Why is the Tamron 90mm 2.8 marketed as Macro and not as a "portrait" lens?

My question is if a lens like a Tamron 90mm 2.8 could be used also for portrait. If not, why ? What are the difference between a specific portrait lens (like any 85mm 1.4) and a 90mm 2.8 ? Only the different aperture ?

Answer

Macro and portrait lenses are generally designed to do two different things that require different design characteristics.

Macro lenses are designed to focus at very close distances and they typically render a fairly flat field of focus. There are a few very specialized macro lenses that can only focus at the very close focus distances required by macro photography and would not be suitable for other types of photography. Most macro lenses, however, can also double as general purpose lenses. These can be used to focus at more typical focus distances and many photographers have a 90-100mm macro lens that they also use for portraits.

Other lenses specifically designed for portraiture often have a more spherical shape to their field of focus. The Canon EF 85mm f/1.2 L II is one such lens. They typically can not focus anywhere near as close as a macro lens can. There are reasons some photographers prefer to shoot portraits with a lens that has field curvature.

The field curvature that is a characteristic of many lenses purposely designed for portraiture would make most everything except the center of the frame extremely blurry due to the very shallow depth of field if used at the extremely close distances involved in macro photography.

On the other hand, the narrower maximum aperture of most macro lenses remove the option of wide aperture bokeh and shallow depth of field when compared to many prime lenses designed particularly for portraiture. The correcting elements needed to render a flat field of focus also tend to make out of focus blur, often called bokeh, a bit harsh. Macro lenses are also generally designed to be sharpest at close focus distances. Sometimes, but not always, they are also very sharp at longer focus distances.

Such macro lenses with very sharp performance across the frame, even at longer than minimum focus distances, are excellent choices if your goal is to make the highest quality photos you possibly can of flat test charts. They're not always the best choice to get the characteristics many portraitists are after.