Viewpoints

Bellows Factor

As all large format photographers know, bellows factor has to be accounted for with close up/macro photography. The closer the object is to the camera, the greater is the light fall off that occurs. This has to be compensated for. Focused at infinity there is no compensation. Focused at life size there are two stops of light to add.

The problem is that the relationship between focusing distance and exposure compensation is not linear, it is logrithmic. So the calculations are not simple or intuitive to estimate.

Anyhow, to cut a long story short, here is a simple table I calculated to apply the correct bellows factor by measuring the distance between the film plane and the lens using a tape measure.

Step1. Put any longer lens on the camera and focus it to infinity. Using a tape measure find two points - one on the front and one on the rear standard that are exactly the focal length of lens apart in cm. e.g. for a 240mm lens you are looking to be able to consistently measure 24cm between two fixed points. Mark these points if necessary. It is best to pick two points on the rail if you can then it will not be influenced by tilt movements.

Step 2. Focus your picture as normal. When you are ready, measure the distance between these two points and read off the correct exposure compensation from the table. For example on a 240mm lens, if the distance between the two points is 33cm, then you will want to use an exposure compensation of 0.9 stops.

Simple.

Here is the full table. You should be able to copy and paste the table into excel and hide the columns you don't need:

Image distance in cm

Stops

90

110

120

125

135

150

180

200

210

240

300

360

0

 

9.0

11.0

12.0

12.5

13.5

15.0

18.0

20.0

21.0

24.0

30.0

36.0

0.1

9.3

11.4

12.4

12.9

14.0

15.5

18.6

20.7

21.7

24.8

31.1

37.3

0.2

9.6

11.8

12.9

13.4

14.5

16.1

19.3

21.4

22.5

25.7

32.2

38.6

0.3

10.0

12.2

13.3

13.9

15.0

16.6

20.0

22.2

23.3

26.6

33.3

39.9

0.4

10.3

12.6

13.8

14.4

15.5

17.2

20.7

23.0

24.1

27.6

34.5

41.4

0.5

 

10.7

13.1

14.3

14.9

16.1

17.8

21.4

23.8

25.0

28.5

35.7

42.8

0.6

11.1

13.5

14.8

15.4

16.6

18.5

22.2

24.6

25.9

29.5

36.9

44.3

0.7

11.5

14.0

15.3

15.9

17.2

19.1

22.9

25.5

26.8

30.6

38.2

45.9

0.8

11.9

14.5

15.8

16.5

17.8

19.8

23.8

26.4

27.7

31.7

39.6

47.5

0.9

12.3

15.0

16.4

17.1

18.4

20.5

24.6

27.3

28.7

32.8

41.0

49.2

1

 

12.7

15.6

17.0

17.7

19.1

21.2

25.4

28.3

29.7

33.9

42.4

50.9

1.1

13.2

16.1

17.6

18.3

19.8

22.0

26.4

29.3

30.7

35.1

43.9

52.7

1.2

13.6

16.7

18.2

18.9

20.5

22.7

27.3

30.3

31.8

36.4

45.5

54.6

1.3

14.1

17.3

18.8

19.6

21.2

23.5

28.2

31.4

33.0

37.7

47.1

56.5

1.4

14.6

17.9

19.5

20.3

21.9

24.4

29.2

32.5

34.1

39.0

48.7

58.5

1.5

 

15.1

18.5

20.2

21.0

22.7

25.2

30.3

33.6

35.3

40.4

50.4

60.5

1.6

15.7

19.1

20.9

21.8

23.5

26.1

31.3

34.8

36.6

41.8

52.2

62.7

1.7

16.2

19.8

21.6

22.5

24.3

27.0

32.4

36.0

37.8

43.2

54.1

64.9

1.8

16.8

20.5

22.4

23.3

25.2

28.0

33.6

37.3

39.2

44.8

56.0

67.2

1.9

17.4

21.3

23.2

24.1

26.1

29.0

34.8

38.6

40.6

46.4

58.0

69.5

2

 

18.0

22.0

24.0

25.0

27.0

30.0

36.0

40.0

42.0

48.0

60.0

72.0

2.1

18.6

22.8

24.8

25.9

28.0

31.1

37.3

41.4

43.5

49.7

62.1

74.5

2.2

19.3

23.6

25.7

26.8

28.9

32.1

38.6

42.9

45.0

51.4

64.3

77.2

2.3

20.0

24.4

26.6

27.7

30.0

33.3

39.9

44.4

46.6

53.2

66.6

79.9

2.4

20.7

25.3

27.6

28.7

31.0

34.5

41.4

45.9

48.2

55.1

68.9

82.7

2.5

 

21.4

26.2

28.5

29.7

32.1

35.7

42.8

47.6

49.9

57.1

71.4

85.6

2.6

22.2

27.1

29.5

30.8

33.2

36.9

44.3

49.2

51.7

59.1

73.9

88.6

2.7

22.9

28.0

30.6

31.9

34.4

38.2

45.9

51.0

53.5

61.2

76.5

91.8

2.8

23.8

29.0

31.7

33.0

35.6

39.6

47.5

52.8

55.4

63.3

79.2

95.0

2.9

24.6

30.1

32.8

34.1

36.9

41.0

49.2

54.6

57.4

65.6

82.0

98.3

3

 

25.4

31.1

33.9

35.3

38.2

42.4

50.9

56.6

59.4

67.9

84.8

101.8

3.2

27.3

33.3

36.4

37.9

40.9

45.5

54.6

60.6

63.7

72.8

90.9

109.1

3.4

29.2

35.7

39.0

40.6

43.9

48.7

58.5

65.0

68.2

78.0

97.5

117.0

3.6

31.3

38.3

41.8

43.5

47.0

52.2

62.7

69.6

73.1

83.6

104.5

125.4

3.8

33.6

41.1

44.8

46.7

50.4

56.0

67.2

74.6

78.4

89.6

112.0

134.4

4

 

36.0

44.0

48.0

50.0

54.0

60.0

72.0

80.0

84.0

96.0

120.0

144.0

 


Those who get this can print off the table and use it. For anyone interested further, some explanations...

By far the most common way of measuring bellows factor is to use the quick disk. This clever idea involves placing a disk in the picture and measuring its size on the ground glass to calculate the bellows factor. I used this technique for many years but increasingly became frustrated with it for a number of reasons. The disk blows away in the wind - indeed it ruined my picture in the sand once. It is impractical to use if you photograph something on a vertical plane. I keep forgetting to do it and have often closed the lens down and loaded film before I remember to use it - very frustrating. Finally, if you practice macro photography any closer than lifesize as I have the disc is completely useless.

So I decided to try my hand at using a tape measure. By measuring the distance between front and rear standards for any given focal length it should be possible to use a simple look up table to calculate the exposure. After an hour with Stobel's LF book, a tape measure and a bit of trial and error I had worked out 0.5 stop increments for each of my lenses up to 3 stops of bellows factor. They were jotted down in a notebook. I guessed the exposure for distances between these increments.

I used this quite successfully for six months or so until Dave Tolcher saw what I was doing and asked me to send him the spreadsheet I was using. It suddenly dawned on me that I should be able to calculate this properly using maths rather than a scrappy notepad. And also calculate it for any focal length lens (we have different lenses).

So I did a bit of research, found the correct formula and built a simple excel model to deduce the ratio between the focal length and image distance (between the film plane and lens) for each fixed 0.1 increment of exposure compensation.

If you have a different focal length lens than the ones above, simply multiply the focal length by these multipliers to get the correct table:

Stops

Multiplier

0

 

1.0000

0.1

1.0353

0.2

1.0718

0.3

1.1093

0.4

1.1487

0.5

 

1.1890

0.6

1.2311

0.7

1.2746

0.8

1.3195

0.9

1.3659

1

 

1.4138

1.1

1.4641

1.2

1.5157

1.3

1.5691

1.4

1.6244

1.5

 

1.6816

1.6

1.7408

1.7

1.8020

1.8

1.8661

1.9

1.9318

2

 

2.0000

2.1

2.0705

2.2

2.1431

2.3

2.2187

2.4

2.2974

2.5

 

2.3785

2.6

2.4624

2.7

2.5491

2.8

2.6390

2.9

2.7319

3

 

2.8275

3.2

3.0314

3.4

3.2490

3.6

3.4823

3.8

3.7324

4

 

4.0000

 



There you have it. Hope you find it useful.

(c) Jon Brock, 2012