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.
Want to learn more? - do a workshop or one on one with Lloyd Godman
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