ISO Invariance: an experiment

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Tom Axford Veteran Member • Posts: 4,909
ISO Invariance: an experiment
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Two recent threads (this and this) have generated a lot of discussion of "ISO Invariance" and related matters. These discussions have been almost entirely theoretical and largely centred on dynamic range and noise.

However, the experimental approach ("suck it and see") raises a couple of extra points that seem to be often overlooked.

For example, here are two images with the same shutter speed and aperture, but differing ISO settings:

Taken at ISO 6400 and at the exposure indicated on the camera's meter. Default processing in Lightroom.

Taken at ISO 200 and the same exposure as the previous shot. Processed in Lightroom with Exposure +5.0

The first point I would like to make is that to do this experiment, you need to choose not only the camera, but also the software to do the raw processing. I haven't tried processing with anything other than Lightroom, but I would guess that it makes a difference what software is used. For instance, Lightroom allows a maximum of 5.0 stops of exposure compensation.

The second point is that it is obvious from the shots above that not only do the noise levels and noise characteristics differ in the two images, but the colour balance differs also. Both shots were processed at daylight white balance set in the camera, and then carried through to LR.

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