Why the difference between sunny f/16 rule and looney f/11 rule?

Question

There's the sunny f/16 rule that says photographs on a sunny day should use f/16 with the exposure time set to reciprocal of the ISO sensitivity.

However, there's also looney f/11 rule that says photographs of the moon should use f/11 with the exposure time set to reciprocal of the ISO sensitivity. Seems about right (f/11, ISO 100, 1/100 s exposure, 250mm focal length on 1.6x crop sensor, image stabilization on):

At first, I thought photographing the moon should use significantly longer exposure time and a big aperture to collect as much light as possible, especially if done during the night.

However, when you think about it, the moon is actually lit by the sun! Thus, the sunny f/16 rule should apply also to photographing the moon.

However, it doesn't. The f/11 collects 2.1 times as much light as f/16.

My question is, why the 2.1x difference? It can't be due to the atmosphere of the Earth, as the light coming directly from the sun and also the light being scattered by the moon travel the same amount in the Earth's atmosphere.

In this day and age, using f/16 for anything is only a good idea if there is a concrete reason to use f/16. It is not a great choice for general purpose work, due to diffraction and sensor dirt. — rackandboneman, Feb 18 '19 at 09:14
f/11 is a rounded off approximation of f/11.31370849898476‬... that is the actual target aperture of your lens when it is set to f/11. The actual full number, which is a power of the square root of two, is exactly one stop brighter than, or 2X the light allowed through by, f/16. — Michael C, May 31 '20 at 08:08

score 5 · Accepted Answer · edited Feb 17 '19 at 15:34

5

The moon's surface is relatively darker than Earth's, with an average albedo of 11% (our gray cards are 18%). Moon phases other than full moon are side lighted, and darker still. Yet we seem to prefer too-bright pictures of the moon, thinking that captures it shining up there against the black sky background. So f/11 more nearly captures that sensation for us.

edited Feb 17 '19 at 15:34

mattdm

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answered Feb 17 '19 at 14:59

WayneF

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Why the difference between sunny f/16 rule and looney f/11 rule?

1 Answers1