It was totally by chance. There were several factors that are used in the so-called low key photography gathered that day. I used these elements without knowing it.
The lens I was using for shooting that day was very dark. The background of the scene was dark, after all it was the end of the day, and the bird was catching some light. I measured the light on it and took the picture.
The result surprised me. I did not even have to make many adjustments at home later. Now I know how to do this, I’m always trying to take a picture like that every time I see that heron in the chapel pond.
This is the kind of lens I used for the photos:
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