Monday, 3 June 2013

Just how far away is that UFO?

UFOI recently photographed this UFO (pic right). It has a 'classic' saucer shape, though tilted diagonally. So what is it?

Before speculating we need to know its size and distance. Unfortunately, it is very difficult to judge the distance of an unknown object in the sky. With a recognizable object we would know the size and could therefore estimate its distance. Otherwise it could equally be a very close insect, a distant balloon or maybe a stick thrown into the air nearby. I included the bushes in the shot to show that even having known objects in the same photo does not really help, unless they are physically in contact with the object.

There is a way in which you can very roughly estimate whether an object is nearby or far away in a photo. If you look at a photo containing objects at different distances, those nearer will often appear just slightly clearer. That applies even when all the objects visible are perfectly in focus. You can see the effect best in a line of identical objects stretching into the distance. The furthest objects look slightly less well defined because they have fewer pixels to show their details. In addition, distant objects might well be affected by atmospheric haze.

There are a number of confounding factors which make this 'clarity' effect not particularly reliable. For instance, as well as distance clarity is affected by such things as focussing, illumination and the objects appearance. Having said that, in general, a nearby toy balloon will look clearer than a distant plane, if both are equally well in focus and illuminated in the same way (try the UFO gallery for some examples).

UFOSo, while you could never use this technique to say that a UFO 'must be 3km away', you might be able to say that it looks roughly as clear as a nearby set of buildings and is probably a similar distance away. In the original of the photo above, the object does look roughly as clear as the nearby trees implying that it is probably at a similar distance, rather than much closer or further off. If so, that would eliminate a nearby insect or distant aircraft. Luckily, I know what the object is because the photo is actually a grab shot from a video. A few frames on from the photo above, the object looks different (see second photo, right).

You can now see the object is clearly a bird! It is flying left and diagonally downwards. A close examination of the original video suggests that it is a Woodpigeon, which would make it around 27 cm long, something I could not have worked out just from the photo. The bird is further away than the trees though probably not by much.

So, though you can use the 'clarity' of an object in the sky in a photo to give you a very rough idea of how far it is away, there are many possible confounding factors to consider. At best, the method can be useful, with experience, in giving you at least some idea of what type of objects to consider first when trying to identify a UFO in a photo.

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