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Old Dec 13, 2003, 9:46 PM   #1
Zal's Avatar
Join Date: Dec 2003
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Default Do I understand ISO correctly? ...

First of all, let me say that I've been a lurker on these boards for quite awhile. You all should be proud of your community here--it's very friendly, approachable, and incredibly helpful. Now, on to my question...

I believe I undersatnd the relationship between shutter speeds and aperature values. For example, the combination of 1/125 at f2.8 will give the same expsoure as 1/250 at f2.0, or 1/60 at f/4.

Now, my question is how ISO figures into that equation. Will an increase of 1 "stop" in ISO give you 1 stop of faster shutter speed? For example, let's say that a given exposure at ISO 100 is 1/125 at f2.8. If I bump the ISO to 200, will that allow me to get the same expsoure if I shoot at 1/250 at f2.8? Another example: if I meter the scene in ISO 100, and the meter tells me that to get a proper exposure at f2.8, I would have to shoot with a shutter of 1/15. That's too slow to hand hold, so if I increase the ISO to 800 (an increase of 3 stops), will that allow a shutter speed of 1/125? (3 stops faster than 1/15) In other words, is 1 "stop" in ISO actually equivalent to an actual stop of shutter speed or aperature value?

I hope that question makes sense. Thanks for the help.
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Old Dec 13, 2003, 10:03 PM   #2
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You have it right.

Just keep TANSTAFL in mind - There Aint No Such Thing As a Free Lunch. If you use a higher ISO, you will have more noise/grain.
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