Playing around with some monochrome conversions in Lightroom 5... Only darkened the midtones and added some midtone contrast with luminosity masks in CS6...
Equipment: Canon 7D and 100-400mm L lens at 400mm
Settings: 1/1000s at F7.1 and ISO 400
I like the comp and foundation of this picture good. Nice and clean picture of the three rhinos. Good to see the eyes of two of the. But I think you did to much in pp, too much contrast and too much clarity. I think the grass looks a bit unnatural, and the rhinos have for my taste too far to much contrast. I like vignette, but also this a bit to much.
Would crop a bit off the bottom and back off on contrast a little as Gregor has suggested. The comp is good and the detail great so think it could benefit from a more natural look, rather than this 'arty' effect....?
Ed