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

What is the speed of a film?
There are different types of film and each type has a different sensitivity to light.

All films have what is called a characteristic response curve that expresses the response of that particular film to various amounts of light.

Based on this, each film has an ASA  (american standards association) or ISO (International standards organization ) rating which indicates that particular films sensitivity to light. 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. As with aperture and shutter speeds the difference between a doubling or halving of the film speed is one stop.



 
 
 
If we imagine film as a bucket that has to filled to the top to gain the correct exposure, different asa films are like different size buckets. 25 asa is a much larger bucket and needs twice as much exposure than 50asa.


 
 
ASA Rating 25 
ASA
50 
ASA
100
ASA
200
ASA
400
ASA
800
ASA
Relative amount 
of light need  to 

expose film
32 units
of light
16 units
of light
8 units
of light
4 units
of light
2 units
of light
1 unit 
of light
Relative sensitivity of film   Twice as 
sensitive 

as 25 asa
4 times 
as sensitive

as 25 asa
8 times
as sensitive

as 25 asa
16 times
as sensitive

as 25 asa
32 times
as sensitive

as 25 asa
   
1 stop
faster than 25 asa
2 stops
faster than 25 asa
3 stops
faster than 25 asa
4 stops 
faster
than 25 asa
5 stops faster than 25 asa


For the Drawing from Nature  project I used 100asa film stock.

 

 

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