Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
Anders W
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Re: It's about useful comparisons - not which is better
In reply to Martin.au, Mar 20, 2013

Don't forget, with error bars of 1/6th, the gap betwetwo the OM-D and the G3 could be anything from 0 to 2/3s of a stop. Given the results so far I'd be guessing it may be smaller than 1/3.

The DPR RAW comparison widget is working now so I ran the check for equal exposure of the E-M5 versus the G3 studio scene samples with both cameras set to ISO 200.

At that camera ISO, the G3 has a DxO "measured ISO" of 187 versus 107 for the E-M5. If the two cameras have received the same amount of light on the sensor, the ratio between the "measured ISOs" should be the same as the ratio between the saturation levels of the RAWs. The saturation levels can be measured (by downloading the RAW files and checking them out via RawDigger) as the green channel average (since the green channel is the strongest) divided by the clipping point. The G3 saturation level thus measured is 0.155 and that of the E-M5 0.097.

The ratio between the "measured ISOs" is 187/107 = 1.75 and the ratio between the saturation levels 0.155/0.097 = 1.60. It follows that the G3 received slightly less light on the sensor than the E-M5 but that the discrepancy is small (about 1/8 EV).

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