Film
Speed
Each film has an ASA or ISO rating which indicates that
particular films sensitivity to light and from this it is
possible to calculate the exposure needed for a particular situation.
Relative
exposure
needed |
ASA or
ISO |
Scheiner |
Din |
Weston |
1unit
of light |
800 |
41
degrees |
30°/10 |
640 |
2
units of light |
400 |
38
degrees |
27°/10 |
320 |
4
units of light |
200 |
35
degrees |
24°/10 |
|
8
units of light |
100 |
32
degrees |
21°/10 |
|
16
units of light |
50 |
29degrees |
18°/10 |
|
32
units of light |
25 |
26
degrees |
15°/10 |
|
Comparison
of Film speed systems
Besides ASA, and ISO there ore other
tables for film sensitivity, which include, Scheiner, Din, Weston
Scheiner It is essential to understand
this when exposing the film to light, because a film with
a higher ASA needs less light than one with a lower film
speed rating. 100 ASA is twice as sensitive to light as
50 ASA; meaning 50 ASA film would need twice the total amount
of light as the 100 ASA film. There are also other scales
for film sensitivity.