There are a couple of moon exposure nuances. One is fairly obvious, the other is not.
Most beginners tend to over-expose the moon because it seems like it should be bright. You are used to seeing the moon at night ... so compared to everything else, it is bright. But during the daytime, the moon is technically just as bright... but seems a lot dimmer beause your eyes are adapted to daylight instead of nighttime.
The obvious one is that just because there's a "Looney 11" rule doesn't mean you have to shoot at f/11. That "rule" is just a quick way to know the correct baseline exposure without having to trust a light meter (in-camera metering is fooled by all the blackness of the sky and tends to over-expose unless you use spot-metering). You can use any f-stop available by just trading stops of aperture for stops of exposure.
The less-obvious one is an issue called "atmospheric extinction". It's the reason you don't go blind looking at the sun at sunset (but you would if you looked at it for a long time at mid-day). It's also the reason the moon looks dimmer and more orange at moonrise or moonset.
Extinction is the notion that when you are photographing the moon, you are looking through a lot more "air" than nearly any other subject you normally shoot. If you're shooting people, they are not very far away from the lens... you aren't shooting through much "air" and that means issues of air transparency don't impact your exposure (unless you're shooting through a lot of smoke or fog, etc..) With the moon, it the air can look reasonably clear, but tiny particles in the air will cut down on the amount of light you get.
This is one reasons why... even though the moon is lit by the sun - so you'd think the "Sunny 16" rule would work -- you use f/11 instead. There's enough air to reduce light transmission. That's "extinction" (extinction of light ... specifically because it is being absorbed by atmosphere). (The other reason is that the moon is not very reflective).
So then the question becomes how much air are you shooting through. If something is in space directly above your head, then you have to look through 1 "air mass" of atmosphere. If you're looking at something 30° above the horizon, the sin of 30 is .5 ... so you are looking through 2 (the inverse of .5) air-masses. At 10° it works out to 5.75 air-masses *except* the curvature of the Earth makes the math a little more complicated so it really works out to be 5.6x.
Anyway, if the air really is clear you get a small impact. If the air-transparency is poor, you have a huge impact. This means there is no rule for how to adjust exposure based on the altitude angle above horizon... just know that the closer things in space get to the horizon, the more light is absorbed.
If you want to learn more about the technical nuances, read this article for a good intro: https://www.skyandtelescope.com/astronomy-resources/transparency-and-atmospheric-extinction/
This may make things sound really complicated... "how will I know if I am really using a correct exposure". This background an idea of what is happening and why. BUT...
Ultimately you can turn on "the blinkies" (exposure clipping warnings) and make sure no part of the moon is over-exposed. (Digital photography has all these wonderful advantages ... and this is one of them.)
Tim Campbell
5D III, 5D IV, 60Da
