Underexposing vs. Raising ISO: a question for the sensor experts

Started Jul 17, 2012 | Discussions thread
DigVis Forum Member • Posts: 97
Re: bobn2...


mnodonnell wrote:

Regarding the tools, I was using Aperture 3 for the RAW conversion and all edits. My technique regarding the lifting is probably not very good.

The proper way to lift these exposures in Aperture is to use the Exposure slider. The slider itself only goes to 2, but the number can be dragged (or edited manually) up to 9.99. The exposure slider will give you what is identical to a pure multiplication of the linear (raw) data.

I did think this was a high DR scene, but I must admit I don't know to measure it. I did slightly lift the shadows in the ISO 100 control scene.

You should probably try to avoid the Shadows slider (in the Highlights & Shadows adjustment) in a test like this. It will apply non-linear and local adjustments that may not match exactly between the exposures.

My knowledge of gamma is limited. I understand conceptually that it has to do with manage the differences between the linear manner in which the camera captures data and the non-linear way that our eyes perceive brightness.

It has to do with compensating for the non-linear way monitors (used to) map voltage into brightness, restoring the linear output signal for your eyes. One other effect is that the non-linear gamma curve allows a greater DR to be encoded in 8 bits. The sRGB curve (which is not a pure gamma curve) encodes 11.7 stops of DR, while the AdobeRGB does in theory encode 17.7 stops. A linear curve would be limited to 8 stops when encoded using 8 bits.

What I had assumed is that most (all?) PP software is actually displaying the gamma-corrected image, so I probably have to change a setting somewhere to see a linear conversion, correct?

Don't worry about gamma if you are using Aperture. As far as I know Aperture does all edits in the linear domain and applies the gamma curve only on presentation and for the final .jpg (and when exporting to an external editor).

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