Sequence viewing > black & white film - processing Index - Resource - © Lloyd Godman

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.

 

 

 

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