reticle plate 14 
is approximately 6 mm, the width of the light receiving area of each line 
sensor 21c (i.e., the width of the focus measuring area Z) is 4 mm, the 
composite focal length of the objective lens group 11 and the focusing 
lens 12 is approximately 240 mm, and the width of the reference pole B is 
70 mm. 
In the table below "Distance" represents the distance from the automatic 
level 10 to the reference pole B, "Image Size" represents the width of the 
image of the reference pole B on the equivalent surface 14C, "Image 
Size/Telescope view" represents the percentage of the telescope view F 
occupied by the image of the reference pole B, and "Image Size/Sensor 
Detecting Area" represents the percentage of the focus measuring area Z 
occupied by the image of the reference pole B. 
TABLE 
______________________________________ 
Image Size/ Image Size/Sensor 
Distance 
Image Size Telescope view 
Detecting Area 
(m) (mm) (%) (%) 
______________________________________ 
3 5.5 93 138 
5 3.3 56 82 
10 1.65 28 41 
20 0.83 14 21 
30 0.55 9 14 
50 0.33 6 8 
______________________________________ 
It will be appreciated from the above table that the percentage of space 
that the image of the reference pole B take up of the focus measuring area 
Z becomes very small when the distance between the automatic level 10 and 
the reference pole B exceeds 10 meters. The smaller the percentage of the 
image of the reference pole B with respect to the focus measuring area Z 
is, the higher the chances are that a focusing error occurs due to the 
disturbance of light and the like. 
The magnification of an eyepiece is defined by the following formula: 
EQU M=Ld/Fe 
wherein 
"M" represents the magnification; 
"Ld" represents the least distance of distinct vision (generally 250 mm); 
and 
"Fe" represents the focal length of the eyepiece. 
If the focal length of the eyepiece 15 is 9.6 mm, the magnification of the 
eyepiece 15 is twenty-six times (.times.26), i.e., 250(mm).div.9.6(mm)=26. 
Thus, the image of the reference pole B is observed through the eyepiece 
15 with the size of the image on the equivalent surface 14C being 
magnified twenty-six times. 
In order to overcome the aforementioned drawbacks, according to the 
automatic level 10 to which the present invention is applied, the width of 
the focus measuring area Z, i.e., the width of the light receiving area to 
be used on each line sensor 21c, is varied depending on the object 
distance. This is the main feature of the present invention. 
The CPU 47 outputs drive signals to the lens drive motor 42 through the 
motor driver 45 in accordance with the focus condition information and 
defocus amount information received from the focus-condition detecting 
portion 48. This activates the lens drive motor 42 to move the focusing 
lens 12 in the direction where the object image, formed on the equivalent 
surface 14C, is brought into focus. The encoder 40 outputs a corresponding 
number of pulse signals in accordance with the amount of rotation of the 
lens drive motor 42, and the outputted pulse signals are sent to the 
encoder pulse detecting portion 46. The encoder pulse detecting portion 46 
detects the position of the focusing lens 12 from the number of pulse 
signals received from the encoder 40, and subsequently, sends the CPU 47 a 
signal indicating the object distance at which the observed object image 
is focused on the equivalent surface 14C or the reticle plate 14. 
The CPU 47 varies the width of the light receiving area to be used on each 
line sensor 21c by actuating only a predetermined number of photodiodes 
from all the photodiodes of each line sensor 21c, in accordance with the 
effective area defining data read out from the RAM 50. Four sets of 
predetermined effective area defining data, i.e., first, second, third and 
fourth effective area defining data, are predetermined and stored in the 
ROM 51. The light receiving area to be used on each line sensor 21c is 
correspondingly varied as one of four predetermined areas EA.sub.1, 
EA.sub.2, EA.sub.3 and EA.sub.4 (FIG. 12), in accordance with the first, 
second, third and fourth effective area defining data, respectively. The 
four predetermined areas EA.sub.1, EA.sub.2, EA.sub.3 and EA.sub.4 
respectively correspond to first, second, third and fourth focus measuring 
areas Z.sub.1, Z.sub.2, Z.sub.3 and Z.sub.4 (FIGS. 4, 5, 6 and 7). The 
oblique-lined portion on each line sensor 21c shows the non-used area 
where the photodiodes provided therein are not actuated in the focusing 
operation. 
The above-noted first through fourth effective area defining data are 
predetermined to correspond to the respective four divided sections of the 
object distance range (e.g., from zero meters to 50 meters or more) within 
which the encoder pulse detecting portion 46 can detect an object 
distance. One of the first through fourth effective area defining data is 
read out from the ROM 51 and stored in the RAM 50 in accordance with the 
above mentioned signal, indicating the object distance that is received 
from the encoder pulse detecting portion 46. In accordance with that 
effective area defining data stored in the RAM 50, the CPU 47 varies the 
width of the light receiving area to be used on each line sensor 21c. 
When the object distance, detected through the encoder pulse detecting 
portion 46, is equal to or less than 10 meters, the first effective area 
defining data is stored in the RAM 50, and the entire light receiving area 
(the light receiving area EA.sub.1 in FIG. 12) is used for the focusing 
operation. At the same time, the corresponding pair of visible brackets i 
are activated to be visible as the AF frame zm. 
When the object distance detected through the encoder pulse detecting 
portion 46 is greater than 10 meters but less than 30 meters, the second 
effective area defining data is stored in the RAM 50, and 50 percent 
(i.e., 25 percent on each side of the center) of the entire light 
receiving area (EA.sub.2 in FIG. 12) is used for the focusing operation. 
At the same time, the corresponding pair of visible brackets j are 
activated as the AF frame zm. 
When the object distance detected through the encoder pulse detecting 
portion 46 is greater than or equal to 30 meters but less than 50 meters, 
the third effective area defining data is stored in the RAM 50. Thirty 
percent (i.e., 15 percent on each side of the center) of the entire light 
receiving area (EA.sub.3 in FIG. 12) is used for the focusing operation. 
At the same time, the corresponding pair of visible brackets k are 
activated as the AF frame zm. 
When the object distance detected through the encoder pulse detecting 
portion 46 is greater than or equal to 50 meters, the fourth effective 
area defining data is stored in the RAM 50. Twenty percent (i.e., 10 
percent on each side of the center) of the entire light receiving area 
(EA.sub.4 in FIG. 12) is used for the focusing operation. At the same 
time, the corresponding pair of visible brackets n are activated as the AF 
frame zm. 
It will be appreciated from the foregoing that the farther the object to be 
focused is located from the automatic level 10, the narrower the focus 
measuring area Z, determined by the light receiving area to be used on 
each line sensor 21c, is set. Since the focus measuring area Z, i.e., the 
light receiving area to be used on each line sensor 21c, is narrowed or 
enlarged depending upon the distance of the object to be sighted (e.g., 
the reference pole B) from the automatic level 10, the chances that a 
focusing error will occur due to the disturbance of light and the like is 
greatly reduced; i.e., the percentage of the size of the image of the 
object to be sighted with respect to the focus measuring area Z is always 
high. 
It is noted that the width of the focus measuring area Z cannot be narrower 
than a certain width. This will be understood from the following. 
When the telescope 8 of the automatic level 10 is directed to the reference 
pole B, since this directing operation is performed by manually swinging 
the telescope 8, (the telescope 8 being hand-held) the automatic level 10 
shakes to some degree. This makes it difficult to precisely place the 
image of the reference pole B in the middle of the focus measuring area Z 
indicated by the AF frame zm. In other words, it would be quite difficult 
and consume too much time to place the image of the reference pole B in 
the middle of the AF frame zm in the case where the AF frame zm is too 
small. The same thing can also be said in the case where the width of the 
narrowed AF frame zm is identical to that of the observed image of the 
reference pole B. The minimum width of the focus measuring area 
corresponds to approximately 15 to 30 degrees in the angle of view of the 
telescope 8 of the automatic level 10. 
The image of the reference pole B can be autofocused on the equivalent 
surface 14C through the objective lens group 11, the focusing lens 12, the 
horizontal plane establishing optical system 13 and the beam splitter 18. 
In this state, even if the optical axis O of the telescope 8 of the 
automatic level 10 does not precisely lie on a horizontal plane, the fine 
horizontal line h on the reticle plate 14 is automatically adjusted to lie 
substantially horizontal by the horizontal plane establishing optical 
system 13. Therefore, when another sighting point is located after the 
telescope 8 has been rotated about the vertical axis 17X, the sighting 
point is located in the horizontal plane including the initially sighted 
reference position. 
When the object light, reflected on the beam splitter 18 and passed through 
the equivalent surface 14C, is incident upon the focus detecting sensor 
21, the focus-condition detecting portion 48 calculates the defocus amount 
for the image of the reference pole B in accordance with the signals 
output from the pair of line sensors 21c. This detects the focus condition 
for the reference pole B, i.e., whether the reference pole B is in an 
in-focus, out-of-focus, front-focus or rear-focus condition. The result of 
this detection is input to the CPU 47, and subsequently, the CPU 47 
outputs lens drive signals to the focusing lens drive mechanism 9 through 
the motor driver 45 to actuate the lens drive motor 42, so that the 
focusing lens 12 moves along the optical axis O through the screw shaft 43 
and the nut 44. During movement of the focusing lens 12, the encoder pulse 
detecting portion 46 continues to feed the position of the focusing lens 
12 back to the CPU 47 in accordance with the pulse signals received from 
the encoder 40. The CPU 47 controls the lens drive motor 42 to stop the 
focusing lens 12 at a position where the image of the reference pole B is 
in-focus on the equivalent surface 14C. 
When the reference pole B is brought into focus, the CPU 47 activates one 
of the pairs of brackets i, j, k and n as the AF frame zm. Only the pair 
of brackets i are turned ON when the detected object distance of the 
reference pole B is less than or equal to 10 meters. Only the pair of 
brackets j are activated when the detected object distance of the 
reference pole B is more than 10 meters but less than 30 meters. Only the 
pair of brackets k are activated when the detected object distance of the 
reference pole B is greater than or equal to 30 meters but less than 50 
meters. Only the pair of brackets n are activated when the detected object 
distance of the reference pole B is greater than or equal to 50 meters. 
As can be understood from the foregoing, according to the automatic level 
10 to which the present invention is directed, since the light receiving 
area to be used on each line sensor 21c is narrowed or enlarged depending 
upon the detected object distance of an object to be sighted through the 
telescope 8 of the automatic level 10 by varying the number of photodiodes 
to be used on each line sensor 21c, the influence of disturbance of light 
on the pair of line sensors 21c can be effectively reduced at any object 
distance. Focusing error during an autofocusing operation is greatly 
reduced, and, precise focusing can be achieved. 
Although, in the above embodiment, the four predetermined focus measuring 
areas Z.sub.1, Z.sub.2, Z.sub.3 and Z.sub.4 are selectively used to 
correspond to the four divided sections of the object distance range (the 
first section (0 m&lt;L.ltoreq.10 m), the second section (10 m&lt;L&lt;30 m), the 
third section (30 m.ltoreq.L&lt;50 m), the fourth section (50 m.ltoreq.L); 
"L" represents the object distance), other numbers of sets of 
predetermined focus measuring areas having different widths may be used 
(e.g., 2, 3, 5 or more than 5). For instance, if six sets of predetermined 
focus measuring areas are provided, these six focus measuring areas may be 
selectively used to correspond to six divided sections of the object 
distance range, or the focus measuring areas may be continuously narrowed 
or enlarged as the detected object distance increases or decreases, 
respectively. 
Although one of the pairs of brackets i, j, k and n is activated as the AF 
frame zm through the LCD plate provided in the optical path of the 
telescope 8 in the above embodiment, the pairs of brackets i, j, k and n 
may be all printed on a transparent plate provided in the optical path of 
the telescope, or only one AF frame may be printed on the transparent 
plate. 
Obvious changes may be made in the specific embodiment of the present 
invention described herein, such modifications being within the spirit and 
scope of the invention claimed. It is indicated that all matter contained 
herein is illustrative and does not limit the scope of the present 
invention. 
For example, although the present invention has been described with 
reference to a level, the invention is not so limited. Rather, the 
invention may be utilized in a telescope of any type of surveying 
apparatus or equipment. Thus, the discussion of the level herein is merely 
a representative example of one of the types of 
machinery/equipment/apparatus in which the present invention can be 
advantageously utilized.