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The thing that, for me, elevates a normal picture, a snapshot, into a portrait is that you get some kind of connection with the subject. Essence of a good portrait, really, is a truth about somebody. If you can have that connection, then that's what's going to move the viewer. I think you want to create as little barrier between you and the subject as possible, so you can really reveal something about them.
I use a DSLR with a 50-mil lens on, because I'd like to keep the perspective nice and natural. But you could use an iPhone. IPhone has a very wide angle lens. I think you probably want to zoom in a little bit, as long as the perspective you give is pretty natural. But ultimately, it's the connection between you and the sitter that is going to move people when they look at your pictures.
There's some basic building blocks to portray, the first being location. Does the location aid you in any way, or is it fighting you? I mean, does the location say anything about the person that you're going to photograph? I would tend to try and keep it as clean as possible.
I would say, as a general rule, don't be seduced by good weather like this. Find somewhere in the shade that's got flattering light, so you can really make the subject stand out.
When you're thinking about composition, what you're really talking about is balance. Look at all the things in the frame and think of them as shapes, not objects. There are portraits everywhere. Look at magazines, books, and in newspapers and take some inspiration from the way other people compose.
Having your picture taken, you're going to be pretty vulnerable. I've done this for 20 years and I've only ever met three people that actually liked it. Once you've got your location and the light and your composition in your head, roughly what you want to do, it's really important that you kind of relax your subject. But try and bring out something in them. What you want to do is just tell the truth of the person that you see in front of you. Have some kind of connection.