Index
Photosensitive Material
The election of the photosensitive material is very important, since it determines
physical final support that our images will have and the time of exposition of the take.
According to the quantity of light we hope to provide for the take we will choose
between paper or movie. Both have advantages and inconvenients:
| |
Advantages |
Inconvenients |
| Paper |
The load/ discharge and the developing could be carried out with illumination of
security. |
Very low sensibility (ASA 4 aprox.).
Slower time of exposition.
Minor neatness copies. |
| Film |
Greater sensibility.
More rapid time of exposition.
Positives of greater neatness. |
The load/ discharge and the developing should be carried out totally to dark. |
The first option is valid for very illuminated scenes and cameras
with a pinhole of great diameter, while the second option is preferable for take
with little light and cameras with small pinholes.
Calculation of the exposition
In order to calculate the time of exposition of a take we are supposed to have a quite
approximate idea of the quantity of light that reflects the scene. The most useful
apparatus in order to measure this light is the photometer, if you haven tone you
can have an idea by trying several times with different times of exposition. So, we have
several possibilities for the calculation of the exposition:
- With a manual photometer
- The manual photometers are the most useful to carry out pinhole takes. They are formed
by a photosensitive cell that measures the light reflected by the scene and a mobile
circular scale, where, once adjusted the obtained value, shows us a series of combinations
of diaphragm and speed of valid exposition for that measurement.
These photometers
allow measurements of multiple light and in extreme conditions facilitate
the election of a combination of correct diaphragm-speed. Yet so, few photometers have
into account in their scales diaphragm values as small as the usual ones in pinhole
cameras (180, 256, 360...).
In order to solve this problem we can construct a mobile scale with two ribbons
of cardboard so that, slipping one on the other, we get a continuation of the scale of the
photometer. Here is an example:
Diaphragms
| 22 |
32 |
45 |
64 |
90 |
128 |
180 |
256 |
360 |
512 |
Speeds of exposition
| 1/8 s. |
1/4 s. |
1/2 s. |
1 s. |
2 s. |
4 s. |
8 s. |
15 s. |
30 s. |
1 m. |
2 m. |
4 m. |
8 m. |
The calculation of the exposition will be carried out measuring the
light of the scene with the photometer and transferring that measurement to the mobile
scale, from where we will take a combination of diaphragm-speed as reference, and we will
transfer it to our own scale, getting a value of speed for our diaphragm.
We are supposed to remember whenever it is necessary to apply the compensation for the failure of reciprocity to get the time of correct
exposition.
- With the photometer of a camera
- If a manual photometer is not avalaible, we can use the photometer that great part of
the reflex cameras - their operating mode is very similar -, provide but instead of
returning a ok value of the quantity of light (EV, for example), it shows us the speed
of exposition for the diaphragm that we indicate in the objective directly.
The
procedure in order to calculate the speed of exposition in a pinhole take is the same that
is followed with a manual photometer, with the difference mentioned
above.
ATTENTION: many modern reflex cameras divide the frame in zones and they ponder
and/or dimidiates the measurements of each zone, so they return a value of exposition
that is NOT a direct measurement of the light of the frame. We recommend to consult the
manual of the camera and find out the type of balance and/or average it applies.
- Without photometer
- A precarious situation can be solved with much patience and several tests.
In many
cases the camera is basic, the sensitive support is photographic paper, and there is no
photometer, but the dark room is near the scene me wish to photograph. If these are the
circumstances, we can get a good take after some tries. One can follow these instructions:
| Light of the scene |
Time of exposition |
| Sunny day with clear atmosphere. |
From 4 to 10 minutes. |
| Slightly foggy day or scene with some shades. |
From 10 to 20 minutes. |
| Day with clear clouds that disperse the light. |
From 30 to 50 minutes. |
| Day with dark clouds. |
From 1 to 2 hours. |
Compensation of the failure of reciprocity
The last adjustment to carry out in order to get a correct exposition is to balance the
failure of reciprocity, that is produced by exposing sensitive materials for a long
time.
The reciprocity is a property of the sensitive materials that makes that the impression
of a take (I) depends directly of the intensity of the light (i) and
on the time of exposition (t):
With times of exposition longer than 1 second, the failure of reciprocity produced can
be balanced by increasing these times. The factor of compensation to utilize relies
on the concrete type of support that we utilize. If we could utilize the table of
compensation of the maker of the support we will have a good guide. Otherwise the only
possible option is to fit the compensation by means of tests.
The table that follows is the one which proposes Manuel Serra (PhotoVision Magazine -
Spain - No. 15. Page 9) for 4 ASA photographic paper, although can also be used for
film:
| Exposition Suitable |
Factor of Compensation |
Corrected Exposition |
| 1 sec. |
x 1,25 |
1,25 sec. |
| 5 sec. |
x 1,5 |
7,5 sec. |
| 15 sec. |
x 2 |
30 sec. |
| 45 sec. |
x 2,5 |
1 min. 52 sec. |
| 2 min. |
x 3 |
6 min. |
| 5 min. |
x 4 |
20 min. |
| 10 min. |
x 5 |
50 min. |
| 20 min. |
x 6 |
2 hours |
| 40 min. |
x 8 |
5 hours 20 min. |
Practical example
We will take here the example of construction of a camera
proposed in the page of construction of cameras in order to
facilitate the understanding of this example.
Imagine that the scenery to photograph is a landscape and the light of the sun
falls sidelong. The sensitive support chosen is 100 ASA film.
- We will measure the general light from the position of the camera.
The most advanced photographers apply their own system of multiple mensuration
and of balance of measuring. We won't give further details given the variety of methods
and the validity of them.
The measurement that we get is 18 EV.
- Bringing 18 EV to the scale of the photometer me get the combination of diaphragm-speed:
f 22 and 1/ 15 sec.
- Adjusting the previous combination in our mobile scale me get a series of equivalent
combinations, we will choose the one which diaphragm of our camera: f 256 and 8 sec.
- Finally we balance the failure of reciprocity making an approach to the
Manuel Serra's table: 8 sec. x 1,6= 12.8 sec. = approaching 13 sec.
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