long exposure - How to prevent hot pixels?

I've just returned from a trip.

One of the locations was very dark site. I took a photo with 4 minutes exposure for the foreground. The photo came out unusable. There are so many red, green, and white hot pixels. It's beyond photoshop clean up.

Is there a way to photograph such a dark foreground without getting hot pixels? I've heard that stacking multiple photos with lower (relatively) ISO + 30 sec shutter speed might work. How does that work and are there any other ways to prevent hot pixels other than using a film camera?

Answer

You can't really prevent hot pixels on long exposures, you can only deal with them. For a single four minute exposure the easiest way is to use what is known as Dark Frame Subtraction. Different manufacturers have different names for in camera versions of it.

Canon, the brand I shoot, calls it Long Exposure Noise Reduction. After an image is taken the camera exposes another frame while the shutter remains closed. The data from the sensor obtained from the "dark" exposure is then subtracted from the shot taken with the shutter open. Be aware that if your exposure is four minutes, then it will take an additional four minutes to create the dark frame. During this time you will not be able to take your next photo.

There are also photo processing software applications that can subtract a single dark frame created by you from a batch of multiple images. Most full suites, such as Adobe Photoshop or RawTherapee support dark frame subtraction. The easiest way to create a dark frame is to make an exposure with the same settings but with the lens cap left on the camera. Just be sure the cap does not leak light! If there is any light source behind the camera you may also want to mask the viewfinder to prevent light from leaking around the raised mirror and making it into the mirror box.

Even in your case with an image you took a while back, you might be able to improve the image that you now consider unusable. Set the camera to the exact same settings that you used when you took the image. Place the camera in an environment (especially temperature) as close to the environment where the original image was taken. Be sure to allow enough time for the camera to adjust to the ambient temperature. Then take a dark frame with the lens cap in place and use that frame in an application such as Photoshop to subtract the hot pixels in the dark frame from the previous file. FOr how to do it in Photoshop, see this link.