Focal length is a big part of a portrait vision. I prefer longer focal lengths. This portrait was shot at 189mm.

Given the choice I'll nearly always shoot longer and longer rather than shorter when it comes to making portraits. There is something about the compression, blended with lighting that helps accentuate the topology of a face, that makes a portrait seem more real to me. When the first digital cameras with interchangeable lenses hit the market in the early part of the century almost every model came complete with an APS-C sized sensor. I was amazed at the number of people who thought nothing of shooting portraits with their 50mm lenses on those cameras. While the 50mm focal length translates to about a 75mm equivalent on a full frame camera I think that's still more than a bit short for good compression when making portraits. 

We could be pedantic and suggest that the 75mm user back up a bit and then crop and that the results would match a longer lens but we know there's other stuff at play. In the early digital days part of the equation was a resistance to cropping in order to ensure there were enough pixels left over to make a decent image. I'd say that if one is shooting on full frame cameras and cropping square the same reservations apply. 

In the best of all possible worlds I'd use something in the range of 135 to 200mm for a studio portrait and I'd also specify that my background be yards and yards behind my subject instead of mere feet. The further away the background the easier it is to drop out of focus and also to light as a totally separate plane. If we put the background very far back we soon see another reason to go longer with our portrait lenses. A shorter lens will show the edges of the background sooner and will limit our ability to push it back as far as we can. In essence, a long portrait lens, delivers more options for the relationship between the subject and background. 

There are a few downsides to using a long lens for a portrait. If you light faces in one of the current styles that calls for flat and even light across a face you'll find the compression makes the face seem wider; fatter. This is rarely a benefit to the subject. When we compose with long lenses I try to create light with quicker gradations to shadow in order to create a more three dimensional rendering to the face. I'm trying to bring back a normal geometry to the face with my lights. 

One more thing about lighting. I like to make sure that the bottom edge my main light is up well above chin level on my subject so a shadow drops in under the chin and gives a visual depth between the chin and the neck/throat. Alaina certainly does not have a double chin to worry about but many corporate subjects do and it's benevolent to make sure that a well placed shadow, created by correct lighting, does its part to conceal certain...flaws. 

The photograph above was done with a Sony 70/200mm f4.0 G lens on an A7ii camera body. 

Of course, you can always ignore these conventions and shoot wider. But eventually you'll come to hate the look and probably give up photography altogether. Wide angle portraits can be that bad... You'll notice that even Bill Brandt only dabbled with wide portraits a handful of times....