ISO variations: shitter speed cheating vs headroom choices

Started Mar 17, 2012 | Discussions thread
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BJL Veteran Member • Posts: 8,900
ISO variations: shitter speed cheating vs headroom choices

In the debate about variations between ISO settings on different cameras, there sem to be two different ways that cameras vary:

1. Different shutter speed at the same f-stop, ISO setting and scene lighting.

2. Same shutter speed in the above scenario, but different placement of the tonal levels, such as more headroom between mid tones and maximum level.

The first is arguably cheating when a lower than normal shutter speed is used, as seems to be the case in some Canon vs Nikon comparisons. This needs adjustment in high ISO performance comparisons: I would like graphs where the scale at the bottom is actual shutter speed rather than ISO speed (these two scales should roughly agree in a situation light bright sunlight at f/16, or four stops dimmer than bright sunlight with f/4.)

The second situation is just a matter of there being legitimate room for different decisions about tonal placement and highlight headroom. It would help in image comparisons to equalize the tonal placement, but no adjustment is needed in graphs of signal to noise ratio or so-called "dynamic range at a higher than base ISO speed".

In particular, DxO measures ISO speeds on the latter basis of highlight headroom, and then shifts it's DR and SNR curves in a way that only makes sense with the former type of discrepancy, thus incorrectly penalizing a camera with equal good low light SNR but less risk of highlights being blown by hitting the maximum digital output level: the left shift applied to a camera with more highlight headroom at higher ISO settings artificially pushes the DR or SNR curves down.

So how do the E-M5 and G3 compare in these two different aspects of ISO speed discrepancies?

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