F Stops

Started Aug 11, 2012 | Discussions thread
dosdan Contributing Member • Posts: 500
Re: Acute.

Bingo. So, what's the reason for using a higher ISO? Simple:

  • We use the ISO control to adjust the output brightness of the photo for playback and in-camera jpgs.

  • We use the ISO control on the camera in combination with the camera's meter to indirectly set the aperture, exposure time, and/or flash power, depending on the exposure mode we're using (Auto, P, Av, Tv, M, etc.).

  • We use the ISO control for a given aperture and exposure time to balance read noise and blown highlights.

And that's what it's all about.

One other reason:

  • We use analogue gain before the ADC to improve ADC noise performance at the expense of reducing clipping headroom. It may also help to reduce LSB linearity issues, although the ADC is probably already dithered sufficiently for this.

If the ADC or combined sensor/PGA/ADC has low enough ADC noise (and sometimes the PGA, if that's an issue) , then analogue gain before the ADC is less necessary or completely unnecessary. A full "ISOless" implementation would offer improved clipping headroom (i.e. an additional, superior form of highlight recovery) when the current exposure is not sufficient for most/all sensels in the sensor to reach FWC i.e. in situations when you would normally boost ISO on a conventional DSLR.


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