After reading about a generic focal reducer
(FR/speedbooster) over at Eric Cote's Mirrorless Journey
blog I decided to make the "big investment" and purchase one of my own. The unit is made to mount a Canon EF lens to a micro four thirds body however due to the popularity of mounting legacy glass to Canon EOS bodies, there are a multitude of adapters available which can be used in conjunction with the speedbooster. I have a two old Zuiko lenses that I had used occasionally with an OM to M43 adapter and by purchasing an obscenely cheap Canon EF to OM adapter
I was able to adapt my Zuiko glass. Since the FR is less than $100 I figured it was a low risk purchase and I was very interested to assess its performance firsthand.
According to the eBay listing, to calculate the focal length of your lens once the speedbooster is attached you multiple its focal length by 0.72. This also applied to the aperture and overall you gain about a stop of light. For example, an OM 50mm f/1.8 renders a field of view similar to a 100mm lens on a full frame camera (50 x 2 <-- crop factor) when mounted to my EM5 using the normal OM to M43 adapter. With the FR the lens becomes a 36mm f/1.3 (50 x 0.72 = 36, 1.8 x 0.72 = 1.296) which has an equivalent focal length of approximately 72 mm on a M43 camera. I used this legacy lens with the adapter for about 99% of my test shots and have really enjoyed this focal length. I like using short telephoto lenses, but the wider angle of view over Olympus' 45mm portrait lens makes this some sort of normal-short telephoto hybrid which is great (especially in tighter settings).
I saw no degradation in image quality when using the FR and I actually found it was better than using the 50mm f/1.8 alone with the OM to M43 adapter. Throughout heavy use I have found that the legacy 50mm is not the best performer wide open and it is often necessary stop down to at least f/2.8 (especially when faced with high contrast scenes) to get acceptable results. With the speedbooster attached I feel confident using the lens wide open which is great because of the extra stop of light you gain. This is crazy because I thought the speedbooster was going to make things worse. Below is the original RAW (converted to JPEG) and a 100% center crop of the image introducing this paragraph. In addition, I have provided another sharpness example and to my recollection these were both taken wide open. As you can see, the image has low overall contrast, but I think this is a characteristic of the old glass and not necessarily brought on by the FR.