Site Updated
June, 2004

EXPOSURE

f16 RULE
In conventional photography, where no light-meter exists or when it  fails, a 
time honored approach to estimating exposure is the f16 rule which suggests:
in bright sunlight, select a shutter speed as near as possible to the film speed   in use e.g. 1/125 second for 100 ISO, and set the lens aperture to f16
(or any reciprocal combination of aperture and shutter speed).

Practice in estimating the amount of available light
 under different lighting conditions
is a valuable aid to understanding exposure

See inside film box for exposure recommendations in pictograph form:

ISO 100
Bright Sun, Distinct Shadows = f16 conditions
Set aperture at  f16
Set shutter speed at 1/125 second

 ISO 100
 Bright Sun, intermittent cloud = f11 conditions
 Set aperture at f11
 Set shutter speed at 1/125 second

ISO 100
Cloudy Dull, diffuse shadows = f 8 conditions
Set aperture at  f 8
Set shutter speed at 1/125 second

ISO 100
Overcast / Open Shade = f 5.6 conditions
Set aperture at  f 5.6
Set shutter speed at 1/125 second

In pinhole photography we extrapolate from this principle and apply the knowledge to extremely small apertures e.g. f160 (project camera), extremely slow photographic emulsions e.g. ISO 4 (photo-paper) and prolonged exposure times under different lighting conditions.

The main advantages of this approach will be to predict exposure time, hopefully increase the number of successful attempts at pinhole image making and, over time, gain an understanding of lighting conditions.

 f16 RULE applied to pinhole:
see also f-stop progression and interval of time/shutter speed

available light

f16

f22 

f32

f45

f64 

 f90

f128

f180

f256

f360

f512

f720

aperture

f16 conditions

 1/4

1/2

1

2

4

8

15  

30

60

120

240

480

 time seconds

f11 conditions

1/2

1

2

4

8

15

30

60

120

240

480

960

f8 conditions

1

2

4

8

15

30

60

120

240

480

960

1920

f5.6 conditions

2

4

8

15

30

60

120

240

480

960

1920

3840

NB time intervals (shutter speeds) have been rounded down for convenience.
The time interval progression, strictly speaking, is: 16.32.64.128.256.560 seconds.
In practice, amending 16 seconds to 15 facilitates conversion to minutes:
15.30.60.120.240.480.960.1920.3840 become 1/4.1/2.1.2.4.8.16.32.64 minutes
This ‘liberty’ is appropriate in extended pinhole exposure times due to the confounding variables of actual pinhole diameter, actual relative f-stop, actual ISO of the materials employed/reciprocity and accuracy of timing device used.
 

using photo-paper with a nominal ISO of 4, the project camera with a relative
 f-stop of f160 (use f180) the (approximate) predicted exposure time is:    

 30 seconds

bright sunlight

 f16 conditions

  1  minute

intermittent cloud

 f11 conditions

  2  minutes

overcast/cloudy

 f 8  conditions

  4  minutes

open shade

 f5.6 conditions

RECIPROCITY
When used outside the designated aperture/shutter speed range photosensitive emulsions are subject to reciprocity law failure.  In practice it will be found that  the prolonged exposure time(s) required with small pinhole apertures may require  additional time to achieve the required density/exposure.   Film (especially colour positive/transparency emulsions) are subject to reciprocity failure. Photo-paper intended for tungsten light source/ relatively long exposure times is more forgiving The following compensation factor table is adapted from  Jim Shull's The Hole Thing

indicated exposure

  x compensation factor

actual exposure

1 second

1 .25

1 .25 seconds

5

1 .5

7.5

15

2

30

45

2.5

112.5

2 minutes

3

6 minutes

5

4

20

10

5

50

20

6

2 hours

40

8

5.5


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