If you are looking for fast lenses for the MicroFourThirds system, you may may already think about the Voigtlaender Nokton 25/0.95 mFT.

If you are looking for REALLY fast lenses with focal lengths above 25mm (e.g. for low light or outdoor portraits with shallow depth-of-field (DOF), concerts etc.), currently you have to adapt lenses made for other mounts like the Voigtlaender 35mm/f1.2 ASPH, the Leica Summilux 35/1.4 ASPH, the Noctilux 50mm/0.95 ASPH (which currently costs about 8000 EUR - if you are lucky enough to get one at all) or the Summicron 75mm/2.0.

Another option is to look for industrial lenses (e.g. from medical equipment) modified to fit to the MicroFourThirds mount. This idea is not new but in the past those experiments often failed due to the large flange back of (D)SLRs which did not harmonize with the focus point of those lenses, so most of these lenses could only be used as macro lenses. In other cases the screen image circle was too small for the sensor or you had to accept strong vignetting. Due to the smaller flange back and sensor format of MicroFourThirds cameras, chances are much better to get a lens that matches expectations.

Recently I saw such a lens in an Ebay auction and decided to give it a try. It was described as 1:1.0 / 55mm from KOWA which is a industrial lens manufacturer in Japan. Usually you see these lenses without any mount / focus adjustment / iris. In this case they are quite cheap but the chance to get it properly mounted to your camera and fine adjusted is quite small. The sample that I found on Ebay was modified to match the MicroFourThirds mount, provided a focus adjustment ring and a variable iris that could be tuned from open aperture (f1.0) to f11. Attached to a Panasonic Lumix GH2 it looks quite cool:

So - what can you expect from such a modified lens?

• First thing I detected was, that after the modification it is not a 55mm lens anymore! This focal length may apply to the original front element but together with the additional elements added at the back end, it now has a focal length of about 90mm. So what you get due to the crop factor of microFourThirds is already an (extremely fast) tele with a 35 film equivalent of 180mm!
• The modification allows to set focus from about 1m to infinite. This makes it much more useable compared to other lenses of this type attached to DSLRs which usually allow only macros
  
• At open aperture it is very bright but also very soft and "dreamy" with low contrast but this is quite typical for lenses of that category
  
• If you stop it down to f2.8, it becomes already acceptable sharp in the center and still provides a shallow depth of field with nice bokeh.

(KOWA 55/1.0 at f2.8; click on the picture to see original sizes and EXIF data)

Now let's see how it performs at (extremely) low light:

The following sample was taken with just one candle as light source. Another precondition was to use an exposure time not longer than 1/15s. The distance to the objects was about 4m. The aperture of the lens was completely open (f1.0). Under these conditions the ISO sensivity of the camera had to be set to ISO 3200. The result is quite "dreamy" and smooth but under these conditions acceptable.

(KOWA 55/1.0 at f1.0; GH2 set to 1/15s & ISO 3200, click on the picture to see other sizes and EXIF data)

Now under the same conditions (also same distance) a picture taken with the Voigtlaender Nokton 35/1.2 ASPH (Leica M mount adapted to mFT). The image is cropped in order to get the same display window

(Voigtlaender Nokton 35/1.2 at f1.2; GH2 set to 1/15s & ISO 3200, cropped to the same display window, click on the picture to see original sizes and EXIF data)

Finally - to get an idea what these light conditions mean to typical apertures of standard zoom lenses (in this case the Lumix G Vario 14-140mm):

(Lumix G Vario 14-140mm at 87mm and f5.8; GH2 set to 1/15s & ISO 12800, image pushed 3 additional stops in ACR 6.3 so effectively equivalent to ISO 102400, click on the picture to see original sizes and EXIF data)

This is another sample taken with the Kowa with pretty wide open aperture:

(click on the picture to see original sizes and EXIF data)