rating the asa

[darryl walthall]

sorry I have asked this question before but I thought I could save the answer in my folder and it appears that the answer has been deleated from my folder.

My question is this : if I lower the asa/iso is that over exposing or under exposing. If I increase the asa /iso, is that overexposing or underexposing and by how much . If a film stock is "box" rated at say 400 asa/iso and I rate it at 100 asa/iso, am I overexposing the film or underexposing and by how much ?

Please clarify and this time I will transfer the answer into my notebook for safe keep and later reference .

Thank you very much.

Darryl Walthall

[John Pytlak RIP]

If the film manufacturer rates the film at EI500, and you expose it as if it was rated EI250, you are OVEREXPOSING the stock by 1 stop. If you expose the EI500 film as if it were a EI1000 film, you are UNDEREXPOSING it by a stop.

 

Nominal 1/3 stop steps are used for rating film speed. Here is the series used, from slowest to fastest film:

 

25, 32, 40,

50, 64, 80,

100, 125, 160,

200, 250, 320,

400, 500, 640,

800, 1000, 1280

 

Do you see the pattern?

 

(Hint, if you change the speed by a factor of two, you have changed by one stop)

 

The Kodak website has a wealth of information on the topic:

 

http://www.kodak.com/US/en/motion/support/h1/exposure.shtml

 

http://www.kodak.com/US/en/motion/support/h1/structure.shtml

 

http://www.kodak.com/country/US/en/motion/.../h2/index.shtml

 

http://www.kodak.com/US/en/motion/students...0.1.4.9.6&lc=en

 

Of course, Kodak also offers a wide variety of films: B)

 

http://www.kodak.com/US/en/motion/products...d=0.1.4.4&lc=en

 

http://www.kodak.com/US/en/motion/products...0.1.4.4.4&lc=en