rating films stocks and the iso

[darryl walthall]

can someone explain the ralationship between over and under exposing a film stock as it relates to the iso . If, for example, I am using a film stock that has an iso rating of 1000 and I rate it at 500, is that over or under exposing and why and by how much of an over or under exposure is is; 1 stop or 2 ?

[Dirk DeJonghe]

Exposing a 1000 iso stock as 500 iso is overexposing it one full stop. There is one full stop difference everytime the ISO is doubled.

 

say: 100 125 160 200250 320 400 500 630 800 1000

 

Also exposing 500 ISO stock as 1000 ISO is underexposing, exposing 320 ISO stock as 160 is overexposing one full stop.

[David Mullen ASC]

As to why rating a 1000 ASA film at 500 ASA is overexposing by one stop instead of underexposing by one stop, just think about it.

 

1000 ASA film is "faster" and needs less light. 500 ASA is "slower" than 1000 ASA and therefore needs more light, right? So if you treat a fast film like it's a slower film, you would be using more light for a film that doesn't need it, and thus overexposing it.

[John Pytlak RIP]

Film exposure:

 

http://www.kodak.com/country/US/en/motion/...exposureP.shtml

 

Exposure table:

 

http://www.kodak.com/country/US/en/motion/....shtml#incident