Are there any "rules" for using ISO?

Started Apr 3, 2013 | Discussions thread
mike703
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In reply to GreenMountainGirl, Apr 4, 2013

GreenMountainGirl wrote:

I was using Av most of the time until I wasn't getting fast enough shutter speeds, so switched to shutter-priority.  That is when I began having trouble with exposure, and made some wrong turns in the process!

That helps to explain some things.  Shutter priority is a dangerous mode to use in low light because you will very easily underexpose if your lens won't open wide enough.

The reason is that the range of apertures available for the camera to select from (after you have chosen the shutter speed) is very small.  Suppose you decide that you want a shutter speed of 1/500 sec and you have set ISO800, and it is dusk.  The correct aperture may need to be f/2 (i.e. very wide) to let enough light in for a correct exposure... but if your lens only opens up to f/4, which is quite likely, the camera will still take the pic using f/4.  There will be a warning light somewhere indicating underexposure of two stops, but it is easy to miss.

Conversely if you use Av mode and select aperture, the camera will always be able to find a shutter speed that gives a correct exposure, because shutter speeds vary over a much wider range.  In the above case if you opened your lens to f/4, the camera would choose 1/125 sec and exposure would be correct.  That may be slower than you want to avoid camera shake, but at least exposure is correct, and you can increase the ISO to let the shutter speed get faster.

So slightly perversely the best way to get the fastest shutter speed that you can for given lighting is to

(i) use Av mode and open the lens up completely and

(ii) set the highest ISO you are comfortable with.

Then, the shutter speed selected by the camera will be the fastest you can get for that light.

Best wishes

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