In this part, I would like to introduce the first constraints to the rule, at least what I believe to be the first constraint: time of day (actually, the interweb in pretty silent about this topic) and time of year.
As mentioned in part 1, sunny 16 applies to mid day, i.e. when the sun is the highest.
Since today, everything is available world-wide, I would like to add that this statement holds true only to a certain range of geographical latitudes! Actually, my understanding is, that the sunny 16 rule is a rule valid for latitudes between about 30˚ - 60˚ N/S.
To my understanding, the rule is all about taking pictures of your family during summer vacation. The rule reflects the amount of illumination available to the scene.
As a physicist, I would like to inform you about the hardness of sun light, i.e. light being collimated, in other words, the light rays are all parallel.
At a perpendicular angle of incidence, the light rays are closest to one another, meaning the illumination (light yield) is at a maximum.
At an angle parallel to the incidence of the light, the illumination is minimal.
Confused?! Take an orange and put it in sunlight. You will observe that the level of illumination drops, dependent on the angle of illumination.
The same happens on Earth. Dependent on the time of day, the sun will illuminate the earth's surface more or less, that's the influence of the longitude. The same holds for the latitude, although this changes with the time of year.
I figure, when traveling, one has to adapt the sunny 16 rule according to the latitude and the time of year.
I would do the following in general:
- latitudes between 0˚ and 30˚: stop down by one stop
- latitudes between 60˚ and 90˚: open up by one stop
- in spring/autumn, between 6:00 and 9:00 local time: open up by one stop
- in spring/autumn, between 15:00 and 18:00 local time: open up by one stop
Remember, I still talk write about a front-lit scene.