All digital SLRs can take fine pictures at night, but some do a better job than others, thanks to special features built into them to handle long exposures and noise. The following sections describe the key components that you need to work with so that your night shots are sparklingly clear.
Fast or slow lens ?
A lens that has a large maximum aperture (generally, anything faster than f/2.8) is a must only if you’re hand-holding (that is, shooting without a tripod) your night photos or, perhaps, if you want to shoot with flash and need your speedlight’s illumination to reach as far as possible. An f/2 or f/1.4 lens might let you take some night pictures at 1/30 of a second hand-held or, with image stabilization, at slightly slower shutter speeds.
The more sane shooters take night photographs with the camera securely mounted on a tripod so that you can use virtually any lens at any desired f-stop and still get a clear photo. Correct exposure in such cases is simply a matter of using a sufficiently long shutter speed. That said, using a long shutter speed isn’t always an option, as I discuss in the following section.
Shooting night scenes at higher shutter speeds
You might want to avoid very long shutter speeds and prefer the shorter shutter intervals a higher sensitivity (higher ISO setting) affords for any number of valid reasons:
Perhaps you don’t want to spend 30 seconds or more making a single exposure because of time constraints or physical conditions.
You might want to take as many photos as possible in available darkness.
You want to avoid the light streaks from moving cars or other illuminated objects that a long exposure likely produces.
Bumping up the ISO can let you take your night photos with a reasonably short exposure. Some dSLRs have ISO settings no higher than ISO 1600, but others go up to ISO 3200, or as much as ISO 25600! A few cameras top out
at a specific ISO notch but include push settings that have names such as H1 or H2, which elevate the effective sensitivity a notch or two more. You can use these settings in a pinch, but expect some quality loss when you do. You can also adjust your camera’s exposure value (EV) compensation controls to underexpose an image (effectively providing a higher ISO setting) and then try to salvage the photo in your image editor.
Of course, high ISO settings tend to amplify non-image signals in an image, producing extra noise. Your camera’s noise reduction feature can cut down on the annoying speckles that appear at those high ISO settings. See the following section for more about using noise reduction.
Consider using noise reduction anytime you take photos at night using long exposures or high sensitivity settings because the objectionable texture overlays unprocessed images.
Noise comes from several sources. One might be the amplification of the weak signal that the sensor produces under low light conditions. The sensor itself also produces noise while it inevitably heats up during a long exposure, and the sensor mistakes some of that heat for incoming photons. Some digital cameras exhibit an amp glow or amp noise phenomenon, which is a reddish or purple glow in the corners and edges of very long exposure images (usually several minutes or more) caused not by heat, but by electro- luminescence of electrons in the readout amplifier of the camera’s sensor. You don’t need to be a rocket scientist to understand that; the actual specialty you need is astronomy (astronomers experience amp glow when they take astro-photographs).
The bottom line is that your camera’s sensor can generate noise at high ISOs with exposures of any length and at relatively low ISO settings with long exposures.
To cancel some of those noisy speckles, digital SLRs include internal noise reduction circuitry. This circuitry works by making two exposures: an actual picture of your subject, and a blank or dark exposure for the same length of time. This second picture contains only noise of the same type that appears in the real picture. The camera’s noise reduction circuitry compares the two and zaps the pixels that are common to both (the noise). This solution is along the order of the sculptor who starts with a block of marble and removes everything that doesn’t look like a statue.
Because the noise reduction feature works by taking two shots, any photos that you take with the noise reduction feature turned on take twice as long. You scarcely notice the delay with shutter speeds of about 1 second or shorter, but when you’re snapping off 30-second exposures, you definitely notice the extra half-minute pause during this dark- frame-subtraction process. If you’re in a hurry or forget to switch on the noise reduction, you can also perform some post-shot noise reduction by using an image editor, such as Photoshop, or with a third-party application, such as Neat Image or Noise Ninja.
The Reduce Noise filter in Photoshop and Photoshop Elements lets you control the strength of the noise reduction, how much detail you want to pre- serve, whether you want to eliminate color noise, and how much sharpening of the remaining detail you want to apply. The Adobe Camera Raw plug-in supplied with both programs also includes noise reduction features that you can apply to RAW files.