Beautiful portrait. It would be nice if the pupil was just a tad better separated from the iris (maybe a little lighter in the eyes). Lighting is very good.
The shot is obviously posed but I guess the lighting is diagetic. The reflection in the eyes is blue so I assume this is daylight from one direction? It would be nice to see a little more fill light but I don't think this was ever meant to be a studio portrait!! The composition is wonderful, I do this a lot with portraits, focusing on the key features of the face and leaving plenty of eyeroom, I think I would probably have cropped it even tighter than this for a really intense look. Nice shot though and really good quality, the Sigma is a nice unit and almost makes me regret being such a Canon Junkie!
I've just been twiddling with the image on PS and it could be cropped tighter but would need more of the forehead in shot to balance it out perfectly. Looks nice in B&W, and by upping the contrast till you loose the definition of the face against the wall you get more detail in the shadows and a nice overexposed moody look to the shot.
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