For this particular exercise we are to look at the difference that focal length would have on a portrait. Normally the best focal length to use is something around the 50mm (full frame) or 35mm if using a crop sensor. A wide angle is not normally recommended due to the unpleasing distortion effect it has on the subjects face.
Some pro photographers often opt for an even longer focal length to 50mm such as 105mm or even 210mm. Scott Kelby often gives demonstrations on many of his blogs and videos using a 210mm Nikon (now Canon) F2.8 lens for portraits.
I took these images on my Sony A6000 which is a crop sensor, so any focal length mentioned can be multiplied by 1.6. All images where taken in AV mode at f5.6. I used a 16-50mm and 70-210 zoom both made by Sony.
NOTE: My wife was about to leave for the airport so the lighting and composition is far from best, but due to delays in my assignment I need to move on.
This first image of Elaine was taken at 16mm or 25mm full frame. You can clearly see the face is a little distorted and not giving a very pleasing look. Due to the wide angle its also very hard to gain any depth of field as can be seen by the light in the background.
In this next image the focal length is 24mm crop or 38mm full frame. Slightly more depth of field than the first image, but still not enough for a good separation between subject and background. The face and body now looks a little more natural.
At 36mm crop and an equivalent of 57mm on a full frame this is a much better image and close to a look we would see by the naked eye. 50mm is the normal length most cameras use to best capture the natural look.
Its harder now to see any real difference between this image taken at 50mm crop or 80mm full frame. The 80mm is a common choice for medium format lenses as used by Phase One or Mamiya.
At 70mm or 110mm equal to full frame, this is now more in the telephoto category than using a medium prime lens. The advantage with using such a lens is greater DOP, and one important point is the photographer is able to stand further away from the subject which makes the person feel at ease with no lens or camera close in on the face. The overall image still looks very natural.
Using now a longer telephoto, this image is at 100mm or 160mm. Again when comparing to the other three images above its hard to make out any real change apart from a better DOP.
For this shot I had to actually change position as I had ran out of room in order to keep the composition the same. This is at 135mm crop which is 216mm. Although this image has no interesting background, if shot outside the image would have nice fall off due to a wide aperture and longer lens.
This last image was taken at a focal length of 210mm in crop sensor, or that would be 335mm on a full frame 35mm camera. I would tend to maybe use a little less focal length given the choice, but if outside shooting subjects hard to get to such as at a sporting event then this would be a good choice for bringing the subject closer.
Using various focal length lenses as above certainly shows how they can be used to give to same effect while simply moving further away. A medium tele-photo lens is good to give a working distance between photographer and subject, which is ideal for making the subject feel more at ease. It also allows the photographer space to move around and compress the background if needed.
The wide angle lens is not really an option for portraiture. This lens leads to too much distortion of the face and anything which is close to the lens, such as hands or feet. The best option would be any focal length lens ranging from 50mm up to around 200mm for best results.