BetterPhoto Q&A
Category: New Answers

Digital Shots at Night

The picture came out too bright with the flash and too dark without. Both ways produced a glare off the moon.

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February 22, 2003

Several issues can cause the "glare" you mention (actual term for the "halo" around the moon in the photo you uploaded is "flare"). From most to least likely:
a. Lens flare from a very bright light source compared to its surroundings shining directly into the lens. Some lenses are more prone to flare than others; it's a function of lens quality, its AR coatings, and its design.
b. Flare caused by pollutants in the earth's atmosphere, or by very high, thin clouds that are nearly impossible to see with the human eye. During long exposures at night, the flare from this as captured on film or digital CCD is often more than you see with your eye.
c. If you're using film, some films also flare under these conditions with "light-piping" across the emulsion. Fuji Provia is notorious for this, but isn't the only one. I have a number of photographs of cityscapes and other urban "street shooting" at night in which this has occurred with bright street lights and traffic signals.

Regarding your usage of flash . . . outdoors at night, light from a flash dissipates very rapidly with distance from the flash and doesn't have that much range. It falls off with the inverse square of the distance. The amount of illumination at 20 feet is 1/4th that compared to illumination at 10 feet. At 30 feet it's 1/9th the illumination at 10 feet.

First, forget about trying to use a flash in photographs like this. The one thing I can think of that would help the extreme brightness difference is a graduated neutral density filter. Neutral density filters are a neutral gray that won't affect color balance. A graduated filter is darker in one half compared to the other half and the transition between the two halves is gradual. The lens mount on a "grad" filter also allows turning it so you can orient it to put the darker and lighter halves where you want them in the image.

In this case, the darker half would be oriented for the sky portion of the photograph to cut down the brightness of the moon compared to the rest of the scene which would be capture through the lighter portion of the filter.

An ND-grad filter is most often used just before sunrise and just after sunset when detail and saturation in both sky and ground are desired. In these situations, illumination of the ground is dim with the sky still comparatively very bright, and the difference between the two is too much to capture detail in both . . . the sky will blow out with very little color saturation, or ground detail will be lost as one deep sihlouette-like shadow.

You have a similar situation in a middle of the night version with sky brightness confined to the moon's disc, not the entire sky.

-- John

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February 23, 2003

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February 24, 2003

If you want the moon and your landscape to be shot at the same time, you must balance the night sky and the landscape lighting values (the sky should be 1 stop brighter than the landscape).

Of course, you can photograph the moon by itself and composite it into your night-time image

Regards

Darwin

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February 25, 2003

Regarding the moon:
For photographs that will show crater detail on the moon's surface, exposure depends on the phase of the moon:
(a) a full moon is the same as for a directly sunlit subject/object on earth, i.e. the Sunny-16 rule . . . shutter speed = 1/(film speed) @ f/16 aperture.
(b) Add one stop exposure for a gibbous moon (about 3/4 moon; halfway between full and half).
(c) Add two stops exposure for a half moon.
(d) Add three to four stops exposure for a crescent moon. How much depends on how thin a crescent it is. Bracket if in doubt.

This will create a mid-tone moon and is straight from Kodak Technical Publication P150. Many want it brighter as that is how we perceive it against a dark sky. To do this, add one stop more to all the exposure guidelines (e.g., for full moon use an "f/11" rule). Want it brighter yet with some loss of highlight detail? Use Darwin's f/8 rule and open up one more stop. I'm not saying this is wrong, it's a matter of what you want the moon to look like.

Keep exposure time as short as practical. IIRC, over about 1/8th or 1/15th second and the moon's details can begin to blur from a combination of earth rotation and moon orbital motion. Too long an exposure will produce a nice streak with a slight curve to it. Exact limit before motion blur sets in depends on focal length. The risk is greatest with long telephoto lenses.

Moonscapes:
For an earth landscape illuminated by the moon at night, it's called the "Looney f/4 Rule" but the exposure calculation has a slight twist to it. Exposure time for direct illumination from a _full_moon_ (and only the moon) with clear sky is aperture of f/4 with an exposure of 1/(film speed) * (seconds in a day). Number of seconds in a day? 60s/m*60m/h*24h = 86400. For ISO 100, exposure time at f/4 would be (1/100) * (86400) = 864 seconds, about 14-1/2 minutes. 15 minutes is plenty close enough. You can consolidate this to:
exposure in seconds = 86400/(film speed)

If using film, be certain to read the film data sheet and adjust *this* exposure time to compensate for reciprocity failure. Something other than full moon?
Half moon: add one stop
Quarter moon: add two stops

Got something other than clear sky? Estimate how many stops to open up to compensate for this as you would for the same type of cloud cover obscuring the sun.

Bracket exposure if possible!!

Why 86400/(film speed) @ f/4 for exposure? The sun is about 1.4 million times the brightness of a full moon. 1.4 million divided by 86400, the number of seconds in a day, is approximately 16. Opening up a lens from f/16 to f/4 allows in 16X the amount of light.

Last, but not least . . . with a standard focal length (50mm lens with 35mm camera) . . . expect "star trails" when exposure exceeds about 30-45 seconds. These are cause by the earth's rotation. Longer focal lengths will show star trails sooner (shorter exposures) and shorter focal lengths will tolerate longer exposures before star trails occur.

-- John

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February 25, 2003

UB.

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September 15, 2010