As a general rule, you should consider any photo with perfectly shaped (most of the time they show up as circles, octagons or triangles) "orbs" a false-positive from out-of-focus dust. Don't forget about the coma effect as well, if an "orb" seems elliptical, and it is curved around the central axis of the photographic lens, it is still just dust.

Things to look for in photographs that may merit them as potentially anomalous or "paranormal" is clear movement of the orb, which would make the possibility that it was caused by dust smaller. Even if the orb shows clear movement, it could still be rain or snow, so be sure never to attempt to try to take photos in either of these conditions (even if it's just sprinkling, it only takes one drop!) Also, if an orb appears to be overlapped by another in-focus object in the picture, it helps proves by inspection that the orb is not out-of-focus dust, because the orb is behind the overlapping object, which means that it was past the focal point. In the case of orbs that appear brighter than normal, etc, just use your own judgement.

If the photo in question has passed the test so far, that is a good sign it may be anomalous. Movement like in the first photograph is very interesting, because it is not just in one direction, but it changes directions during the exposure time (1/30th of a second!). Movement of this type shows that, depending on the distance from the camera, the orb was potentially moving at high speeds. Be careful, though, because falling rain/snow can look like fast moving orbs. Vibration/oscillation of the orb can also make it not so easily explainable. If an orb appears to be overlapped by another in-focus object in the picture, it theoretically helps prove that the orb is not out-of-focus dust, because the orb is behind the overlapping object, which shows that it was past the focal point. Orbs being brighter, different colours, or oddly shaped doesn't prove that they aren't dust, so don't try to pass the pink dust orb you captured as the "spirit of a girl". When taking pictures in hopes of capturing an anomaly, it is essential to take two photos in a row from the same angle. If photo 1 shows something anomalous, it can be compared to photo 2 to see if it was anomalous, or just an optical effect.

After years of experience, I have seen exceptionally few photographs that did not have some sort of natural explanation. In fact, I only take photographs during investigations for documentation purposes and basically to record where equipment is placed, etc.

Designed

Orbs are a very controversial item in the paranormal investigation field. The hard fact is that most, if not all, "orbs" are caused by water droplets, dust, snow, rain, etc.. A common misconception about dust orbs is that it is dust actually in the camera or on the lens. This isn't true, in fact, dust orbs are caused by air-born dust particles directly in front of the camera lens. Below is an in-depth article on photographic/video analysis and contamination factors that cause false-positive orbs. Please note that the following applies for any camera with a flash (35mm & digital), and all night shot video cameras.

         Diagram 1
        

Point "A" represents the camera lens's inverted focal point (the point that an object must be past to be in-focus), and "B" represents the way light travels into the lens. The dust particles that are closer than point "A" will be out-of-focus, and appear as false-positive dust orbs.

         Diagram 2
        

Point "P" represents the distance the dust particles are away from the camera lens, and "A" again represents the camera lens's inverted focal point (the point that an object must be past to be in-focus). This diagram shows how dust particles become out of focus as they get closer to the camera lens.

Diagram 1, on the left, is a representation of how dust in front of the camera lens appears orb-like when a flash is used. "B" represents how light travels through the camera lens, and "A" represents the focal point of the lens, or the point objects need to be past in order to be in focus. As you can see, there are dust particles closer to the lens than point A, therefore the dust will appear out-of-focus, as seen in Diagram 2. When the flash fires (Diagram 1), light is reflected off the dust, back into the camera lens, causing the dust to be illuminated. Because of the angle of the flash and the camera lens, the closer the flash is to the lens, the more prone it will be to dust orbs. The same applies for using night shot video cameras, the IR light reflects off the out-of-focus dust, causing them to look like "orbs" or plasmoids.

Some common characteristics of dust orbs in photographs may be showing some sort of nucleus, elongation around the central axis towards the edges of the photos, a rectangular or octagonal shape, but is not limited to these. Elongation around the central axis towards the edge of the photo is caused by a lens curvature error known as "Coma", cameras with very small lenses and short focal lengths (such as digital cameras) are more prone to coma than other cameras with longer focal length lenses, such as SLR cameras. Orbs of a rectangular or octagonal shape are caused because of a phenomena that can be defined as "When an object with a similar shape as the aperture of the camera lens is brought out-of-focus, the object will begin to take the shape of the aperture". In other words, if the aperture of the camera is an octagon (the aperture is the hole that opens to let light through the lens), an out-of-focus dust orb will begin to take the shape of an octagon, particularly towards the centre of the image. Another common phenomena that is often mistaken for something paranormal is called lens "Flare", and occurs when a bright light source (most often the Sun) reflects off a portion of the lens, and creates "ghost" images (not really ghosts, but internal reflections) of the aperture of the camera. Generally, flare can be ruled out of most photos taken at night, but can be a problem with shooting pictures during the day.

There is nothing paranormal about the dust orb phenomena, it is natural to any camera with a flash and any video with a light source that isn't in front of the camera (ie, infrared light, spotlights). It seems that many groups still consider dust orb photos to be paranormal, because they don't want to take the time to collect REAL anomalous data. What keeps these groups holding on to the dust orb as phenomena is the instant gratification they get by collecting hundreds of so-called "spirit orbs" in a single night of investigating. The truth is no matter what the environment, there are always air-born particles in the air (except in a vacuum...) that can contaminate your data. You can sometimes even see the flash reflect off the dust particles when a picture is taken. This phenomena isn't just limited to dust.