Are there any "rules" for using ISO?

Started Apr 3, 2013 | Discussions thread
nelsonal Senior Member • Posts: 2,464
Re: Are there any "rules" for using ISO?

GreenMountainGirl wrote:

baloo_buc wrote:

There is certainly a rule:

Increase the ISO when other means of adjustment (shutter speed and aperture) could not be used anymore.

In most cases you can set the aperture as wide as possible and the shutter speed as high as needed and then increase the ISO speed.

This is what I do, but still tend to get some underexposures.  The photos are taken in early evening when the light is falling away.  For instance, took some photos at f/5.6, 1/250sec,  ISO 320 which turned out OK.  But when I raised to f/16 and 1/500sec, ISO 800 the picture was underexposed.

Just so you know, going from f/5.6 to f/16 is reducing the aperture/exposure by 3 stops, the shutter speed adjustment was a 4th stop reduction, and your ISO increased brightness by a stop and a third so you're second photo was 2 and 2/3s less bright than the first.  Leaving the aperture at f/5.6 with the other two settings (1/500th and ISO 800 would have resulted in a second picture that was similarly bright as the first).

If the second exposure was selected on manual, you'll want to add stops somewhere to compensate for stops you're removing, if it's on an auto-metered setting (my guess would be spot) it would appear to have metered on something considerably lighter colored (like a sunlight reflection or patch of pure white hair in the light).

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