Night Photography: DOF at High ISO

Night photography forces a trade-off. You need a wide aperture for light, and that thins your depth of field. You need a high ISO, and that adds noise. The two are often discussed together, but they are separate problems, and one common idea about them is wrong: high ISO does not widen depth of field.

Noise does not change depth of field

Depth of field depends on aperture, focal length, distance and sensor size, and nothing else. ISO only changes how much the signal is amplified. Noise can hide a small focus error, because grain breaks up the edge between sharp and soft areas, and it can make out-of-focus transitions look smoother. But heavy noise reduction does the opposite, softening real detail. So do not count on noise as extra margin. Aim for exact focus.

How thin it gets

At night you often shoot wide open. With an 85 mm lens and the subject 2 m away on full frame, the zone in focus is about 4.3 cm at f/1.4, 5.5 cm at f/1.8, 8.6 cm at f/2.8 and 12 cm at f/4. A 50 mm at f/1.4 gives about 13 cm. That is less than the depth of a face, so for night portraits focus on the near eye, use eye detection if your camera has it, and expect the ears to be soft at f/1.4. The portrait aperture guide has more.

Stars

For stars plus a landscape, the depth of field issue is the foreground, and the astrophotography post has a table of how far away a foreground must be to stay sharp when you focus at infinity, together with the exposure rules. Find focus with live view magnified on a bright star or planet. A common trick is to focus with the lens stopped down a little, to make the star easier to judge, and then open up to shoot. Some lenses shift their best focus as the aperture changes, so confirm at your shooting aperture. See astrophotography depth of field for the details.

Tripod cityscapes

On a tripod, the priorities flip. Long exposures handle the light, so you can stop down for depth and sharpness: f/8 to f/11, ISO 100 to 400, and exposures of several seconds to a minute. Stopping down also gives bright lights a starburst. The number of points depends on the aperture blades. A lens with an even number of blades gives that many points, and a lens with an odd number gives twice as many, so an 8-blade lens makes 8-point stars and a 9-blade lens makes 18-point ones. Focus with the hyperfocal distance or manually, and magnify live view to check.

Handheld street

Zone focusing helps in the dark, but not as much as people expect at wide apertures. A 35 mm lens at f/2.8 focused at 3 m is sharp only from about 2.5 to 3.7 m. At f/8 it covers about 1.9 to 6.8 m, which is a real zone, but f/8 needs a much higher ISO at night. A compromise is f/4 to f/5.6 with ISO 3200 to 12800, or autofocus with face detection. Aim for 1/125 to 1/250 s, to freeze movement and avoid camera shake. The zone focusing post has a table of ranges.

Focusing in the dark

Autofocus needs contrast, and night scenes often lack it. Manual focus with magnified live view is the most reliable method: zoom to 5x or 10x, turn the ring to the sharpest point, take a test shot and check at 100 percent. Recent mirrorless cameras with phase-detect autofocus can work down to very low light levels, with ratings around minus 4 EV or lower in manufacturers' figures, usually with a fast lens. Face and eye detection with a wide area is often more reliable than a single point when contrast is poor.

Noise versus blur

When you have to choose between a higher ISO and a slower shutter, take the noise. A noisy sharp frame can be improved in software, with tools such as Lightroom's Denoise, DxO PureRAW or Topaz, while a blurred one cannot be rescued. How much ISO is acceptable depends on the camera. Modern full-frame cameras often give good results well past ISO 6400, and smaller sensors have less headroom.

Aperture and focus decide depth of field, the shutter speed deals with motion, and ISO makes up the rest. To see the depth of field for a given lens and distance, use the calculator.

Related reading: Aperture vs Distance: What Affects DOF More?, Background Blur (Bokeh): How to Maximize It, Lens Compression: How Distance Changes Perspective.