In all of the images in this sequence I have tried to maintain the same framing in terms of how Niall's head and shoulders fill the frame and the same direction of shooting. The only variable is my having to progressively move backwards to manage the framing. In all of the shots I have also maintained an aperture of f/4 to eliminate the influence of variation in the aperture. Setting the camera on aperture priority and an ISO of 100, all of the shots were either 1/60s or 1/90s. Finally I selected spot metering to ensure that Niall's face is properly exposed and that the background brightness did not influence the exposure of his face.
The first image starts at the wide end with 24mm
Not the most flattering image, the wide angle has forced me to get in very close and has accentuated the size of his nose and almost hidden his ears. Moving to 50mm the image is far more acceptable, apart from his eyes being shut (did not use a flash)
However, even at 50mm his head is still too large compared to his shoulders. 50mm is more appropriate for a full body shot. Stepping up to 70mm and switching to my telephoto zoom brings a great improvement
At this point the image is far more natural and Niall was feeling more comfortable as I was further away. With the shorter focal lengths I was very much invading his personal space. From this point I start to enter telephoto focal lengths, starting with 100mm
In this sequence the quality of the foreground portrait is not greatly changing, however, there is significant and progressive change in the background. The longest focal length offers the smoothest Bokeh and eliminates the distraction of the tree trunk to the right of the head.
My preference from the images is that provided by the 300mm, however, this is a difficult lens to sue for portraiture as it pushes me too far away and I loose rapport with the subject. 200mm is a good focal length, but is in my view still too long, although very good for shooting candid portraits at events. from a practical standpoint 100mm or thereabouts provides a good compromise between isolating the subject from the background, but being close enough to engage.
My favorite and most used lens for occasions when I want to take a portrait with a willing subject is my 135mm f/2. This very fast lens gives me great creative control and excellent image quality. Alternatively my 85mm f/1.8 is also very good and on a crop sensor camera behaves much like the 135mm.