If you use an interchangeable-lens mirrorless digital camera, you know that one of the pleasures of mirrorless shooting is the ability to use lens-mount adapters to photograph with any number of vintage lenses (manual focus or autofocus), as well as some modern-day lenses from your favorite optical manufacturers. When using a crop sensor camera, you might have stumbled across adapters with the monikers “Accelerator,” “Speed Booster,” or “Turbo” in the name. What are these specialized adapters, and what do they bring to the table when you use them to adapt a lens to your crop sensor camera?
A lens adapter allows you to mount a lens with a certain style bayonet on a camera with a different style lens mount. For example, you can get an adapter to mount a Nikon lens on a Sony mirrorless camera. You’ll always be adapting lenses made for larger or larger-mount cameras to smaller crop sensor cameras like APS-C and Micro Four Thirds or, even medium format lenses to full-frame (or smaller) cameras.
Standard adapters are devoid of optical elements and are basically metal and plastic tubes that serve simply to connect a lens and camera with different lens mounts. We dive deeper into this subject in this Explora article.
When you are using a crop sensor camera, one of the things that photographers bemoan is that, due to a smaller sensor, both their adapted (and native) lenses have fields of view that are smaller than those on full-frame cameras. A "nifty-50"mm lens on an APS-C (1.5x crop factor) camera will have a field of view more akin to a 75mm lens on a full-frame camera. The 35mm focal length lens on an APS-C camera is needed to get the traditional 50mm “normal” field of view of the classic 50mm lens.
While this change in focal length is not always a bad thing, if you have a favorite lens from your past, such as a trusty 50mm prime and you want to use it on your new crop-sensor camera while maintaining the same familiar field of view, you can do that with an optical adapter that virtually eliminates the smaller lens’s crop factor.
Some adapters include optical lens elements inside of them. Depending on the brand, these adapters are called Accelerators, Speed Boosters, or Turbos. These specialized adapters not only change the effective focal length of a given lens—Wait? What? How?—but they also increase your lens’s maximum aperture—Double wait? Double what? Double how?
For the purposes of this article, I will refer to these optical adapters as “Speed Boosters.” Speed Booster is a brand trademark from Metabones—an optics company that made some of the first such adapters. The Speed Booster name has become relatively synonymous with this type of optical adapter.
Effective Focal Length Reducer
The Speed Booster is, for all intents and purposes, an effective focal length reducer. Many of you are familiar with the lens teleconverter that effectively increases a lens’s focal length; the Speed Booster does the opposite.
I dive deep into focal length in this article and focal length is defined as:
The distance from the lens’s optical center (or nodal point) to the image plane in the camera (often mapped by a "Φ" on the top plate of a camera body) when the lens is focused at infinity
I have often said that a 50mm lens is a 50mm lens regardless of what camera it is placed in front of—or if it is just sitting on the shelf. That is still an accurate statement, but, when you put a Speed Booster on the back of a lens, its optics change the field of view of the lens’s image circle.
The teleconverter and Speed Booster work just as your elementary school science class magnifying lenses did.
Common teleconverters have 1.4x or 2x magnification—effectively doubling a lens’s focal length. With a 2x teleconverter, a 200mm lens captures the same field of view as a 400mm lens would (200mm x 2 = 400mm).
Conversely, a 0.7x Speed Booster will increase the field of view of a given lens—a 50mm lens with a Speed Booster will have the larger field of view of a 35mm lens. (50mm x 0.7 = 35mm)
The effective focal length reducer part of the Speed Booster math is easy. Right?
Effective Aperture Increaser
This is where the Speed Booster gets a bit magical. The selling points on the B&H website on Speed Boosters claim “1-stop increase in lens aperture.” If you are photographing with a lens with a maximum aperture of f/2, you know that you physically cannot turn the aperture ring (or electronic controls) farther than that f/2 setting, so what gives? How does a “magnifying glass” allow us to have a wider aperture?
I dive into the basics of aperture in this article and, if you look at the formula for f/stop (a numeric ratio used to show the size of a lens aperture) you’ll find:
f/stop = focal length / diameter of effective aperture (entrance pupil) of the lens
Getting into how the “entrance pupil” is measured would lead us down a long and wordy mathematical rabbit hole of optical engineering, so if you just know that the entrance pupil is the diameter of the on-axis light passing through the lens from the front and not necessarily the size of the first element of the lens, we can push the “I believe” button and press on. Before you think I am leaving you high and dry here, the measurement of the entrance pupil is pretty much completely irrelevant to the process of creating art with a camera.
If we stick with the math, the effective aperture of the lens does not change (the entrance pupil is what it is), but we have effectively changed the focal length and that, in turn, changes the f/stop number.
A 50mm lens alone:
50mm true focal length / 25mm entrance pupil = f/2
The 50mm lens with a 0.7x Speed Booster:
35mm effective focal length / 25mm entrance pupil = f/1.4
I mentioned earlier how the Speed Booster was opposite of a teleconverter and the formula here, when applied to a teleconverter, shows the biggest negative of a teleconverter—reduced effective maximum aperture. Using the formula above, a 2x teleconverter will change a 200mm f/2 lens into a 400mm f/4 lens—two stops of effective aperture gone due to the change in effective focal length.
Effective Light Increaser
When you see that same “1-stop increase in aperture” selling point, you might be tempted to believe that the Speed Booster will brighten your image. Guess what? It does… kind of! How in the world does that happen?
If you shine a light through optical glass, will it be brighter on the other side? Nope. That does not happen. A light source is a light source is a light source. We cannot make the sun or a light brighter—certainly not by putting glass between a camera and the light source—so, again, what is this brightness increase the Speed Booster gives us?
Think back, again, to the magnifying glass in grade school. The sun is not bright enough to burn a hole in a piece of paper… unless you concentrate the photons into a much smaller area. Right? Well, the Speed Booster is basically taking the light made to cover a larger area (a larger medium format or full-frame sensor) and concentrating it to cover a smaller area. Brighter? Technically, no. More concentrated? Yes.
Depth of Field Non-Factor
So, before you get all excited and think that the Speed Booster’s magical changing of your lens’s effective focal length and effective aperture will lead to a change in its depth-of-field performance, take a deep breath and know that your lens’s depth-of-field performance will be unchanged by the Speed Booster.
Again, I’ve taken a deep dive into the mathematics of DoF in this article, but you can skip those equations and know that depth of field is affected by focal length, aperture, circle of confusion, and focus distance. In the case of employing a Speed Booster, the changes to effective focal length and effective aperture mathematically cancel out any changes to the depth-of-field equation for a Speed Boosted lens.
Using the examples above, and your favorite depth-of-field calculator, you will find that a 50mm f/2 lens has the same depth of field for a given focus distance and sensor size as a 35mm f/1.4 lens.
The Potential Downsides
While the Speed Booster allows you to get your familiar full-frame field of view from your favorite adapted lenses, as well as giving you an extra stop of light, there are some potential downsides.
Just like with a teleconverter, or even a lens filter, the Speed Booster adds more optical elements between your subject and camera’s sensor. Light is bending more than before. This can affect image sharpness, as well as add optical aberrations. Having mentioned reality, in my experience, any degradation of performance from the Speed Boosters I have tried is very minimal—pixel peeper stuff—and “real-world” performance is unaffected. (See below for some testing results.)
Another negative, Speed Boosters are not featherweight. While they don’t add noticeable heft to your camera bag, even a standard, non-optical lens adapter is just one more thing to carry around and keep track of, and the Speed Booster-type adapters do have some heft.
1 Speed Booster + 1 Lens = 2 Lenses
Besides the tweaking of optical performance (some good, some bad), the Speed Booster has the added benefit of turning a single lens into two different optics.
Using the example, again, of the Nifty 50 on an APS-C camera, you can use the lens with a standard adapter and get the telephoto 75mm portrait-friendly focal length field of view. Swap the adapter for a Speed Booster, and your Nifty 50 is back to being the familiar 50mm you grew up with, but now on a smaller, crop sensor camera—the best of both worlds!
Real World and Test Target Results
How does the Speed Booster perform in the real world and when pointed at a test target?
First, I photographed the proverbial “brick wall” with the FUJIFILM XF 35mm f/1.4 R and the Nikon NIKKOR 50mm f/1.2 equipped with the Mitakon Zhongyi Lens Turbo Adapter V2 for Full-Frame Nikon F Mount Lens to FUJIFILM X Mount APS-C Camera.
Next, I photographed a test target in the studio with the same FUJIFILM 35mm and Nikon 50mm. This time I added a “control” to the experiment by photographing the target with the Nikon and a non-optical Novoflex Adapter for Nikon Mount to FUJIFILM X Mount Digital Cameras . Without a Speed Booster, the Nikon 50mm has a 35mm equivalent field of view of a 75mm lens on the FUJIFILM camera. To match the field of view of the FUJIFILM 35mm and Speed Boosted Nikon 50mm, I moved the camera farther from the test target.
Impressive Results from the Speed Booster
I felt that, of both optical lens adapters I tested, the Metabones Nikon F-Mount G Lens to FUJIFILM X-Mount Camera Speed Booster ULTRA and the Mitakon Zhongyi Turbo Adapter gave great results when attached to the Nikon 50mm f/1.2 lens on the FUJIFILM X-T3 camera. Center sharpness was virtually unaffected through the aperture range. Corner sharpness is where the Speed Boosters suffered a bit because they also introduced a bit more color aberration. Overall, I would be comfortable shooting in real-world conditions with a Speed Booster when I want to get a “true” field of view from an adapted lens. Plus, the extra stop of brightness is a nice bonus!
Do you have questions about Speed Boosters or lens adapters? Let us know in the Comments section, below!