Long exposures create images with surprising effects. But they cause the sensors to heat up. Thermal noise increases, with a risk of hot pixels.
Long-exposure photography produces stunning blur effects: in the sky, on bodies of water, or with all kinds of moving objects. At night, it reveals lights and colors that are barely visible to the naked eye. But most digital cameras limit exposure times to 30 seconds in automatic or semi-automatic modes. Beyond that, you’ll need to switch to manual mode and use the bulb function (a term inherited from the pear-shaped pneumatic shutter release that was used in the past).
On some cameras, such as the Nikon D850, the shutter speed flashes at 30 seconds when the exposure time calculated by the exposure mode exceeds 30 seconds. The maximum duration remains set at 30 seconds.
Maximum Exposure Time
According to the manufacturers, the maximum exposure time varies widely: it is 15 minutes for Nikon Z, 30 minutes for Panasonic, and 60 minutes for Fujifilm. Canon allows you to set it for up to 99 hours, 59 minutes, and 59 seconds, while for Sony, the only limit is the battery life.
In manual exposure mode, for exposures longer than 30 seconds, shutter speed control becomes entirely manual.
“Bulb” indicates that you must hold down the shutter button continuously for the entire duration of the exposure.
Using a cable release is therefore essential to avoid any risk of camera shake.
Thermal Noise and Hot Pixels
These very long exposures cause the sensor to heat up, which generates thermal noise.
This phenomenon affects image quality in two ways. First, a general graininess becomes apparent, and second, this heat causes hot pixels. These pixels appear as white or colored dots, as if certain photosites had gone haywire due to the heat during the shot. At the same time, the camera’s power consumption skyrockets.
Hot pixels are barely visible and negligible if the scene contains a lot of detail. If the image has large areas of solid color, they will be more obvious—such as in a long-exposure shot of a sunset over the ocean. You’ll need to scrutinize the entire image to spot them. Here, next to the large white spot caused by the light from a boat in the distance, you can make out two hot pixels.
Native Sensitivity
To minimize this thermal noise, it is essential to set the camera to its native ISO sensitivity, which is the lowest setting (typically ISO 64 or 100, depending on the brand). It is best to avoid values extended downward, as this artificial reduction is purely software-based and reduces the dynamic range of images in the highlights. These hot pixels are rarely visible on the camera’s rear screen, but they stand out when the image is viewed at 100% on a computer.
While post-production software automatically detects and removes them, a few may still remain.
The lower the ISO, the longer the exposure time. A camera’s lowest ISO setting is its native ISO.
Here, on a Nikon D850, it is ISO 64.
Dark Frame
How can you avoid hot pixels? When shooting, a cool ambient temperature helps the sensor generate less heat. In addition, most cameras have a noise reduction option for long exposures. When enabled, the camera takes two consecutive photos of equal duration.
If the exposure time is 5 minutes, the total shooting time will be 10 minutes.
The first photo is the actual image.
The second, called a dark frame, is taken automatically with the shutter closed. The camera will often display a message such as “Job nr” (for Job Noise Reduction). This dark image contains no details of the scene but captures only the thermal noise and hot pixels generated by the sensor’s heat. At the end of the process, the camera uses this second image to clean up the actual image.
When noise reduction is enabled in the menu (here, on the Nikon D850), exposures of 1 second or longer are paired with a second exposure called a “dark frame” to correct for noise. This second exposure is indicated by the “Job nr” mention.
Dust & Scratches
The main drawback of this method is that it doubles the exposure time. Fortunately, hot pixels aren’t always a problem: in a scene rich in detail or texture (such as the foliage of a tree), they blend into the image and remain invisible. On the other hand, they become glaringly obvious on large, dark areas, such as a night sky or a smooth sea at sunset.
On newer cameras, these thermal artifacts are becoming increasingly rare.
If a few stubborn pixels remain, the quickest post-processing solution is to use a simple dust & scratches correction tool in Photoshop.
Time-Stacking
You can get around the problem of overheating by using time-stacking (or stacking images).
Instead of taking a single 10-minute exposure, you can schedule a series of shorter exposures (for example, 20 images of 30 seconds each). An intervalometer can be used to schedule such a series. The images are then merged using software such as Photoshop. By combining these images, you achieve exactly the same smoothing effect on the water or clouds.
This approach has four advantages: it prevents the sensor from overheating, eliminates the need to wait for a dark frame in the field, reduces noise through image merging, and provides a safety net in case an unwanted element (such as the headlights of a car entering the frame) spoils one of the shots. To merge the images in Photoshop, follow steps 2 through 9 from the article on super-resolution, as step 1 is not necessary here.
The 20 30-second exposures are merged in Photoshop to create an image with effects similar to a single 10-minute exposure. The difference between this image and the one at the beginning of the article is due to the later shooting time.
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Text and photos: Philippe Bachelier, teacher of Printing techniques at Spéos