Thursday, 20 February 2014

Wall anomaly

Wall anomalyThe vast majority of anomalous photos fall into a small number of well known categories and are all photographic anomalies (see here). So, it is always interesting to come across something different. In the photo here (right) you see a brick wall. But on it there are at least two obvious fuzzy white vertical 'threads'. No such object was seen at the time of exposure. So what are they?

Given that the photo is sharp, the presence of such fuzzy features is certainly anomalous. The 'threads' seem to be attached to the wall at their top end and hang outwards. This is why I call them 'threads'. The most easily viewed example is just to the left of the centre of the photo.

A clue to the identity of these 'threads' comes from the wet appearance of the wall, a familiar look this winter! A second clue is the exposure time, which is 1/80s, which is relatively slow. The 'threads' are, in fact, droplets of water hitting the wall and bouncing after falling from a bridge some 20m above. With that kind of fall, the droplets were travelling quite fast, enough to show as blurred trails or 'threads'. If those bricks are around 6.5 cm high (standard house brick size), and the 'threads' are a similar length, then the drops must have been moving at around 5 m/s or around half the terminal velocity for a typical water droplet. I only mention this rather tedious point as an example of what can be worked out simply from looking at a photo!

So, in fact, it was a photographic artefact caused by the motion blur of a rapidly moving object. So it's a variation on a well-known cause of anomalous photos with most motion blurred photos being caused by camera shake. Note how important it was, in this case, to understand the layout of the location and the exact circumstances, like the weather!

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