There are lots of things that can cause digital noise but a high ISO in low light conditions is a one of the worst contributes.
Digital cameras have ISO setting ranging from 100 to 3200.
By default, your camera will usually automatically pick the best ISO setting for whatever you’re taking a picture of. If it is bright sunshine, it will use a low ISO number. If it is dark or the lighting is poor and there is a lot of shadows, it may automatically set the ISO to a very high setting so that it can pick up as much light and information as possible.
As you can see here and one if the dangers of this is what is known as matrixing or pareidolia: this is the tendency for incorrect perception of a stimulus, as an object, pattern or meaning known to the observer, such as seeing shapes in clouds, seeing faces in inanimate objects or abstract patterns.
So, you have taken a photo, it was dark, and your camera or phone has introduced a lot of high ISO digital noise, which has then been concentrated and smoothed into mottled patterns in the shadows of the photo by the JPEG compression. You see something that looks a little odd in the photo, you enlarge and zoom in. Those blotches there, the 2 dark ones near each other that could be eyes, that patch below could be a mouth. Is it a face?? An example of pareidolia in a high ISO jpeg image. The shape of what appears to be a skull is partially visible in the image noise in this zoomed-in copy of the image.
Pareidolia can cause people to interpret random images, or patterns of light and shadow as faces. Here you can see that faces appear in these images of mist and smoke. These tend to be more of an issue with outdoor investigations when you are subject to temp and weather conditions changes and more moisture in the air.
So be sure to do test shots keep an eye on the weather as mist due and even condensation on the lens can cause this effect. Also, your own breath can cause this artefact on your image. If you are taking photos hold the camera away from you when you do, and not up to your eye, as when you exhale you can cause this effect as the flash reflects off your breath in cooler conditions
Also, be aware of smoking or vaping as this kind of smoke dose hang around in the air longer than you expect although you may not be able to see it anymore with the naked eye like moisture particles it will get illuminated by the flash and can create all sorts of patterns the brain interprets as faces or figures.
Here are some examples of how a high ISO low light and Jpeg compression can give you the illusion of an apparition or figure in an image. Here is what appears to be a figure of a woman standing in a graveyard at Holy Trinity.
When you zoom in you degrade the image more and you can see what appears to be an apparition, when in fact it was a combination of 2 gravestones and the hedge behind but because of the digital noise and matrixing we think have captured a ghost.
Here we have an image that was in the press of what appears to be the figure of a women standing on the roof of Sunbury Lunatic Asylum.
Again, here we can see the combination of low light high ISO and a distorted cropped image of what appears to be a woman on the roof,
In fact the ghostly female patient turns out to be a chimney stack!!!
