Lesson 12ª

 

 

 

 

 

   

 

Film speed

Film speed is the capability of the film to react to light. This is determined when films are manufactured. Since digital cameras don't use film but sensors, ISO equivalents are used. ISO determines how sensitive the sensor is to the light present in a scene we want to shoot.

There are several systems which indicate film speed; ASA  (American system) and DIN (German system). These were old systems which have been replaced by a system called ISO. The numbers and values are the same as in ASA.

These are some ISO values:

25,    50,    100,     200,     400,     800,    1600,     3200,   6400

The higher numbers indicate a higher film speed (higher sensitivity to light) while lower numbers represent a lower film speed (lower sensitivity to light). THis means that, for example, an ASA 100 value is less sensitive to light than an ASA 800.

Between every ISO value, there is 1 point in difference. What does this mean? For example, ISO 100 is 1 point less sensitive (half) towards light than ISO 200. And vice versa, ISO 200 is 1 point more sensitive (twice) than ISO 100.

The ISO in electronic (SRL) and digital (DSRL) cameras comes in thirds. This enables us to have more control over the image.

100,   125,  160,   200,    250,   320,   400,    500,    640,    800

The advantage of digital cameras is that I can change the ISO value in each photograph, while in regular cameras (film cameras) we need to finish the film we are using in order to be able to change to another film speed.  

The film speed is one of the 3 values, next to the diaphragm and shutter speed, which control the exposure of a photograph. As a matter of fact, film speed is the first value that we need to consider when taking a picture.

picture

In this photograph, I used a ISO 200. As you can see, It came out dark because there is very little light in this indoors environment. This image is too dark or under-exposed.

 

picture

I shot this photograph just after I had shot the previous one. I used an ISO 1600. ear in mind that the shutter speed and diaphragm I used were the same. In this case, the photograph is properly exposed and it received the correct amount of light.