I am using a Panasonic Lumix G Vario 35-100mm F/2.8, a MFT telephoto lens.
I am learning about photography by reading articles and reviews and in one review it was mentioned there is a great portrait lens, namely the Panasonic Lumix G 42,5mm F/1.7, also a MFT lens.
I learned a bit about sensor size (full frame, ASP-C, MFT, ...) and the relationship of f#, focal length and aperture, etc., though I'm still not very fluent to be honest.
Considering that the sensor size is the same (MFT) and that the focal length of the portrait lense 42.5mm is within the range of my telephoto lense, I ask myself why would I want to buy the portrait lense then?
The different I see is the f.#, but what does that mean? Could someone explain to me in simple words please? And it would be nice to have some examples (on some articles they let me see what the difference is, but not sure if one can show a difference of F/1.7 and F/2.8?).
There is no such thing as a portrait lens. Just some are known to produce more appealing portraits. It can be used to shoot any other thing where the focal-length desired is the same. Note that I didn't say needed since portraits can and are shot with different focal-length.
A 42.5mm lens on a MFT gives what people consider a flattering perspective, making people's face's proportion and nose length shorter than perceived with the human eye. Your 35-100mm lens, set to 42.5mm will give exactly the same perspective.
With an F/1.7 aperture though, which your 35-100mm lens cannot manage, produces an image with a more shallow depth-of-field. This is considered desirable for portraits since the background gets more blurred and isolates the subject more than an F/2.8 lens.
The bottom line is that you can make portraits which are flattering with your 35-100mm, but a 42.5mm lens could be used to isolate your subject more from the background, given the same situation. It will not necessarily mean your portraits will be automatically better since there are so many factors that influence the results such as subject, lighting, position, distance, etc.
Specifically regarding those two lenses, when it comes to the quality of the resulting images, there is a small difference between the two. The 42.5mm is an extremely sharp lens and shows very little corner shading (aka vignetting) while the 35-100mm is quite sharp - just not as sharp - and shows moderate vignetting until stopped down to F/4. Such difference in sharpness would be visible in large prints only. Although the vignetting is easily noticeable, it is one of the easiest artifact to eliminate in software.
First, those numbers are probably f/2.8 for the 35-100mm and f/1.7 for the 42.5mm.
Secondly, the 42.5 has a larger aperture which allows the camera to collect more light in a given period of time. This means that you can get a good shot with less light, or have a faster shutter speed with the same amount of light.
Additionally, a prime lens (one which only handles a single focal length), is usually sharper than a zoom lens at the same length. (Though you should check comparisons if you can to be sure.) This article explains the benefits in more detail. Here's an excerpt:
Ask any professional photographer about the benefit of a prime lens and the answer will be the same: it takes clearer photos. Since they don't have a lot of moving parts like zooms, the glass inside of a prime lens is very precise.
Advantage number two: prime lenses are pretty light.
Here's the best part: you don't have to break the bank to get a superior lens for your digital SLR camera.
... why would I want to buy the portrait lense then?
Thinner depth of field from a smaller/lighter/cheaper lens. Courtesey of camerasize.com:
The 45.2mm is much smaller. The Olympus m.Zuiko 45/1.8 is even smaller than the Panasonic 42.5. And both of them are around the US$300 mark, new, while the Panasonic 35-100/2.8 II new is about $1k.
Granted, it's only 1.3 stops advantage in the max. aperture, but that's still more than double the light, and a thinner DoF if you need it than an f/2.8 lens can give.
Some people also prefer the discipline of shooting with a prime vs. the framing versatility of a zoom—one less factor to worry about pre-visualizing—but this is mostly a matter of personal taste.