Understanding the Impact of a Speed Booster on Aperture and Focal Length

by | Sep 6, 2018 | Gear and Reviews

In this blog, I’m delving into the intricacies of how a speed booster affects aperture and focal length, specifically focusing on my recent experience with the Metabones Speed Booster for my GH5.

What is a Speed Booster?

A speed booster is an adapter that enables you to mount different lenses onto your camera. It essentially ‘speeds up’ a lens by increasing the amount of light it lets in and also affects the lens’s effective focal length.

The Metabones Speed Booster

I recently acquired the Metabones EF-MFT Mount T Speed Booster ULTRA 0.71x for my GH5. This particular model is quite an investment, costing around £450 used (around £700 new). But it’s a game-changer for anyone looking to expand their lens options, especially for micro four-thirds cameras like the GH5.

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Benefits of the Speed Booster

  1. Focal Length Adjustment: It makes a lens wider than it would be normally.
  2. Increased Light Sensitivity: Allows more light into the sensor, effectively ‘speeding up’ the lens.
  3. Mount Versatility: Enables mounting of EF mount lenses onto a micro four-thirds camera body.

Real-World Implications

Focal Length Calculation

To understand how a speed booster affects focal length, let’s take the Canon 50mm F1.4 lens as an example:

  1. Multiply the lens’s focal length by the magnification factor of the speed booster (50mm x 0.71 = 35.5mm).
  2. Then, multiply this result by the crop factor of your camera’s sensor (35.5mm x 2 for micro four-thirds = 71mm).

Aperture Calculation

Similarly, for aperture:

  1. Multiply the lens’s aperture by the magnification factor (F1.4 x 0.71 = F0.99, rounded to F1).
  2. Adjust for the sensor’s crop factor (F1 x 2 = F1.9 as a full-frame equivalent).


The Metabones Speed Booster is a powerful tool for filmmakers and photographers using micro four-thirds cameras like the GH5. It not only expands your lens choices but also enhances low-light performance and depth of field control. The calculations for focal length and aperture may seem complex, but they’re essential for understanding the true capabilities of your lenses with a speed booster.

Whether you’re looking to utilize old lenses or maximize your camera’s potential, a speed booster like the Metabones can be an invaluable addition to your gear. If you have any questions or experiences with speed boosters, feel free to share!

Happy shooting!

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