An Exposure Value (EV) of 0 is defined as an exposure of 1s @ f/1.0 and ISO 100 (or any equivalent combination of exposure time, aperture and ISO).

When photographing natural scenes in very dim conditions (i.e. at night, away from artificial lights), the following exposure guidelines may come in handy:

- Subject under starlight only: EV -6 (e.g. 30s @ f/2.8 and ISO 1600, or 16min @ f/4 and ISO 100)
- Subject under crescent moon: EV -5
- Subject under half moon: EV -4
- Subject under full moon: EV -3
Northern Lights (Aurora Borealis) also typically fall in the EV -6 to EV -3 bracket.


In order to avoid recording the apparent motion of the stars (i.e. IF WE WANT THE STARS TO APPEAR AS FIXED POINTS OF LIGHT instead of 'trails'), a good rule of thumb is:

Exposure time (s) < 400 / eq_FL, where eq_FL is the 'full frame'-equivalent Focal Length of the lens being used (mm)

The following table summarizes my quick-and-dirty recommendations on the maximum exposure times that allow to obtain sharp point-like stars (i.e. no star trails) for a range of useful focal lengths, as well as the corresponding f/stops and ISO values that are necessary to expose for EV -5 (the required ISO for a different f/stop and/or EV can be easily calculated on the basis of these values):

Lens

DX camera

FX camera

FL (mm)	F/stop	Max exp. time (s)	ISO for EV -5	Max exp. time (s)	ISO for EV -5
14	2.8	20	1250	30	800
20	2.8	12	2000	20	1250
24	2.8	10	2500	15	1600
35	2	8	1600	10	1250
50	2	5	2500	8	1600

IF INSTEAD WE DO WANT TO OBTAIN STAR TRAILS, it will be necessary to aim for much longer exposure times, so that the trails come out nice and long, and not merely like 'blurred points'. In most cases, nice start trails require exposure times of several minutes. The length of the trails actually depend on the lens' angle of view (AOV), and a good rule of thumb is:

Exposure time (min) > AOV (º) / 4

Lens

DX camera

FX camera

FL (mm)	AOV (º)	Min exp. time (min)	AOV (º)	Min exp. time (min)
14	90	22	114	30
20	70	17	94	25
24	60	15	84	20
35	43	10	63	15
50	31	8	47	12

An alternative way to achieve this with those digital cameras that are prone to generating unwanted 'thermal noise' during very long exposures, is to stack a large number of shorter exposures (e.g. 30 one-minute exposures for a total of 1/2 hr). In practice, this is done as follows:

(i) Take N relatively short exposures (e.g. 1 min each) at f/4 and ISO 200 for the stars, leaving only a very short interval (e.g. 1 - 3 s) in between - this is best done with a programmable intervalometer, such as Nikon's MC-36. Note that the correct exposure for the stars themselves only depends on Aperture and ISO; Shutter speed controls the length of the trail, and of course the exposure for the foreground (e.g. 1 min at f/4 and ISO 200 gives EV -3).
(ii) If necessary, take one additional lighter exposure for the foreground (e.g., if there is no moonlight: 1 min @ f/2.8 and ISO 800 gives the appropriate EV -6).
(iii) (Optional) Take one final exposure of equal length, but with the lens cap on, to be used as a 'reference dark frame' to remove any 'hot pixels' and residual thermal noise.
(iv) Later, in Photoshop, stack all photos as Layers, with blending mode = LIGHTEN for all 'real' pictures, and blending mode = DIFFERENCE for the last 'reference dark frame'.

Finally, in order to enhance the blue colour of the night sky, and counterbalance the residual level of artificial light contamination (which is unfortunately almost impossible to completely avoid), it is often best to set a Tungsten White Balance.