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How To Measure TFOV Of Binoculars




I've been doing a lot of measuring of fields of view lately in preparation for a new binocular article. This is an excerpt of some of the data used to check Max FOV. You may find it useful. Clear skies, and if not, Cloudy Nights. edz

True Field of View


TFOV is how much an area of sky you can see at once. FOV is measured in degrees. From horizon to horizon is 180 degrees. Human eyes can take in about 50° to 60° degrees of sky without any optical aid. The distance across Orion's Belt stars is a little less than 3 degrees, considered a small field of view for binoculars. When panning the sky looking for objects it is much better to have a 4° to 6° Tfov at a minimum. Oberwerk 15x70 have a 4.4° Tfov. Orion Ultraview 10x50 have a 6° Tfov. Minolta Activa 7x35 have a 9°+ Tfov


Even a 3.5° Tfov begins to make it difficult to orient yourself to the night sky. I have used two different binoculars with fields of view of 3.3° and 3.2°. They were more difficult to use than even 4.2° (Pentax 12x50) and 4.4° (Oberwerk 15x70). For a while I used a pair of 25x70 binoculars that had a 2.6° fov. The 25x70s were mounted for all viewing and although magnification was outstanding, the fov was restrictive and made it difficult for finding objects. Binoculars with a field of view 2 degrees or smaller would be similar to looking thru a telescope eyepiece. You would need a finder scope mounted on top of the binoculars to find anything.


Star Fields You Can Use for Measuring Max FOV of Your Binoculars


True measure of the FOV can be obtained by noting the limits of star fields and referring to SkyAtlas 2000.0 or Uranometria 2000 for a measure of the field limits. (Refer to the Accessories - Books and Charts section to see my review of SkyAtlas 2000.0 and other Books for Astronomy). I was doing this so often in various areas of the sky that I decided it best to list the measures I was using. I have numerous measures I have taken to obscure stars, getting more exact, but that would lead to confusion. These are the easy to see, easy to remember fields that anybody can use.


The data columns listed for SkyAtlas and Uranometria are the measurements in mm determined from SkyAtlas 2000.0 or Uranometria 2000 of the actual measurement in the charts between the two points. The baseline is 82mm per 10° in Sky Atlas 2000.0 and 184mm per 10° in Uranometria. That never changes, the scale remains the same, unlike Right Assencion. RA of 15min at the equator covers a lot more distance than RA of 15min at Dec70°N. A degree is a degree, no matter where you are in your chart. When the measurements varied between the two charts by 0.1°, in general that is only a variance of 2% to 3%. The resultant FOV listed is a rounded off average of the two chart books.


There is no need to take the time for a star to drift across the fov of your binoculars. Simply sight stars at the very edge of the field of your binoculars. There is no better indicator of Max fov for your binocular than to look thru it and measure what you see. Much better measurement can be recorded with mounted binoculars. Mounting the binocular adapter on top of an Orion slow motion adapter helps tremendously. Use several readings on different fields to verify your results.

Fields for Measuring Max FOV







SkyAtlas 2000.0

Uranometria 2000.0


FOV °

point A

point B


82mm = 10°

184mm=10°






82


184







mm

fov°

mm

fov°

UMi 1

3.2

B 7 UMi

y 13 UMi

front of the cup of Little Dipper

25.5

3.11

59

3.21

UMi 2

2.7

z 16 UMi

n 21 UMi

back of the cup of Little Dipper

22

2.68

51.5

2.80

UMi 3

4.8

z 16 UMi

B 7 Umi

top of the cup of Little Dipper

38.75

4.73

89

4.84

UMi 4

5.4

n 21 UMi

y 13 UMi

bottom of the cup of Little Dipper

44.25

5.40

101

5.49

UMi 5

3.9

z 16 UMi

15 Umi

top of Little Dipper but only to #15

32

3.90












UMa 1

5.4

a 50 UMa

B 48 UMa

front of the cup of BIG Dipper

44

5.37

98.5

5.35

UMa 2

4.5

d 69 UMa

y 64 UMa

back of the cup of BIG Dipper

36.5

4.45

83

4.51

UMa 3

10.0

a 50 UMa

d 69 UMa

top of the cup of BIG Dipper

82

10.00



UMa 4

7.9

y 64 UMa

B 48 UMa

bottom of the cup of BIG Dipper

64.5

7.87












Dra 1

2.9

B 23 Dra

v 24 Dra

west side of Draco head shortest

23.75

2.90

53.5

2.91

Dra 2

3.4

v 24 Dra

z 32 Dra

north side of Draco head

27.75

3.38

63

3.42

Dra 3

5.4

z 32 Dra

y 33 Dra

east side of Draco head longest

44.25

5.40

99

5.38

Dra 4

4.1

B 23 Dra

y 33 Dra

south side of Draco head

33.5

4.09

76

4.13










Cas 1

4.9

B 11 Cas

a 18 Cas

brightest two west side W

39.5

4.82

90

4.89

Cas 2

3.6

y 27 Cas

d 37 Cas

two closest stars in the W

28.9

3.52

66.5

3.61

Cas 3

3.0

y 27 Cas

n 24 Cas

center of W south to eta

25

3.05

56

3.04










Per 1

3.7

a 33 Per

d 39 Per

Mirphak to brightest east

30.25

3.69

68.5

3.72

Per 2

4.8

a 33 Per

y 23 Per

Mirphak to brightest northwest

39

4.76

87.75

4.77

Per 3

2.1

z 44 Per

o 38 Per

Perseus foot just N of Pleaides

17

2.07

38.8

2.11










Tri 1

2.0

B 4 Tri

y 9 Tri

the short base of Triangulum

16

1.95

36

1.96










Tau 1

3.2

a 87 Tau

e 74 Tau

Aldeberan across the open V, the Hyades)

26.25

3.20

59

3.21

Tau 2

4.0

a 87 Tau

y 54 Tau

Aldeberan to the point of the V the Hyades)

32.75

3.99

73

3.97

Tau 3

2.4

a 87 Tau

71 Tau

part way along bottom of V

20

2.44

45.25

2.46

Tau 4

2.7

74 Tau

63 Tau

part way along top of V

22

2.68

50

2.72










Aur 1

2.8

e 7 Aur

n10, z8 Aur

The Kids triangle (7, 10, 8 Aur) in one view

23

2.80

51

2.77










Ori 1

2.8

z 50 Ori

d 34 Ori

Orion's Belt stars

22.5

2.74

50.75

2.76










Gem 1

4.5

a Gem

B Gem

Castor to Pollux

36.5

4.45

83

4.51

Gem 2

3.5

a Gem

75 Gem

75 is just north of Pollux

29

3.54

65

3.53

Gem 3

2.6

1 Gem

n eta Gem

M35 is between these in Gemini's foot

21

2.56

48

2.61

Gem 4

4.4

1 Gem

13 Gem

the whole foot

36

4.39

81

4.40










CMi 1

3.3

a 10 CMi

B 3 CMi

Procyon to companion

27

3.29

61

3.32

CMi 2

4.3

a 10 CMi

B 3 CMi

Procyon to companion

35.25

4.30

79.5

4.32

CMi 3

4.6

a 10 CMi

B 3 CMi

Procyon to companion

37.75

4.60

85

4.62

CMi 4

5.3

a 10 CMi

B 3 CMi

Procyon to companion

43

5.24

98

5.33










Ser 1

3.3



triangular head of Serpens Caput



61

3.32










Her 1

6.6

pi 67 Her

n 44 Her

top of the Keystone of Hercules

54

6.59



Her 2

6.6

pi 67 Her

e 58 Her

east side of the Keystone of Hercules

54

6.59

122

6.63

Her 3

7.3

n 44 Her

z 40 Her

west side of the Keystone of Hercules

60

7.32

135

7.34

Her 4

4.1

e 58 Her

z 40 Her

bottom of the Keystone of Hercules

33.25

4.05

75.5

4.10










Lyr 1

3.4

d1d2 11,12

e1e2 4,5

NE stars in Lyra

28

3.41

63

3.42

Lyr 2

2.0

B Lyr

y Lyr

across bottom of Lyre

16

1.95

37.5

2.04










Cyg 1

6.1

a Cyg

y Cyg

Deneb to Sadr top to center of cross

50.125

6.11

113

6.14










Del 1

2.7

y 12 Del

B 6 Del

body of Dolphin

22

2.68

49.25

2.68

Del 2

3.1

y 12 Del

z 4 Del

body of Dolphin + #4

25.25

3.08

57

3.10

Del 3

4.4

y 12 Del

n 3 Del

body of Dolphin + #3 in the tail

35.6

4.34

80.25

4.36

Del 4

5.8

y 12 Del

2 Del

extending further down the tail

47.75

5.82

106

5.76

Del 5

1.7

y 12 Del

9 Del

smaller parts of the diamond

14

1.71

31.25

1.70

Del 6

1.3

y 12 Del

11 Del

smaller parts of the diamond

10.5

1.28

24

1.30




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