The photograph below was taken in the mid-afternoon of April 19, 2007, with a Nikon Coolpix 2500 digital camera. The exposure time was 1/2506.2 seconds. It shows a small aircraft, probably a Bombardier Dash 8, on its approach to the nearby airport. Above the airplane is another object that appears to have the shape of a flying saucer.
The focus of the shot was the aircraft, and the object above it was not noticed until the photo was displayed on the computer screen. Such an object is sometimes labelled a UFO when it is, in fact, nothing more than a bug or a bird that was in the field of view precisely when the camera shutter was open. An object that is incorrectly labelled a UFO is called a BLURFO. The name refers to the fact that such objects are usually blurry, hence the contours are not clearly defined.
In this particular instance, it can be argued that the object is not a BLURFO but a UFO.
Click on the image below to see the original photo.
The following figure shows the object cropped from the original photo in the left panel, and the image enhanced with equalization in the center panel. The latter shows additional details, including the suggestion of a toroidal distortion near the object that is often seen near UFOs in other photos. The object is usually in contact with the ring of the torus which is thought to represent some kind of field effect with visible optical properties. The right panel marks the location of the torus with a graphic overlay for the benefit of those who find it difficult to spot.
Similar toroidal optical distortions can be seen here in other photos of UFOs.
The aircraft was a Dash 8 which has a wingspan of about 26 m. The Dash 8 has a typical landing approach angle of about 6 degrees. Since it was about 5 km from the airport, its elevation was about 500 m. The unknown object is assumed to be no lower than the same altitude of 500 m and, of course, no higher than the cloud ceiling which was around 6000 m (information provided by the Weather Underground).
On the photo, the aircraft measures about 10 mm across the wingspan while the unknown object measures about 2 mm at its widest point.
The apparent width of the object is about 2/10 of the width of the airplane, so if it were at 500 m, it would actually be (2/((500/500)*10)) x 26= 5.2 m across. However, if it were at 6000 m, the object would have a diameter of about (2/((500/6000)*10)) x 26= 62.4 m.
So the possible flying saucer had a diameter in the range of 5 to 62 m.