In my experience, the best Leica lens for portrait is Summilux-m 1:14/50 ASPH. I also have a 90mm Summicron lens but I prefer 50mm over 90mm when it comes to portrait photos. With 50mm, I could get close enough to a model therefore I can interact with a person.
With 1.4 wide open aperture, I could shoot a model with really low light. This is a huge benefit as I’d like to shoot at night.
Before Leica M10, Leica M body did not really support high ISO such as 3200. Technically it was supported but not in practical sense as noise was too bothering. But with Leica M10, high ISO like ISO3200, you wouldn’t be able to notice too much noise. Sometimes I even use ISO6400.
Han river is my favorite spot for a model shooting. Not only I like water of any kind but also I like to shoot with lights in the background which makes beautiful bokeh. With wide open aperture, Summilux makes a stunning bokeh.
Converting a color image into monochrome creates different emotional responses to the viewers. Before Leica, lenses I used to use has soft feeling when it is used in wide open aperture such as F/1.4 but with Leica Summilux lens, it creates very sharp image. In my experience if you want to have a beautiful black and white photo, you need a sharp, high contrast image.
Sometimes, I use illumination from a mobile phone screen. I don’t use any light when I shoot but this light from a phone creates a unique feeling.
28mm for a portrait photo?
I never thought I could enjoy 28mm for a portrait photo because I strongly believed you need at least 50mm or over to take a good portrait photo. But it turns out when you get close enough, enough to feel the breath of a person you’re going to take a photo of, it creates very dramatic looks with more background. This creates scene more like a cinema.
Leica Q support high ISO but it shows more noise than Leica M10 even at ISO3200. But the good news is it has a bright lens. F/1.7 aperture, you don’t necessarily need high ISO more than 3200 in most cases.
Also it has a beautiful bokeh just like other Leica lens. Sharp image with rich contrast makes both color and monochrome image satisfying.
My first choice of portrait photography is Leica M10 with Summilux-M 50mm but I always carry Leica Q as a backup. In fact it’s more than just a backup. I capture a lot with Leica Q to produce cinema like feeling.
I truly believe Leica Q completes Leica M experience. (check out this link for more).
I found that I am not the only one to think that 28mm is best for portrait. This guy I found through internet surfing also prefers Leica Q as his choice for street photography. He could up close and very personal with strangers with Leica Q.
Check out this article for Leica Q for street photography.