The Metabones Speed Booster is a device that sounds impossible. But it isn’t, and RedShark contributor Craig Marshall thinks it’s a good investment
Anyone who has worked with either still or video lenses has heard of and maybe even bought a ‘Focal Extender’ or ‘Teleconverter’. These devices are typically fitted between the camera and the lens to extend your focal range. For example, I want to get up close to the subject with my 300mm telephoto so I fit a 2x Extender and voila, I’m looking through the equivalent of 600mm. Sounds great! Unfortunately, there’s a catch.
Good quality Extenders have traditionally been very expensive. They usually cost as much as your lens and consist of several optical elements in several groups so they typically halve the amount of light reaching the camera’s film plane or sensor. There’s another problem too, unless your Extender is ‘optically perfect’, it will simply amplify any imperfections in your current lens: distortion, chromatic aberration, etc. Focal extenders still have their place but what’s this mumbo jumbo about a Focal Reducer?
Before I answer that, let’s look at a typical case study. I use a popular Sony NEX videostills ‘hybrid’ camera with a large APS-C sensor and a ‘bayonet’ E-Mount lens system. A digital HD camera with a removable lens makes good sense, especially if you already have some SLR 35mm Full Frame lenses in your collection as lens to camera adapters are cheap and readily available. Over the years, I have acquired a selection of vintage Carl Zeiss Contax prime lenses. They’re big, they’re brass, they’re fast and most important to me they’re affordable. I also have a Zeiss Mutar II 2x Extender. When fitted to any Contax lens, it reduces light by only one stop but doubles the focal length without any noticeable optical distortion and yes, it was expensive.
To adapt my Full Frame SLR lenses with their C/Y mount to an APS-C camera with a NEX E-Mount, I can use an after-market adapter so for around $15, I can buy a simple mechanical device which will do the job. (Canon to NEX?, Nikon to NEX? Yes, these adapters are commonly available too) However, there is a trade-off. Adapting an FF lens to an APS-C format camera will result in approximately 1.5 times the FOV (field of view). Now, in my case if I fit the adapter plus the 300mm telephoto to my NEX E-Mount camera, suddenly I’m looking through the equivalent of 450mm. Nice. Add my 2x Extender and now I have 900mm. Even nicer! But what if I want to use a wide angle lens? I put on a 24mm FF lens and suddenly it’s only a 36mm. Not so nice. An 18mm wide angle reduces to a 24mm wide angle. What if there was an alternative? There is and it’s called a Focal Reducer.
Instead of using a ‘dumb’ adapter, you can use one with an optical element but unlike the Focal Extender we discussed earlier, this cunning device does not reduce the light by one stop but instead it INCREASES the light by one stop! What, is this more Work of the Devil? No. Focal reducers have long been coveted by opticians and astronomers but until recently, they have been both difficult to make and expensive to buy. In fact, the remarkable candle lit scenes in Stanley Kubrick’s “Barry Lyndon” (1975) employed NASA’s outrageously expensive 1960’s Carl Zeiss 50mm lens which featured an inbuilt focal reducer. After that film’s release, this optical dinosaur became known in the trade as the famous ‘Barry Lyndon Lens’. Its speed was and still is, a stunning F0.70!