"Dawn and Dusk Shooting - Camera Metering" Fri 17-May-13 10:33 PM by nrothschild
Of interest to astrophotographers and general landscape photographers is the problem of metering in deep dawn or dusk.
Modern Nikon camera meters are generally rated to meter down to 0EV in Matrix and Center Weight mode, and 2 EV in spot metering mode.
Hint: at the margins, be wary of using spot metering- it will fail 2 stops brighter.
I've been studying a long series of images I took from sunset to about 1.5 hours after sunset. I had a lot of distractions, aside from the basic task of shooting the scene so I decided to let the camera meter the rapidly dwindling light. One less thing for me to do.
On this particular day, within a week of the spring equinox, my camera metered properly right down to Nikon's spec of 0 EV. Beyond that point the camera increasingly underexposed as the light dropped further. The exposure will "freeze" at 0 EV no matter how dark the scene.
EV 0 is 15 stops below sunny/16. In my particular case, my D700 meter leveled out at ISO 800 f/4 3s.
The point at which the exposure stopped adjusting occurred exactly 50 minutes after sunset. That was 23 minutes after the end of Civil Twilight and 8 minutes prior to the end of Nautical Twilight.
It was a clear evening with no clouds, ideal conditions. Less ideal conditions would result in that level of darkness arriving sooner in time.
At different times of the year it darkens more or less quickly, depending on the angle the sun cuts through the horizon. For that reason I mentioned the relationship to Civil and Nautical Twilight.
By definition, Civil Twilight ends at the instant in time that the sun reaches -6° altitude. Nautical Twilight ends when the sun reaches -12°.
Civil twilight is often used for legal reasons, such as the time that vehicle headlights might be required.
The end of Nautical Twilight is considered to be the time when, on a moonless night, a mariner can no longer clearly distinguish the horizon from the sea. It is then quite dark but not as dark as it will get.
That time of (near) maximum darkness is defined as Astronomical Twilight, when the sun reaches -18° altitude.
I mention the various twilights because they are published in tables and accessible by all decent astronomical software. It is a better reference point, year round, than some single specific time after sundown.
The length of time between sunset and the end of Nautical Twilight varies throughout the year. It also varies depending on latitude. At 40 degrees latitude, it ranges from about 57 - 72 minutes throughout the year.
Just remember that at least 8 minutes before the end of Nautical Twilight you need to take over the wheel, in the case that you are using an Auto Exposure mode .
If you do not know the ending time for Nautical Twilight, I would suggest figuring 30 - 40 minutes max after sunset. That allows for weather related accelerated darkening but you can judge that at the scene.