Sequence viewing > Photographic Exposure - Resource - © Lloyd Godman

What is ISO

Sensitivity ratings were defined as a means of identifying the sensitivity of film. Different films had an inherent sensitivity to light. ISO - is a sensitivity rating originally given to photographic film that referenced this sensitivity difference. The sensitivity ratings have since been carried over into digital photography and applied to the CCD sensor that receives the light and records the image in a digital camera as a series of varying electrical voltages.

So ISO applies to both film and digital capture.

If we imagine film or the CCD as a bucket that needs to filled exactly to the top to gain the correct exposure - different ISO settings are like different size buckets.

25 ISO is a much larger bucket and needs twice as much exposure than 50 ISO which is a smaller bucket - and 100 ISO is an even smaller bucket etc.

 

Below is a table that outlines ISO settings, relative amounts of light needed etc.

Each setting is twice or half as much as the next one, and the difference in each of these is called a stop.

ISO Rating
25 
ISO
50 
ISO
100
ISO
200
ISO
400
ISO
800
ISO

1600

IS0

3200

ISO

6400

ISO
Relative amount 
of light need  to 
expose film
256 units
of light
128 units
of light
64 units
of light
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 ISO
4 times 
as sensitive
as 25 ISO
8 times
as sensitive
as 25 ISO
16 times
as sensitive
as 25 ISO
32 times
as sensitive
as 25 ISO
64 times
as sensitive
as 25 ISO
128 times
as sensitive
as 25 ISO
256 times
as sensitive
as 25 ISO
 
 
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
6 stops faster than 25 asa
7 stops faster than 25 asa
8 stops faster than 25 asa

Many professional SLR cameras (both film and digital) and also hand held light meters allow for 1/2, 1/3 or 1/4 stop settings between each full stop setting. For instance the Canon 5d MII has the following ISO settings in 1/3 of a stop - the full stops are in red. Today when we use a digital camera and rotate the ISI settings we see a series of numbers, so it is easy to lose track of the full stop settings.

100 125 160 200 250 320 400 500 640 800 1000 1250 1600 2000 2500 3200 4000 5000 6400

 

 

 

Digital

As mentioned the CCD sensor that receives the light and records the image in a digital camera as a series of varying electrical voltages. Faster ISO settings require less light for a correct exposure allow the photographer to shoot in situations with less light hand held at a shutter speed that does not require a tripod.

However with the increased sensitivity come a degradation of image quality. With digital this is termed noise and the image appears less smooth than an image shot at a lower ISO. So shooting with a slower ISO will produce a higher quality image, than a lower, but a higher ISO allows you to shoot hand held in a lower light level.

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 ISO (International Standards Organization) or ASA (American Standards Association) or 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 ISO needs less light than one with a lower film speed rating. 100 ISO is twice as sensitive to light as 50 ISO; meaning 50 ISO film would need twice the total amount of light as the 100 ISO film. As with aperture and shutter speed scales, the difference between ISO settings is one stop.

Film is made from thousands of tiny grains of silver. While faster film allows the photographer to hand held the camera without camera shake in lower light levels, it also increases the grain and overall sharpness of the image. But unlike digital with negative film, this is more pronounced in the high light areas of the scene where the negative is denser.

 

 

Image shot on 400 ISO

Detail of sky -

Image shot on 1600 ISO

Detail of sky at same magnification as the image above - note the speckled noise

 

 

8

 

investigation

ISO setting


 

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