Today we’re going to talk speedboosters, also called focal reducers and how they affect the image quality of a shot. First we’re going to see how a speedbooster works, then we’re going compare images shot using a Speed Booster Ultra 0.71x from the well-known Metabones brand with the lower end Viltrox EF-M2, how the two of them stack up. And finally we’re going to see which type of lens you should use with each speedbooster.

A speedbooster is a powerful adapter that allows you to reduce the size of an image created by a lens.

A typical use-case is when you already own Canon lenses which are designed to produce big images for big sensors like DSLRs, and you want to reduce the size of the image in order to fit a smaller sensor such as a Micro Four Thirds sensor on the GH5 or the Pocket 4K.

Most speedboosters are made with 5 to 6 pieces of glass which altogether transform a big image into a smaller image that is :

  • wider
  • sharper
  • brighter

So how the heck can you get a brighter image by adding glass between your lens and your sensor ? Well the principle is actually very simple.

How does a Speed Booster work ?

Think of when you’re using a magnifying glass to focus the rays of the sun and start burning a sheet of paper. You are increasing the concentration of light on a given surface, making it brighter.

With a speedbooster Ultra, the aperture increases by 1 stop, which means that with a F2.8 lens you can boost the aperture to F2.0 . Also, the field of view gets wider since, instead of seeing just a portion of the image created by the lens , you are seeing the full image since it now fits the whole sensor properly. Each lens is designed for a specific sensor so if the image created by your lens is too big for your sensor, what you’re seeing is a lower quality crop of the full image. This is why by scaling the full lens image on your small sensor, the speedbooster increases the sharpness of your image a bit.

And by the way, the principle of a 1.4x or 2x extender which allow you to double your focal length is the exact opposite. Instead of decreasing the size of the lens image circle, you are increasing it, which means you will see a softer, darker and zoomed in image.

Also, a speedbooster has an internal chip that allows the camera to control the lens’s aperture, stabilization and focus.

If you need help choosing your magnification factor, check out this article !

Also, I compared two different brands Metabones and Viltrox, you can check out the results here :