Wilt wrote in post #18661923
Pointless! Unless you measure at the same time of year, there can be variances. And on a given day, there may be more/less water vapor or smoke in the air to dispell light. And unless you use the SAME meter in both places, you cannot account for differences in individual meter calibration!
There is also color-specific concerns. The illumination also varies in color balance, even at different latitudes in the same air quality. The green channel in a Bayer digital camera is what is of concern in exposure index, because the green channel is always more sensitive in a Bayer camera, and is what is in the most immediate danger of clipping. The green channel is what the camera targets for ISO exposure index, and the red and blue channels just fall where they do, almost always with less signal in the sensor, under the color filters.
So, a highly green-weighted value of sun strength is actually the most precise for Bayer camera concerns regarding "ISO" and "exposure".
It is a pity that cameras don't do more to guide us in RAW-relevant exposure; as I said in another post in this thread, people are wishing for lower ISO, have them in their cameras, under the hood, and don't use them. Most of the newest cameras are capable of ISO exposure indices down to 64 (Canon) or to 44 or less, at base ISO! What a camera with a saturation-based base ISO of 44 doesn't have is extended highlight headroom, like negative film. It does as well as slide film, though, when you are in control of exposure vs RAW file and don't need headroom insurance.