Photography is one of the many forms used by paranormal investigators to try and capture evidence of spirits and ghosts. It is believed in the paranormal field that it is possible to capture the energy of a spirit on film or on a digital camera photo if the timing and environment is right. However it is very rare when it happens and there are a lot of things captured in photographs that get easily confused with evidence of a spirit.
The first and most common object misunderstood for spirits are called 'orbs'. 'Orbs' are the round spheres of white or color that show up near people's heads in photographs or during night shots in cemeteries. Many people believe that these 'orbs' are balls of energy from a spirit that is trying to manifest itself in our world. In reality, almost all of these so-called 'orbs' can almost always be explained as natural phenomena and easily duplicated. In most cases an 'orb' is just a speck of dust, moisture, rain, mist, saliva, or a bug on the lens of the camera or in the air. The flash from the camera will reflect off of these objects and form an 'orb' in the photo. Even when these items aren't visible to the naked eye, they are there and visible to the camera's eye. The environment that most people take photographs in is always filled with particles and natural phenomena that can show up on film. Another common cause for 'orbs' in digital photography is the camera's need to understand everything it sees. When a digital camera takes a picture it uses something called pixels to fill in each tiny piece of the picture. If for some reason the camera can't understand a pixel it will fill it in with something that looks like an 'orb' in order to complete the full picture. Sometimes people even see a face in the 'orb' proving it must be a spirit. But actually this is also a natural phenomenon called 'matrixing'. Our brains automatically take things that have no real form and organize them into something we can recognize. It's sort of like looking at a cloud and thinking it looks like an animal or when we look at the moon and see the face of the 'man in the moon'.
Another common form of spirits in photographs is 'mists' or 'fogs'. Sometimes when people take a photograph a mysterious white 'fog' will show up in the picture that wasn't visible to the naked eye at the time. People mistake this for a ghost or spirit trying to form itself in front of you. This again is something that is easily explained and duplicated as something natural. Many different things can cause a 'fog' in your photographs. And actually fog is one of the most likely causes. When taking photographs outside at night the flash from your camera can reflect off of fog that is rising from the ground. In many cases this fog is not visible to the naked eye, but the flash from your camera will bounce back off it, making it appear ghostly in your picture. Another cause of 'fog' in your photographs can be your cold breath showing up in the picture. On a cold night, you won't necessarily see your breath but it will reflect the flash of your camera. The same is true for smoke caused by someone with a cigarette nearby. Some other causes of 'fog' in your pictures can be a fingerprint or smudge on your camera lens, a piece of your hair or an eyelash in front of the lens and in some cases the strap from your camera.
Reflection is another common cause of ghostly looking objects showing up in your photographs. The flash from your camera can reflect off of many different types of surfaces and shine back at the camera causing distortions to the photograph. Objects like mirrors, windows, glass, jewelry, fine china, and even door knobs in or near the subject in the photograph can cause flash bounce backs that result in a bright streak of light or ghostly looking image in your picture. It won't always be evident, but it is the most likely cause for the distortion and has actually been duplicated and proven by professional photographers.
All of these natural causes should be considered first before jumping to the conclusion of a 'spirit' being captured on your camera. In most cases it is almost impossible to know for sure whether or not something is spiritual or not. So in these cases, the scientific approach is to go with the most logical cause, and those are natural ones. However, photography is still used in paranormal investigation because they can help lead to evidence of spirits. Sometimes when other signs of paranormal activity, like cold spots, electronic voice phenomena (EVP), or electromagnetic field (EMF) fluctuations are present, photography from the same time frame can be used along with this other evidence to help prove or disprove something paranormal occurring.
For your best chance of capturing a spirit on your camera you should try a few practices that are used by experienced paranormal investigators. First you should be aware of your surroundings. Dirty buildings, dusty rugs, weather, and other natural phenomena should be taken into consideration when reviewing your photographs. Second you should make sure that your equipment is set up properly. Make sure the batteries in your camera are new, and if you're using film that it is not defective. Clean your camera lens, and make sure nothing is in the way of your shot. While taking photos, try taking a series of three or more shots of the same exact picture. This way you will have more than one photo to use as a comparison. A series of pictures can help you show movement or environment to help better understand what is in your photograph. You may also want to try using a camera that is equipped with 'night shot' or 'infra-red' lighting. This won't eliminate the distortions that a flash can create, but it will cut down on it. After you've inspected your pictures and ruled out any natural or technical phenomena, then you might actually have evidence of something paranormal. For more reassurance you should have several different people look at the photo objectively and have them give an opinion of what it might be. If all or most of them agree on the same result without being told what it might be beforehand, then you just might have captured a ghost or a spirit on your camera.