Ghosting

Term: Ghosting
Description: Ghosting is a photographic term used to describe the unwanted double-exposure of an object within a single frame. It occurs when too much light enters the camera lens and causes identical objects to appear more than once in the same photo. Most commonly, ghosting will appear as two or more faint outlines in laid over each other that blur together without merging into one another. Ghosting can also be used as a creative tool rather than an unwanted artifact because it gives them an opportunity to capture multiple perspectives of a single shot in one photograph.

Motion Blur vs. Ghosting: The Difference between These 2 Artifacts

In HDR photography, we are usually taking bracketed sequences to create the final HDR image. Since bracketing involves multiple consecutive shots, any moving objects in your scene will be moving across each bracketed image, a common artifact in HDR photography known as ghosting. Do not confuse ghosting with motion blur as motion blur and ghosting are two different artifacts created by two different pieces of the overall HDR process. In this article, we will explain the difference between motion blur and ghosting in HDR photography.

What is Ghosting in Photography?

As mentioned before, ghosting is caused when objects moving across multiple images are combined into one image. Ghosting is an actual artifact created from the overall HDR processing. The most common example of ghosting is a bird flying through the scene. In a bracketed sequence, the shots are taken consecutively. Therefore, in the first shot of the bracketed sequence, the bird will be in Position A. In the next shot, the bird will be in Position B and in the last shot, the bird will be in Position C. The bird has moved right across the frame, so when we combine and stack these 3 images, we basically get duplicates of the bird. Each of these duplicates will look a little faded and transparent, making the duplicates of the bird look like little ghosts, hence the name “ghosting.” You will essentially end up with 3 birds in your final HDR image, so you will have to go back and fix each one of the images in the bracketed sequence. With ghosting, there is nothing you can really do, except to shoot a single-shot HDR whenever possible. If you can capture and compress everything to one single image, then shoot a single-shot HDR. If you must do a bracketed sequence, you will need to mask and clone out any ghosting in the HDR image in Photoshop. However, one factor that can help reduce ghosting in HDR photography is to use a DSLR with a faster frame rate, which we will discuss in a future article.

In the image below, the moving cars look a little transparent and faded, hence the name “ghosting.” There is also motion blur in this image, which we will explain next.
01-hdr-photography-ghosting-example

Motion Blur

The other type of artifact that we will get in HDR photography is motion blur. Motion blur is not necessarily a factor of the HDR process but rather the slow shutter speed on a camera. Motion blur occurs when the shutter speed is slow and there is movement within a single frame, not necessarily across the frame. For example, if your shutter speed is at 1/30th of a second and there is someone moving in your scene, that person will come out blurry in the image. So, motion blur is caused when the shutter is left open or “dragged” when objects are moving in the scene, while ghosting is caused when we combine three images with moving objects into a single image.

As you can see in the image below, there is a blur where the cars are moving. In addition, the brake lights on the cars are being dragged out because of the slow shutter speed. When you have motion blur in your HDR image, you will always have ghosting as well. We will discuss why this is in the next section of the article.
02-hdr-photography-motion-blur

One without the Other

If the shutter speed on your camera is fast enough, it is possible to have ghosting without motion blur in your image. If the slowest shutter speed on your camera is 1/500th of a second, it will freeze most moving objects; therefore, you will not necessarily have motion blur in the image. However, regardless of how fast your camera’s frame rate is, there will still be a quick duration of time in which moving objects have time to move throughout each frame, thus causing ghosting. Therefore, you will still have ghosting, but you may not capture motion blur in your image.

It is also possible to get a combination of motion blur and ghosting in your image. Unlike ghosting where you can have one artifact (ghosting) without the other (motion blur), if you have motion blur in your image, you will have ghosting as well. For example, if there are moving objects in your scene and your shutter speed is slow, you will see a combination of motion blur and ghosting. Just remember that, depending on the overall shutter speed settings, ghosting can come without motion blur but motion blur always comes with ghosting. Now, there are many photographers who intentionally use motion blur and ghosting in their HDR images to create interesting effects, so ghosting and motion blur are not always bad.

Ghosting and motion blur are both visible in the image below. However, the motion blur also creates a cool effect with the brake lights, giving the impression that the cars are moving quickly.
03-hdr-photography-ghosting-and-motion-blur

Conclusion & Learn More!

Ghosting is caused by the combination of the 3 shots with moving objects while motion blur is caused by the moving object with a slow shutter speed. The speed of the moving object and the shutter speed will determine the amount of motion blur and ghosting that will be caused in the image. Although some photographers intentionally use the ghosting artifact to create effects in their images, ghosting may not always be intentional. If necessary, you can use Photoshop or other editing software to fix ghosting across a bracketed sequence.