Metabones Speedbooster crop ratio explanation

Need help understanding Metabones Speedbooster crop ratio and sensor crop? Our service technician Casey gives advice on Metabones Speedbooster 0.71 magnification.



Need help understanding Metabones Speedbooster crop ratio and sensor crop?
Our service technician Casey gives advice on Metabones Speedbooster 0.71 magnification.

The Metabones Speedbooster and Speedbooster Ultra have a 0.71x magnification. The Speedboosters are designed for Super 35mm sized crop sensors. No full-frame lens focal length measures ‘correctly’ to Super 35mm standards because they are designed for the much larger full frame sensor. Every EF lens on a Super 35mm sensor is cropped approximately 1.6x, meaning a 24-70mm measures closer to 38-112mm.

The Metabones Speedbooster does a reverse magnification of 0.71x to approximate a full frame lens focal length and field of view so the lens measures ‘correctly’ on a crop sensor. The 0.71x is about 90% of a full frame sensor so after the conversion a full-frame 24-70mm zoom is roughly 27-80mm with a Speedbooster on a Super 35 sensor. Essentially, the Speedbooster reduces the crop on an EF lens with a Super 35mm sensor.

Basically this is how the Speedbooster ‘gains’ the extra stop – It’s taking the light hitting the lens to use as much of the full-frame glass as possible and send that to the crop sensor, just as a full-frame camera would with an with a full-frame lens. More light to the sensor = extra stop. A Super 35-sized sensor uses a much smaller portion of the full frame lens glass to make its image (hence: crop factor).

The Speedbooster focal length crop equation is this: (a x 1.6) x (0.71) = (b)
a = Full-frame focal length
b = Lens focal length with Speedbooster

For the purpose of this explanation, I’ve used the standard accepted Super 35mm crop ratio of 1.6x based on the standard full-frame sensor size. The crop factor for different cameras will vary slightly since sensors from different models and brands of cameras can be slightly different sizes.