Originally posted by GUB It never occurred to me to meter anything.
I wasn't thinking of this as a test of metering.
It would be reasonable to presume the meter would have tried to place the greycard on the centre of the histogram in both situations wouldn't it? (spotmeter).
I would expect that metering the grey card would have returned EV 15 if the light were actually appropriate for Sunny 16. Doing an incident light measurement would also have been a good check. Doing so tests the rule against standard-referenced instrumentation.*
The problem with histograms is that they represent the output intent of the rendering/processing engine and not the light intensity. A good example of how that works is to simply add one stop brightness/exposure in a tool such as Lightroom. Instead of the histogram shifting right one stop, the tool changes the curve shape. Similar is done on import, even with JPEGs.
Steve
* Some care should be taken with metering that the measurement is of the grey/gray card and not the rest of the frame. As noted, this is one of the cases where the spot meter is one's friend.