Night photography can quite literally be a shot in the dark. The table above gave some general guidelines for various types of night photographs. Now, the following section discusses some of the key technical aspects of night photography. A simple undertanding of some of the physics of the camera can add light to the picture, and open up an array of photographic possibilities.

(As a word of consolation, the following discussion goes into more detail than is necessary in order to take great night photos. The physics I've presented is meant to enlighten, rather than frighten, the curious reader. In practice, it is perfectly sufficient to simply understand how to make adjustments to aperture and shutter speed and ISO to achieve the desired exposure. Oftentimes, using intuition and creativity are better than following strict rules.)

Key Concept = Understanding Exposure

The exposure of a photograph is affected by three camera settings: (1) aperture, (2) shutter speed, and (3) ISO speed. It is important to understand how these work together to affect the exposure (i.e. the Law of Reciprocity), and how these settings can be determined and quantified for a specific scenario (i.e. Exposure Values).

Law of Reciprocity

The amount of light reaching the film or digital sensor in the camera is determined by the aperture, shutter speed, and scene luminance. This, in a nutshell, is the Law of Reciprocity, formally stated as:

For instance, doubling the amount light exposing the film can be achieved either by:

1. Doubling the area of the aperture (i.e. decreasing the f-number by one stop)
2. Doubling the shutter time (i.e. one stop)
3. Doubling the scene lighting

Likewise, halving the amount of light exposing the film can be achieved by the opposite of one of these operations.

The law derived its name from the reciprocal relationship between aperture and shutter speed. Both are calibrated in stops, or doublings and halvings. To get the same exposure, a change in one variable can be offset by changing the other variable an equal number of stops in the opposite direction. This reciprocity means that a variety of aperture and shutter speed combinations can produce the same exposure. Once an initial exposure setting has been determined, the aperture and shutter speed can be adjusted in stops to get the settings that give the desired photographic effect.

Reciprocity is also helpful in determining what film speed to use, often just called the ISO (on digital cameras, the ISO setting refers to the sensitivity of the sensor rather than film speed). A lower ISO corresponds to a lower sensitivity (i.e. the film or sensor records less light), while a higher ISO corresponds to a higher sensitivity (i.e. records more light). ISO can be adjusted in stops or fractions of stops that can be counterbalanced by an equivalent number of stops of aperture or shutter speed. The low light conditions inherent to night photography often necessitate a high ISO. The disadvantage of using high ISO is that this increases the noise in the photograph.

Back to the Law of Reciprocity. As an example of how the Law of Reciprocity can be used to manipulate camera settings, let's say a photo is correctly exposed with an aperture of f/5.6, a shutter speed of 1/30th of a second, and an ISO speed of 100. We might want to increase the depth of field (i.e. decrease the aperture size) and use a faster shutter speed – both of these adjustments will reduce the exposure. In order to maintain the exposure, each stop in aperture or shutter speed requires the ISO to be increased by one stop. The photo will be identically exposed with an aperture of f/16 (3 stops down), shutter speed of 1/60 (1 stop down), and ISO of 1600 (4 stops up).

But there is a small exception to the rules that is important to low light situations, and hence night photography. Reciprocity between the settings is not a one-to-one relationship at exposures longer than about 1 second. This is because sensor (or film) has a threshold level of photons needed for the pixels (or film grains) to register an image. Under typical lighting conditions, this threshold value is negligible compared to the amount of light entering the camera. However, in low light, when there are fewer photons of light per unit time, the threshold must first be overcome, which adds time to the exposure and skews the linear relationship between incident light and exposure time. This is known as reciprocity failure. Each sensor type (or film emulsion) has a slightly different response to long exposure, but the important point is that in low light when exposure times are greater than about 1 second, the exposure time must be more than doubled to double the exposure. I've tackled this problem just by trial and error with longer exposure times.

Exposure Values

To make use of the Law of Reciprocity, it is necessary to determine an initial set of camera settings that will give the correct exposure. With a little practice, most photographers can intuitively guess a combination of exposure time and aperture.

However, for those with less experience or for those who like to know the underlying principals, this is where exposure values (EV) come in handy. Each combination of camera settings can be defied numerically by an exposure value (EV). Essentially, an EV numerically defines the overall luminosity of the scene to be photographed. Every scene – a Full Moon casting a beam of light over the ocean, the colorful reds and greens of Aurora Borealis, the Milky Way in a starry night sky – has a characteristic EV value. Mathematically,

where N is the relative aperture (f-number) and t is the exposure time. Typical values of EV range from -6 to 16, with EV 0 corresponding to an exposure time of 1 s and a relative aperture of f/1.0. High EV correspond to brightly lit situations, while low EV correspond to dimly lit situations. Most night photography takes place in situations with negative EV.

Each EV defines an exposure and, through the principles of the Law of Reciprocity, is associated with all combinations of camera shutter speed and aperture that produce this particular exposure at a given ISO speed. Hence, once the EV for a scene is known, it is easy to select a desired combination of exposure time and aperture. The tables below (from my book) give some typical exposure values (at ISO 100 speed) for various night photography lighting conditions. Note that increasing the ISO by one stop (e.g. ISO 200 to ISO 400) causes the EV to increase by one increment, while decreasing the ISO causes the EV to decrease accordingly. Exposure value is also used to indicate an interval on the photographic exposure scale, with 1 EV corresponding to a standard power-of-2 exposure stop.

A light meter can be used to determine the EV. However, natural light, as well as many scenes with artificial lighting, is predictable, so exposure often can be determined with reasonable accuracy from tabulated values, as shown above. These charts are good to know about, but with a little practice choosing a suitable combination of camera settings (i.e. EV) becomes quite intuitive, and most photographers don't even think about the EV value their chosen combination represents.

Although all camera settings with the same EV nominally give the same exposure, they do not necessarily give the same photo. For example, a 30-second f/1.4 exposure of the Big Dipper is nominally equivalent to a 16-minute exposure at f/8; the first will show the pinprick stars of the constellation, while the second will show star trails. In night photography, the exposure time plays an important role in the overall outcome of the photo, so often it is just a matter of choosing the appropriate aperture and ISO setting to suite the desired exposure time and EV of the scenario.

Handy Tip

If you are not sure what exposure settings to use, you might want to take a few trial photos with high ISO and relatively fast shutter speeds. When you find the combination that produces the desired lighting, you can "stop down" the ISO (since a lower ISO is desirable as it results in less noise) and "stop up" the shutter speed or aperture by an equal number of stops.

Examples

Now, (finally!), I give a sampling of my favorite photos, each accompanied by its exposure setting.