Intro to Night Photography: Cityscapes — Focus, Noise, Exposure, Effects

At night in a city, virtually any city, the scene before you changes dramatically. In some locations the architecture and spirit of the city is merely enhanced, shown with a different, though recognizable look.

In other locations, the city at night transforms into a new place, sometimes brighter and more adventurous, sometimes more alive and mysterious.

Either way, making images of cities at night can allow you to often capture a “new city” with a different life than it has during the day. Making photos at night has its own challenges, but it's still subject to the same set of constraints as daylight photography; aperture, shutter speed and light sensitivity. The difference is, these constraints often push you and your equipment “to the edge,” compared to photographing the same city during the day.

Last week I discussed night photography trade-offs and constraints, the potential of star trails at times, and the equipment needed to produce high quality night photography in cities.

This week we'll get down to the nitty-gritty of actually making our night cityscapes. We'll talk about focus, noise and camera settings

• Focus — If it's dark enough, auto-focus won't work, or at least it may significantly slow down as it “hunts” for focus. In fact, darkness may often make it hard to focus manually too, but there are focus methods you can use when your focus point is particularly dark. Fortunately, for night photography in cities, these methods aren't needed too often.

Many times you can use auto-focus by focusing on something well lighted at the same distance from you and your subject. Once you focus you can recompose the photo. Also in many cityscapes, what you're shooting is well lighted enough to auto-focus, though not necessarily as fast as in daylight.

For a landscape type shot, such as a skyline, many would simply manually focus to infinity, however, that would cause a significant amount of depth of field to be beyond the horizon in the scene. To help ensure my foreground is in focus, I want to have as much depth of field in front of the horizon as possible. Therefore, I typically will focus this type of photo just short of infinity.

It is important to realize that as you approach infinity, for most lenses, a very small rotation of a lens' focus ring is a big change in focus.

• ISO — Digital noise tends to be most noticeable in plain, solid color areas of your image. They are generally more noticeable the darker they are.

The noise comes in two general types; luminance and chrominance. Luminance noise is the gray or black colored noise or “grain” you see when you look at a print. An image's detail is in this noise, so you have to be careful about removing it. Chrominance noise shows up as pastel colored specks in mid-tone and shadow areas. This noise has little impact on image detail.

Generally, normal use digital camera ISO (sensor sensitivity) settings of from 100–400 can be successfully used without incurring noticeable noise. Upper end cameras can use higher ISO settings. Try to use the lowest ISO setting possible to reduce noise, within the your ability to make the requisite exposure.

• Noise Reduction — Many digital cameras today have the ability to do in-camera noise reduction. Personally, I don't use it. I believe I have better control of noise reduction in post processing. According to how much noise reduction is used, and the camera's method for it, in-camera noise reduction can remove image detail.

While you can choose gross levels of in-camera noise reduction with its settings, when you use noise reduction in post processing, you can more precisely choose the amount of noise reduction and have considerable control over how much detail you lose, and where.

• Shutter Speed — Using a low ISO, and a small aperture to increase depth of field, will result in extended shutter speeds. In cityscapes longer shutter speeds, (Not so long as will produce star trails as per Intro to Night Photography: Cityscapes — Constraints and Equipment.) can produce such effects as light trails from automobiles and trucks similar to those in the image to the right of the Philadelphia Skyline, taken while overlooking I76. The exposure time for this image was 1.5 sec., but light trails can occur at much short exposures of 0.3 seconds or even 0.1 seconds according to the speed of the traffic. The longer the exposure, the longer the light trail.

• Aperture — This is the third setting of the exposure triumvirate. The aperture is chosen primarily to set the size of the depth of field, but there is another important factor which affects depth of field, which, for cityscapes and other night shots at distance, is the distance you are from your subject. The further away you are, the more depth of field you'll have at each aperture.

For example, most people would consider an aperture of f/2.8 pretty wide open for a DSLR lens. Using a full size sensor based DSLR in these examples, paired with a 24mm lens, set to f/2.8 I calculate at 25 feet from my subject, my focus area starts at 13 feet in front of the subject to infinity behind it. If I'm 50 feet from my subject my focus area starts at 34 feet in front of my subject to infinity behind it.

Therefore, it's safe to say for cityscapes at night you really don't have to worry too much about opening up your lens hurting the sharpness of your image.

When I'm doing typical night cityscapes, skylines, architectural shots, buildings, historic sites, anything but close-ups in street shots, I don't worry about aperture very much when shooting with a wide angle lens. I concentrate on ISO to limit noise, and shutter speed to get the lighting effect desired.