Exposure question?

If 1/15th sec.shutter at ISO 400 is a certain exposure, what would be the equivalent ISO values at the following shutter speeds?

1/30th - ?
1/60th - ?

Thanks, trying to figure out my limitations on flash shooting and where to set it's upper limits on Auto-ISO. I would like to shoot ISO 800 but I don't want to deal with the noise.

JP

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John M. Polston
Atlanta, GA