The Aperture or Stop value is based on the amount of light the lens is capable of transmitting which is related to its Focal length and size of the entrance pupil. A lens in which the size of the largest opening is the same as its focal length is said to have an aperture value of 1 or ƒ1
A lens opening which lets in half the amount of light will not be half the diameter but √2 as we are dealing with reducing the area by half thus √2 = 1.4
So a aperture stop value that is half the preceding is represented by a value 1.4x as shown with ƒ1 being the theoretical maximum value ƒ16 allowing only a 1/256 as much light through as ƒ1
and so on ...
The shutter speed determines for how long a given amount of light is allowed to fall upon the film, these have been standardised as follows:
The sensitivity of film to light has been standardised by various methods and test and the one in use today is the ISO standard based on the old ASA\DIN values
They are printed on the film box and cassette
ISO - International Standards Organization
ASA - American Standards Association
DIN - Deutsche Industrial Norm
ASA like ISO which it predates is an arithmetic scale in which doubling of the sensitivity is indicated by a doubling of the ASA value
DIN is a logarithmic scale in which a doubling of sensitivity is represented by an increase of 3 DIN units
|ISO (ASA)||DIN (log)|
Photographers concept of the 'Stop'
It is common for Photographers to talk in terms of 'stop' values when considering exposure but they may not be talking about the aperture stop value 'per say' but rather the concept of a 'stop' being a doubling or halving of the previous value.
You may be considering using 100 ISO film on a Bright Sunny Day (BSD) which would normally require a ƒstop value of ƒ16 and a shutter speed value of 1/125sec for a standard exposure.
If the subject was significantly darker than normal you might decide to "give it a couple more stops exposure.
This can be achieved by,
1. opening up the aperture by two stop values to ƒ8
2. 'reducing' the shutter speed by "two stops" to 1/30
3. you could use a film two stops more sensitive i.e. 400ISO
There a good reasons why you would do one or the other or a combination to mitigate for the 'side effects' of altering each of the factors
NB. Some lens and camera systems show increments in terms of of 1/3rd and some 1/2 intermediate stop values
The relationship between the 'ƒstop' value and the shutter speed is called a Reciprocity relationship.
If you reduce one by a 'stop' and increase the other by a 'stop' the exposure remains the same; well it does between limits.
For example on a Bright Sunny Day (BSD rule) using 25 ISO rated film you could select the following range of values what exposure times would be appropriate if you were using 100 ISO rated material?
|BSD Reciprocity Values using|
|ISO 25||ISO 100|
|ƒstop||time sec||time sec|
At either extreme i.e. very long or ultra short exposures the film or sensor may not respond in a linear manner and you will suffer 'Reciprocity failure' a compensation table is normally supplied within the film container, in the case of a digital sensor you will find that most cameras are limited to 30 seconds and manually timed open shutter is not possible you need to refer to your digital camera manufactures specification