Why 'more or less stopped using ISO'?

Started Feb 19, 2014 | Questions thread
xpatUSA Forum Pro • Posts: 12,851
Re: Why 'more or less stopped using ISO'?

edhannon wrote:

There are three places that brightening can be applied:

  • With amplification between the sensor andq analog to digital conversion (e.g. Pentax *ist line)
  • By using more bits in the analosg to digital conversion than are used in raw output file then bit shifting in firmware (e.g. Pentax K10)
  • By processing the raw data either in camera or in the raw converter.

The last one uses cheaper hardware but results in loss of effective bit depth.

Not sure what 'effective bit depth' is, so I did a quick test:

Took two shots, one at ISO 100 other at 400 (camera Sigma SD9). As expected on this camera, the raw data histograms are virtually identical:

view original to see the numbers

When developed in Sigma's converter, the one is rendered darker that the other, again as expected:

Saved as TIFF 8-bit

Re-opened the darker (ISO100) image and brightened it by 2EV, thereby "losing effective bit depth"? and saved it, also as 8-bit TIFF. Then analyzed the identical-looking images in ImageJ:

view original size to see the histograms

There appears to be no loss of bit depth.

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