Getting the technical moon shot

To get the moon shot using Ansel Adam’s oft-quoted exposure formula, used for his

Sailing the moon

Sailing the moon, Snow King, Jackson Hole

famous Moonrise over Hernandez:

Take the square root of your ISO – that becomes the aperture.

  • ISO100 = f11
  • ISO200 = f14

Then, take the luminance of the object.  In this case, the full moon is 250 cd/ft^2, relatively low on the horizon.  That becomes the shutter speed to put the object in zone V, 1/250 sec.  To bump up the moon to zone VII, a more desirable target to make the moon bright but not blown out, cut the shutter speed to 1/60 sec.

In the digital era, people probably don’t use the zone system too much any more.  But, being a former B&W photographer with a dark room, I still think that way.  And, it still works.

Moon exposure

Moon exposure

I have used this setting with proportionate variation to the ISO and aperture, to great effect over the years.  It’s much easier than trying to meter the moon.  Interestingly, the moon is the exact same exposure as daylight for zone V.  I’ve seen websites reporting luminance measurements that well exceed this calculation, yet it works well every time.  My Sekonic L-508c light meter does not have enough zoom to fill the meter area, as the moon is about a half degree wide and the meter area is one degree.

Also, a full moon is 250 cd/ft^2 when full.  The exposure drops quickly as the moon enters its gibbous, half and crescent phases.  Yet, the above shot is very close to the above calculation.

The shot was taken at f5.0 at 1/500 at ISO 200.  Lets see if the math works out.  My Nikon D300s was set to ISO 200 = f14.  Let’s just say the moon was full, so that’s 1/250

Moon exposure on D300s

Moon exposure on D300s

sec.  To get the moon to zone VII, I want to shoot at 1/60 sec.  (1/250 – 1/125 – 1/60).  Since I was shooting with a 180mm lens, 1/60 of a second would have gotten me a nice, blurry image.  So, I needed to shoot at a higher speed.  I chose 1/500 of a second, 3 stops above 1/60 (1/60 – 1/125 – 1/250 – 1/500).

That meant I had to open my aperture 3 stops to keep the exposure equal.  Lets Just fudge the calculated aperture to f16 for easier calculations for a moment. Drop the aperture to f5.6 (f16 – f11 – f8 – f5.6).  I wanted just a tiny bit brighter on the moon, so I dropped the aperture to f5.0.

Hence, the above shot was taken at 1/500 at f5.0 at ISO 200.  Conveniently, the sky metered just right to make a rich blue, so I just ran with what the camera gave me.  Had the sky been completely dark, the above calculations still would work.  My paraglider would have just been, had he/she been in front of the moon, a nice silhouette.

***

Over 70 years later, Ansel Adams was still right.  I always wonder where he got that equation from.  Probably an optics professor (or friend at Kodak) buddy of his.

One handy reference for shooting the moon is this site, Solar Calculator 2.2.  Find your location, click the moon tab and you’ll be able to see what the moon’s azimuth will be.  It’s pretty handy.

There are lots of phone apps out there to do this, too.  I’ve not figured out which one I’ll purchase, as they’re not cheap.  But once I decide on one, it will be posted here.

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