Metabones Nikon F to Fuji X adapters – part II

The Speed Booster: it’s a kind of magic!

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When Metabones originally announced the concept of its Speed Booster, in January 2013, the initial reactions ranged from rather skeptical to overly enthusiastic. Indeed, the claims for this product are all that is need to start us dreaming: a focal length multiplier of 0.71x (nearly eliminating the field-of-view reduction by APS-C crop sensors), a maximum aperture increase by 1 stop, and the promise of increased sharpness and reduced optical aberrations. Sounds too good to be true, isn’t it?

Yet, all I can say after five weeks of exploration and testing is that so far the Speed Booster has lived up to all my expectations – at least from the optical side. More about that later, when we look at detailed test results.

So what is the Speed Booster?

The Speed Booster is a focal length reducer, something you can think of as the reverse of the well-known 1.4x teleconverter commonly used with long lenses. Such a converter enlarges the image projected by the lens so that it covers a larger area than the film or sensor can capture: the result is a ‘cropped’ image and the reduced field-of-view (FOV) makes the focal length of the lens seem longer. At the same time the incoming light is spread over a larger area, so the amount of light captured by the film or sensor is lower: the effective aperture is reduced. Typically, a 1.4x converter will come with a 1 stop loss of light.

The Speed Booster too inserts between a lens and a camera body, but its optical elements create the opposite effect: the light from the lens is compressed onto a smaller imaging circle, better adapted to the crop sensor. As a result, the field-of-view is enlarged (countering the effect of the sensor crop factor) and the amount of light illuminating the sensor is increased (resulting in a larger effective aperture). Obviously, this can only work combining a full-frame lens (having a large enough imaging circle to start) with a crop sensor not exceeding the size of the reduced imaging circle.

 

Here we see a full-frame lens mounted on a CSC with an APS-C sensor, via a ‘straight’ adapter. The required focal flange distance is restored. The lens still projects a full-frame image circle, of which the sensor captures only a cropped area.

 

The Speed Booster optics reduces the image circle to more closely cover the APS-C sensor, resulting in a field-of-view that matches the full-frame behavior of the lens. All available light now hits the sensor.

 

Here’s another way of looking at what’s happening:

At left, the unmodified imaging circle of a full-frame lens: it adequately covers the full-frame sensor area (green rectangle). The Fuji X-cameras with their APS-C sensors capture only a reduced crop area (red rectangle). At right, the situation with a Speed Booster: the original imaging circle is reduced, and no longer covers the full-frame area. Instead, our crop sensor now captures the scene with a field-of-view close to the full-frame rectangle at left. In addition, the image gets brighter as all available light is concentrated on the smaller imaging circle.

The next two images (shot in my improvised table-top product studio) illustrate the difference in field-of-view. On top in the first image, the scene captured with a ‘regular’ lens adapter; at the bottom the same scene shot from the same position, this time using a Speed Booster. Also note the change in exposure time: the 1-stop gain in effective aperture is real!

The second image shows exactly the same top half part, but when using the Speed Booster the camera was moved in closer to obtain more or less the same size for the main subject. The change in vantage point explains the difference in perspective, causing more of the background to be visible.

(Note that the darker corners are partly caused by the lighting on my makeshift test setup)

Optical design and specifications

The concept of a focal length reducer is not new: the approach is for example commonly used in telescope eyepieces. Some specialized lenses have focal reducers integrated into their optical design. A practical general purpose add-on focal reducer for photographic applications was never realized: one of the bottlenecks was the difficulty to create room for additional lens elements while maintaining the required focal flange distance. It is the difference in register distances between a full-frame (D)SLR lens and a CSC body that created the opportunity to build a Speed Booster.

The Speed Booster is designed by Brian Caldwell, a highly respected optical engineer, and Wilfried Bittner, a specialist in opto-mechanics. Both veterans have worked together on numerous product development projects. They jointly filed a US Patent Application (ref. 2013/0064532A1, should you want to read it) and Caldwell’s name proudly figures on the adapter’s barrel, next to a real serial number (as any precision optical device deserves).

The two inventors have published a white paper on the Metabones website with details and background on the design. You better have a minimal understanding of optical principles and basic lens design to appreciate it, but if you do the comprehensive document is very convincing. And if you rather have a second opinion from another independent qualified expert, I can recommend the initial test report published by LensRentals’ Roger Cicala.

Metabones currently offers two Speed Booster designs: one for APS-C sensors with either Sony NEX or Fuji X mount, and a slightly different one for Micro-Four Thirds.

These are the specifications for the Nikon-F-to-Fuji X version:

• 4 lens elements in 4 groups
• Magnification: 0.71x
• Maximum ‘input’ aperture: f/1.26
• Maximum ‘output’ aperture: f/0.90
• Focal flange distance reduction: 4.16mm

At this moment, there are also Fuji X-mount compatible Speed Booster variants accepting Alpa, Contarex, Contax Yashica and Leica R lenses.

The presence of additional optical elements causes a slight reduction of the flange focal distance for the lens+adapter combination. The Speed Booster therefore is some 4mm shorter than both the F-adapter and G-adapter.

Lens compatibility

The Nikon-F-to-Fuji X Speed Booster will accept almost any full-frame (FX) Nikon or F-mount lens, including pre-AI, AI, AI-S, AI(-S) modified, AF, AF-D and AF-S models. G-type lenses are supported as this adapter comes with its own unique aperture control ring.

The ‘native’ imaging circle for all these lenses covers the 43.3mm diagonal of the full 35mm frame. After the 0.71x size reduction, the resulting 30.7mm circle more than covers the ca. 28mm diagonal of the APS-C sensor.

DX lenses cannot be used, as in general their reduced imaging circle will fall short of the APS-C sensor requirements and cause heavy vignetting.

The Metabones website lists two older Nikon lenses as mechanically incompatible. In the case of the AI-S 20mm f/2.8, the reason is a plastic shroud surrounding the rear element that extends some distance and would hit the front element of the Speed Booster (in this specific case, it’s possible to remove the interfering part and keep a working lens).

Product claims

Let’s summarize the list of features and benefits that the Speed Booster should (and actually does) deliver:

• The effective focal length of the lens is reduced 0.71x. That means that the field-of-view is now 1.4x wider. Together with the 1.5x crop factor from the APS-C center, the final field-of-view becomes 1.5x0.71=1.065 times what you get with the same lens on a full frame camera: almost the same in practice;
• The effective maximum aperture is extended by one full stop: you can now use faster shutter speeds, lower ISO settings (and less noise), shoot with shallower depth-of-field or in darker environments;
• All aberrations of the mounted lens are reduced in the image compression process; combined with the small aberrations introduced by the adapter optics the final outcome should be an increased optical performance (as indicated by MTF curves) in the center of the image, and a no worse than original performance near the outer edges.
• And what happens to the depth-of-field (DOF)? The short and practical answer to this complex question is that a lens used with a Speed Booster on an APS-C crop camera essentially gives the same depth-of-field effect as if used on a full-frame camera body. That means a shallower DOF than obtained without the Speed Booster.

It takes an excellent test setup and a lot of expertise to verify these claims by the numbers, something that is well beyond the means and skills of a ‘normal’ photographer. All measurements executed and documented by Caldwell and Cicala indicate that the claims are not idle at all.

So what do we get out of this?

A number of my prime lenses take on a totally new personality when used with the Speed Booster:

• The AF 50mm f/1.8 becomes a compact 36mm f/1.3;
• My AF-D 50mm f/1.4 turns into a 36mm f/1.0!
• An AF-D 85mm f/1.8 becomes a sharp 60mm f/1.3 lens, great for portraits;
• My old Tokina 17mm f/3.5 now works as a 12mm f/2.5;
• My even older Vivitar Series 1 135mm f/2.3 now serves as a 96mm f/1.6;
• The AF-D 180mm f/2.8 lives on as a 128mm f/2.0;

and so on…

NEXT: controlling the aperture with Speed Booster

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