Merrill "base ISO" revisited

Started Jun 12, 2016 | Discussions thread
PrebenR Veteran Member • Posts: 4,164
Re: Merrill "base ISO" revisited

xpatUSA wrote:

We will not find the term "base ISO" in the ISO Standard for how to determine what they call the "Recommended Exposure Index" (REI) - but another publication does refer to the ISO saturation-based method as being "base ISO".

It is possible to estimate base ISO for the Merrill F20 sensor by prorating published data for the SD14 F13 sensor.

The ISO formula is simple enough: Isos = 78/Hsat where Isos is the saturation-based index and Hsat is the exposure for sensor "well full". The formula gives half a stop headroom which is buried in the constant 78.

Hsat for the SD14 can be calculated from published data as being 0.81 lux-sec for "well full" (middle layer). Taking "well full" as being inversely proportional to actual photo-sensing area, I make it 0.405 lux-sec for the the Merrill.

Which makes the Merrill's base ISO an estimated ISO 193.

Previously I had estimated ISO 160 but I had forgotten that the Merrill sensor uses far fewer transistors than the SD14 so I upped the actual fill factor a bit.

The only ISO is 100.

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