# ISO/ASA

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### #1 darryl walthall

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Posted 07 May 2008 - 06:37 AM

With each change in the ISO, how much exposure loss or gain is created. For example, if I rate my film at a ISO of say 25 on one take and then rate my ISO at say an 8 on the next take, how does this change relate to exposure increments. Is this is a change in a stop or two ?
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### #2 John Holland

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Posted 07 May 2008 - 06:53 AM

Its about a stop and a half.
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### #3 Freya Black

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Posted 07 May 2008 - 07:23 AM

With each change in the ISO, how much exposure loss or gain is created. For example, if I rate my film at a ISO of say 25 on one take and then rate my ISO at say an 8 on the next take, how does this change relate to exposure increments. Is this is a change in a stop or two ?

I'm not sure I understand the question but I always tend to assume that half the ASA means 1 stop.

For example from 500 to 250 ASA is one stop, from 200 ASA to 100 ASA is one stop.
From 100 ASA to 50 ASA is one stop.

I'm not sure how accurate that is but it helps me to think about things.

love

Freya
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### #4 Laurent Andrieux

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Posted 07 May 2008 - 07:25 AM

Rule is a 1 stop gain is equivalent to doubling the ISO sensitivity. and you lose 1 stop when dividing it by 2.

then one must know the intermediate values, usually given by third :

Between 100 and 200 ISO is a full stop, 125 1 third, 160 1 other third, you then can find out that between 8 and 25 ISO is exactly 1 stop (you go to 16) and 2/3 of a stop (20 a third, 25 another one).
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