Prima si prende la fotografia del cielo di una grande città poi quello della stessa parte di cielo ma dove non c'è inquinamento luminoso, come in un deserto, quando si trova nella stessa posizione in cui era la città. A quel punto si sovrappongono le due fotografie e si ottiene un cielo notturno di una metropoli come si vedrebbe senza inquinamento luminoso. A utilizzare questa interessante e spettacolare tecnica è Thierry Cohen, come spiega Francis Hodgson.
The principal operation that has to take place before these pictures can exist is that the sky from one place has to be superimposed upon cityscape from another. The reason is simplicity itself. As every amateur astronomer knows, it is impossible to see this detail in the night sky above a city. Modern lighting provides a level of light pollution so high that looking into the urban sky is like looking past bright headlights while driving. Add to that the atmospheric pollution above any city, and you have a screen only barely penetrable by light. Stand in New York or Rio and look up, even on the most cloudless night, and you won't see Cohen's explosions of light. Yet it is there, blotted out only by man's interference.
[...] Cohen is also a fine technician, who has practised digital photography for longer than almost anyone else. But he is not practising for virtuosity alone. Cohen does not merely replace one sky with another for convenient photographic legibility. By travelling to places free from light pollution but situated on precisely the same latitude as his cities (and by pointing his camera at the same angle in each case), he obtains skies which, as the world rotates about its axis, are the very ones visible above the cities a few hours earlier or later. He shows, in other words, not a fantasy sky as it might be dreamt, but a real one as it should be seen.E questo è il risultato.
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image credit and copyright Thierry Cohen