I too have seen inconsistency with the sunny 16 rule in South Georgia. It's mainly sunny 11 dropping to 8. I just attributed it to varying levels of hazyness; humidty, pollen, and polution along with composition. There have been many times when I thought the meter was underestimating the amout of light on a seemingly sunny day but in most cases the meters were right and my eyes were wrong. Gotta get better at reading the shadows and color lightness I guess.
Thank you for the information that the sun will be recalibrated this coming Thursday. I was beginning to worry about that. After all, the Johnson Curve was done back in 1948, and that data is at least 64 years old now.
And all of this time I have been using the "Sunny 16 Rule" without any thought at all to the accuracy of the spectral intensity of the light source.
Actually, the main thing that has been providing problems out here in Latte Land is the correction factor to apply as a function of the varying transmission characteristics of the different types of di-hydrogen oxide units floating in the air above us almost all of the time.
Ralph Javins, Latte Land, Washington
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the sunny 16 rule is just a rule of thumb and, like alan says, really only for 10 am to 2 pm on summer days -- and your experience here in the Rockies also makes sense -- we're about a mile up here, the air is thinner so there's less light lost to the air -- and here in Utah I usually allow an f-stop to the rule because the thinner air means shadows are a bit darker/contrasty because there's less air to reflect light into the shadow areas.
Incident meter is your friend in all cases. Next best tool the human eyeball and the ability to bracket.