In addition to the focal length, though, the portrait lenses typically also have a fast aperture, at least f/1.8. The Olympus 45mm f/1.8 comes very close, and is in fact the first real portrait lens for Micro Four Thirds. It is a very good lens, and at an affordable price.
However, some may want an even faster aperture, for better background blur and bokeh. So it may be tempting to turn to the Leica 25mm f/1.4. After all, it is currently the fastest (in terms of aperture) autofocus capable Micro Four Thirds lens.
To test how these lenses perform as portrait lenses, I have tried to photograph a static face, a statue. The statue has natural proportions, i.e., the size of the head is the same as for a genuine human. I focused on the eyes, which is the common thing to do for portraits. Here they are:
|Olympus 45mm f/1.8 at f/1.8, focus distance about 1m, 3 feet||Leica 25mm f/1.4 at f/1.4, focus distance about 0.6m, 2 feet|
To better see the differences, I have superimposed both images into one animated GIF:
What we see here is that at 25mm, and at a closer focus distance, the face becomes distorted: The chin and nose looks bigger, and the eyebrows look a bit asymmetric.
This is in fact the whole point of the traditional portrait lens: With a focal length of 85mm (equivalent), it allows you to take a headshot picture at a distance of 1 meter, sufficient to make the face look natural.
With a shorter lens, you need to go closer for a headshot portrait, which makes the facial features look distorted. It is not the lens in itself which creates these distortions, but the fact that you are closer. You would see the same with your own eyes. It is a matter of geometry.
So in the traditional sense, the Leica 25mm f/1.4 is not a portrait lens. Taking headshot portraits with it would give you distorted features. Now, this is not necessarily a bad thing. Some people like to use shorter lenses for portraits, as it makes the pictures look more funky.
You could solve this by using the Leica 25mm f/1.4 at a distance of 1m (3 feet) or more, and then crop the picture afterwards.
|Leica 25mm f/1.4 at f/1.4, focus distance about 1m, 3 feet||Same picture, cropped to 4MP|
This way, you can use the Leica 25mm f/1.4 to take headshot portraits, and avoid the facial distortions. However, there are two drawbacks: You lose a lot of resolution, when cropping from 16MP to 4MP. Also, with a longer focus distance, you get less selective focus and background blur.
Another possibility is to take an "environmental portrait", rather than a headshot photo. With an environmental portrait, you include more than just the head. Here are a couple of examples, taken at shorter focal length than a typical portrait lens:
Lumix G 14mm f/2.5 with a focus distance of about 1 meter
Lumix G 20mm f/1.7 with a focus distance of about 1 meter
The Leica 25mm f/1.4 is not a traditional portrait lens, but you can still use it to take environmental portraits at about 1 meter distance to the subject. At closer focus distances, you will get some distortions to the facial features.
If you are using a kit zoom lens, then set it to 42mm when taking a headshot portrait. That way, you will use the "correct" distance of about 1 meter, to avoid facial distortions.
Other portrait lenses
The Olympus 45mm f/1.8 is a very good portrait lens at a good price. However, if you are looking for alternatives, then there are a few:
In the lower end, price wise, you find the Sigma 60mm f/2.8 DN. It is a medium tele lens, and you can use it for portraits at a distance of about 1.5m. It is very sharp, and has a nice bokeh.
On the opposite side, in terms of pricing, you find the Olympus 75mm f/1.8. It is a long portrait lens, and is widely considered to be one of the sharpest lenses in the Micro Four Thirds lens lineup. With this lens, you need to keep a distance of about 2m to take a portrait. To photograph a group of people, you must walk far away from them. So while this lens is truly brilliant, it is not very versatile.