Good Evening, Bill!
The Looney 11 rule works for a fully illuminated moon ... or at least it's a
good starting point. It works because the moon reflects a bit less sunlight
(about 1 stop) due to it's grey surface.
For my own shots, I simply trust the TTL metering, though you do have to watch
the shutter speeds, during the total phase, and adjust the ISO accordingly.
That's what I failed to do for the recent "blood moon" ... and I know better,
having photographed many lunar eclipses and 5 total solar eclipses.
The 400 or 500 rule is actually the 340 rule ... Fred Espenak (NASA) is the
world's leading expert on such events and runs the MrEclipse.com website. In
it, he states:
"Exposure (seconds) = 340 / focal length (millimeters). For longer exposures,
you need a clock drive to compensate for the Earth's rotation."
It can be found, here:
and scroll down to the section on "Telescope Clock Drives and Polar Alignment"
& start reading.
Also note, that when the rule speaks of focal length, it refers to the
EFFECTIVE focal length on crop-sensor (APS-C & mFT) cameras.
By using the 400 or 500 (some even say 600) you get more blur due to the
rotation of the earth... so which number you use depends on just how fussy you
are. I go with Fred, at 340. He's the guru.
A good article that uses the 500 rule, but which explains things quite well,
can be found here:
For other sites, simply Google "eclipse 500 rule".
I hope this helps. If not, you know where to find me!
PS: Next total Solar eclipse visible in the USA is April 8, 2024. Details here:
though there will be an annular ellipse visible through much of Canada & the US
on Saturday, October 14, 2023,
Details here: https://en.wikipedia.org/wiki/Solar_eclipse_of_October_14,_2023
For solar eclipses over Europe and other places, consult MrEclipse.com
I have had no luck with moon photos, trying to begin with the Sunny 16
rule because the moon is fully lighted by the sun.
I have never seen or heard of the 400 or 500 divided by ISO rule but I
did find a Looney 16 Rule, which has nothing to do with Canadian coinage:
"The basic rule is, "For astronomical photos of the Moon's surface, set
aperture to f/11 and shutter speed to the [reciprocal of the] ISO film
speed [or ISO setting]."