Why bother changing base ISO?

Started Jul 18, 2007 | Discussions thread
RichardPillay New Member • Posts: 1
Re: Why bother changing base ISO?

Jim Kaye wrote:

important thing is that I don't see any difference in the noise level
depending on whether I bump ISO in the camera or change the
"exposure" setting in post. Julia Borg and others have talked about
this in other posts -- changing ISO being equivalent to making
"exposure" adjustments in post -- but I've started shooting this way

-- hide signature --

There may not be any difference in noise level, but there will be a diffeence in the image itself. Try it yourself with a more extreme example - say 5 stops underexposed.

First try a correctly exposed shot at ISO 100. A certain amount of light falls on the sensor, gets amplified and converted to digital. All well and good.

Now the light lessens by 5 stops.:

When you expose correctly by increasing ISO level, you are letting 5 stops less light reach the sensor, and the your change of ISO from 100 to 3200 has told the camera to amplify the light more than in your first shot so that the same amount of "electronic signal" from the sensor reaches the A/D convertor and gets converted to digital.

If instead you were to leave the ISO level at 100, again 5 stops less falls on the sensor (as compared to the first shot). However, this time, the lower signal gets amplified only as much as the first shot. What gets converted to digital is a very much lower signal, with most of your shadow detail lost, and simply not there to be boosted in PP.

The reason you are not really noticing a loss of shadow detail in the shots you've done is that the camera is working with 12 bits of A/D, while JPegs only use 8. You can get away with around 3 stops of underexposure and fix it in PP. More is a problem.

Best Regards,
Richard.

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