In order to get reliably good results when shooting in low-light conditions, it is essential that you understand exposures. It is also a good idea to shoot extensively in aperture and shutter priority and in full manual, so that you're familiar with controlling exposure using the camera's controls.
You also need to determine the limits within which you can photograph a given scene and get usable results. Most of these limits are relative, and will vary from situation to situation, and from photographer to photographer. The only way to find out for sure what your limits are is to shoot, take notes, and shoot some more.
Shutter speed
How slow can you shoot and still get acceptably sharp results? This will vary depending on your skills, how much coffee you drink, how heavy your gear is, what focal length you're shooting, how close your subject is, and how fast the subject is moving. Some very general rules:
- Don't shoot slower than 1/(focal length) in seconds. So, with a 30mm lens, don't shoot slower than 1/30s. This rule is fine for many situations, but you're much better off doing some testing with a given lens/camera combination to see whether you can hold the camera at slower speeds or whether you need a faster speed because of shaky hands or poor technique.
- Subject motion may require much faster shutter speeds. If a person is running or walking, you may need to use 1/60s, 1/125s or even faster. Even a person who is standing or sitting in one place may create motion blur if you go slower than 1/30s or 1/15s.
Once you know how slow you can safely shoot in a given situation, you can shoot in shutter priority and choose the slowest safe shutter speed in very low-light situations.
Aperture
In low light situations, a wider aperture will allow your lens to gather more light. The faster lens, the greater your ability to shoot in the dark. However, you are limited by the lenses available to you, and in your budget. Also, it's important to remember depth of field (DOF). If you shoot at very wide apertures (low f/number), you may not be able to render more than a single eye in focus. If you're trying to depict two or three people interacting, this can be a serious problem.
- Use the widest aperture you can in low light. This is dependent on your lens. There are some lenses which are able to have very fast apertures of f/1.2, f/1.4 and f/2.8 while others are much slower at f/5.6 and f/6.3.
- Be aware of how much DOF you require. If you need to cover multiple subjects, stop down a bit — f/4 , for example, although the appropriate aperture will vary depending on focal length, subject distance, and your needs.
- Be aware that aperture impacts lens performance. Many lenses are softer and more prone to flare when shot wide open. Take this into account.
In low-light situations, you can shoot in aperture priority and keep the aperture as wide as is practical. However, keep an eye on the shutter speed and make sure it doesn't drop too low.
ISO
Raising the ISO will lets you shoot at smaller apertures and faster shutter speeds while still getting acceptable exposures. However, this comes at the cost of added noise.
- Determine how high an ISO you are willing to shoot at. Some D40/x/D60 users are whiling to push as high as ISO 1600 while others may not go above ISO 800. An advantage of the digital age is that some have adopted a post-processing workflow that allows them to shoot at a high ISO with a high degree of confidence that they will get a usable image. You need to find out what your workflow is like for noisy images, and what your standards are for noise control.
- When shooting at high ISO values, do not underexpose. This is essential. If anything, err on the side of slight overexposure. Better to raise the ISO to 1600 than to underexpose a shot at ISO 800.
- Auto ISO. If you use auto ISO, make sure you understand how it works, and how to set it up so that it kicks in when you want it to, not before, and so that it goes to a high enough ISO to do you some good. If you don't pay attention to how it's set up, it can lead to very noisy images in situations where a lower ISO could easily have been used. However, if you do have it set correctly (for your needs), it can be a very useful tool.
Once you know, for a given situation, what your slowest safe shutter speed is, your widest safe aperture is, and your highest safe ISO is, you can choose any combination of settings within those limits and presume that you will get good results. However, don't be afraid to keep chimping and be aware of changes in light, etc.
Metering Options
Metering a scene properly is essential in getting proper exposures. Leaving the camera on matrix metering may work very well during the day, but not so much at night. Matrix metering evaluates the scene based on what the camera 'thinks' is the best exposure, consequently night is very difficult. Bright street lights and dark elements can often get the metering incorrect. Possibly the best option is to go into Spot metering which will allow you to select the object you wish to have proper exposure of. This isn't exactly a perfect science, so try several different exposures out.
- Try Spot metering. This will give you full control over what you want exposed in the scene.
- Check the LCD screen. This will give you instant feedback to how you have to adjust the exposure of the scene.
Using Manual Mode
Most would recommend working in manual mode. This allows you to manage all three variables independently and gives you more control over the scene. A lot of low light photography is trial and error, so often you need to be able to quickly change one setting without changing everything. However, you may prefer to shoot in aperture or shutter priority, and there situations where those have distinct advantages.
Rules of thumb for specific situations
- Downtown San Francisco, street lights, 1/15-1/30, f/1.4-2, ISO 1600
