The inadequacy of pop up flash units, and even high end flash heads on DSLRs make non-flash photography preferable for most photo projects and opportunities. Photos taken without flash allow the resulting shot to have even natural light throughout the image as a whole.
A knowledge of how DSLRs capture light goes a long way toward success in low light photography.
The basics of photography are a working understanding of how light is taken in by the camera. Three main functions coordinate together to get the correct exposure of light to the image sensor. Those functions are the lens aperture opening, shutter speed and the ISO. Modern cameras with automatic settings calculate the settings for the varied scene modes on the camera dial for you, but they do not always interpret the scene or the photographer's intent correctly. Cameras that allow manual adjustment of aperture, shutter speed and ISO give much greater creative control back to the photographer, but the photographer needs to know how these settings work together.
Understanding Exposure by Bryan Peterson is a very well regarded book on the subject of the following three exposure settings. The three settings must be balanced well in order to get a great shot in low light, and in photography in general.
The balance of these three settings in unison, is what is referred to as the Exposure Triangle.
Aperture is like the pupil of the eye, which widens or constricts based on the amount of light hitting the retina. In cameras, the retina would be the image and light sensor. Automatic or scene settings take control of stopping down or closing the aperture to let less light in when scenes are bright. If there is not enough light, the aperture opens wider to let more light in.
It would seem that it should be as simple as opening up the aperture as wide as possible when taking shots in low light. However, in cameras, the aperture also affects the depth of field that is in focus in an image. Opening up the aperture wide is a trick to use to help blur out the background in portrait photography. That would not look good trying to capture a landscape image or any image that needs more than just a few inches in sharp focus. To get sharp focus for a greater depth of field, the aperture needs to be stopped down or closed to its smaller opening widths.
To remember aperture number settings, just keep in mind that it is the opposite of what is intuitive. The smaller hole sizes for light through lenses are the bigger numbers such as F11. Smaller aperture numbers correspond with wider aperture openings.
But what if a photographer needs a greater depth of field in focus when the light is low? That is where shutter speed and ISO help.
Shutter speed controls the amount of time the light hits the sensor. Aperture is how much light gets through, and shutter speed is how long light gets through.
The photographer needs to work out the longest shutter speed acceptable for the scene. Even tiny movements of a the camera will blur objects such branches moving in a slight breeze. If the intent is to blur something such as water moving over a waterfall, longer shutter speeds are not a problem, and can be used for artistic effect.
A shutter speed of 1/250 of a second is likely adequate for most low light scenes needing actions, such as walking, frozen. The photographer could also manipulate the scene by having everyone stand as still as possible. A tripod is needed for shutter speeds slower than 1/60 of a second.
If the low light scene has little or no movement, long shutter speeds can bring in the light. The caveat is that noise also begins to occur in the images when the shutter is opened too long. This is where the ISO setting helps. Remember, wider apertures have shorter depths of field, and longer shutter speeds let any movement show up as blurriness.
Though the letters are out of sequence in the acronym, ISO stands for International Organization for Standardization. ISO is pronounced eye-so. ISO standards are decided upon and set for use in manufacturing devices and products. They apply to many devices other than film and digital sensor settings. In film emulsions the speed ratings of the actual film had an ISO rating. It was standardized so photographers knew that a 400 speed film made by one company was the same as the 400 speed film made by another company as far as light levels needed to take usable images was concerned. In digital photography the image sensor's sensitivity is adjusted according to ISO settings since there is no film being used.
At the basic understanding level:
The issue of noise at higher ISO settings is a problem for most entry and mid-level camera models. Modern DSLRs can often shoot images at an ISO of 1600 or even 3200 and still create sharp images that are practically noise free.
Noise in a digital image is equal to grain in images shot on film. Noise looks like an image was shot through a dusty window. At its worst, it looks like fine snow filled the area where the image was taken. Think of video surveillance stills from old security cameras such as those old grainy black and white stills that showed suspects in a retail or bank holdup. High quality image sensors of professional cameras can shoot at higher ISO settings without the noise ruining an image.
As a rule, photographers should use the lowest ISO setting the scene will permit. The lower the number, the sharper and clearer the image will be. However, when a wider aperture or longer shutter speed is not workable for an image, setting the ISO higher may be the only thing that can be done to be able to capture the image at all. That is, other than adding artificial light to the scene. Photographers should experiment with their camera and ISO settings before going out to shoot low light images that are important to capture.
Finally, be sure to shoot in RAW. The photo file sizes are a lot bigger than JPEG files, but they contain a lot more information that can be manipulated in post-edit using Photoshop or any other photo editing software. Think of JPEG and file formats other than RAW as abridged versions of books—there are things missing. RAW files are the unabridged version.
RAW images contain the maximum amount of detail that is possible to capture with the camera's image sensor and processing unit. Digital information that exists can be edited. If it is not present, software tries to fill in the spaces, making muddy looking final images.