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What's An 'ISO'?

ISO is the standard for film sensitivity. 
There used to be several different standards for film sensitivity depending on where the film was manafactured, and the numbers all changed. This included ASA & GOST. 
Luckily all film produced now, uses ISO. Although some old film cameras, still use these measurements, requiring you to convert them into an ISO value. 

A low ISO or low speed film, is used on bright sunny days. They have finer grain and sharper images. The camera needs to use a lower aperture (EG - f/3.5) and a slower shutter speed (EG - 1/30th), to achieve a perfect exposure. These range from 25 - 100 iso. 

A medium ISO or medium speed film, can be used in the shade, cloudy days or indoors. The grain is a bit more noticeable (Not by much). They tend to be all-purpose speeds for a varierty of conditions. The camera can use a smaller aperture (EG - f/5.6) and a faster shutter speed (EG - 1/100th), to achieve a perfect exposure. These range from 200-400 iso. 

A fast ISO or fast speed film, can be used in dark conditions and dimly lit situations. The grain is a lot more noticeable, and doesn't have the perfect sharpness of a low speed film (Although, this can be quite desirable when using BW film). The camera can use a smaller aperture (EG - f/8) and an even faster shutter speed (EG - 1/500), to achieve a perfect exposure. These range from 800 - 3200 iso. 

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