Cameras like the Canon 5D Mark III and the Nikon D3X have completely changed the playing field when it comes to low light photography. In the earlier days photographers would have no way to change the ASA (ISO as it was called in those days) once they had loaded a film in their cameras. They were pretty much stuck with the film sensitivity until they changed it. That had a limiting effect on their creativity and it ensured that they were careful about their selection of film before heading out.
These days, digital photographers have a much easier time. They could change the ISO (light sensitivity of the sensor) just with the flick of a button. Cameras like the D800 comes with the added advantage of a greater dynamic range, allowing photographers to compose and make images that are closer to what they see with their naked eyes.
Yet, there are certain time-tested and useful tips that you need to know in order to nail good exposures in low light conditions. These are, in no way, alternatives to the ability of your camera to shoot in low light. Rather, these are complimentary to a high ISO capability.
Know your subject
Not all low light situations require the same camera settings. The most important thing to consider before making the exposure is your subject and to set your camera accordingly. E.g., if the scene is backlit expose for the subject while slightly underexposing the shot and use spot meter. If the background is interesting slightly underexposing the scene will not blow out the highlights because of the difference in illumination between the background and the subject. If you need a bit more exposure that can be added back when you post process the images.
If you are trying to photograph the Milk Way, you will need to use a high ISO number coupled with the fastest aperture that your lens permits to collect the maximum amount of light in a short period of time.
Use a Tripod
A tripod is an essential tool when shooting in low light. This is because in low light conditions you invariably need to use a slightly longer shutter speed. Carbon-fiber tripods are sturdy and yet lightweight and they are easier to transport, especially if you mostly shoot outdoors. Attempting to hand-hold an exposure with long shutter speed will induce image blur. Using a tripod gives you the leverage to use a long shutter speed without having to tinker with the ISO number. When using a tripod, shut-off the image stabilization feature of your lens / camera, else it will confront and try to stabilize a non-existing camera shake.
Use the Camera timer / shutter delay option
All DSLR cameras have this feature and so does most Point & Shoots. Using shutter delay / camera timer function is useful when you are using a DSLR. This eradicates any camera shake if you are manually pressing the shutter release. The alternate option would be to use a remote trigger / cable release. You could also use the mirror lock-up feature. However, on the downside, you lose the phase detection auto-focusing feature on your camera.
Use the widest aperture your lens can shoot in
ISO, shutter speed and aperture value have a mutual relationship that we have learnt in the article on exposure triangle. Increasing the ISO has the same effect as using a larger aperture and or slower shutter speed. But at higher ISO little white or dark specs appears. This is the digital equivalent of noise that we are so accustomed to seeing in low light images shot with film.
As a matter of fact noise is omnipresent in all images where signals are converted into digital form and recorded onto the memory cards. In low conditions the quantity of noise simply overshadows the data that makes up the image. This happens because the camera is inherently trying to amplify the available light signal and in the process noise is also amplified.
Shoot in RAW
When in doubt regarding the correct exposure settings, shoot in Raw and underexpose the scene by about 1/3 to a full-stop. In fact when shooting landscapes and other scenes where you are invariably going to find it difficult to expose properly, because of the wide difference in illumination across the frame, RAW gives you the best opportunity to collect as much information that the sensor collects and retain that for post-processing.
If you shoot in JPEG your images are immediately processed from RAW to a compressed file and you lose the ability to tweak them later on without losing data. Remember, JPEG is not a loss-less format, RAW is. JPEG contains only about 8 bits of information where as RAW has 12 to 14 bits. Every time you open and edit a JPEG file you lose some amount of data. This does not happen with RAW.
If you use a higher ISO number RAW shooting enables you to eliminate that noise in post-processing using photo editing software such as Adobe Lightroom and Photoshop. You can alternatively choose not to post-process your images yourself and outsource the task to one of the experienced photo editing professionals on Phowd.
With RAW you are able to change the exposure, retrieve details from blown out highlights and shadows, sharpen the image, reduce noise, eliminate banding as well as use a specific color space as per your requirement. If you need to print the images RAW stores the highest amount of information which means better print quality when processed compared to camera created JPEGs.