Metabones Speed Booster

Metabones is known mainly for expensive but high-quality adapters for mounting legacy lenses on a variety of mirrorless cameras. The Metabones lens adapters are more carefully made than the much cheaper, run-of-the-mill adapters for the same purpose available from dozens of eBay sources. The most visible difference is that cheap adapters have a black-anodized Micro 4/3 rear bayonet machined as part of the aluminium body. This has a number of drawbacks and durability concerns. Metabones adapters, instead, have chrome-plated machined brass bayonets on both the camera and lens sides. These bayonets are much more durable, and similar in construction to the rear bayonet of good quality lenses.

What is a speed booster?

A couple of years ago, Metabones enlisted Brian Caldwell, a lens designer known for designing extremely high-quality, specialty lenses like the Coastalopt 60 mm f/4 Apo Macro, to design a high-quality speed booster (or focal length divider, or focal length reducer) that mounts in the same position of a lens adapter (i.e., between a legacy lens and a mirrorless camera body), and reduces the focal length of the lenses while increasing the lens speed. It may sound like magic, but it is no more magic than old-fashioned focal length multipliers (also known as teleconverters).

Look and feel

Metabones Speed Booster(left) and Metabones adapter (right) with Olympus OM lens mount.
Metabones Speed Booster with Olympus OM 50 mm f/1.8 on Olympus E-M1 (tripod shoe of Speed Booster has been removed).
Metabones Speed Booster with Olympus OM 500 mm f/8 on Olympus E-M1 (tripod shoe of Speed Booster has been removed).

Physically, the Metabones Speed Booster looks similar to a Metabones lens adapter, but is shorter than the latter and has a few internal optical elements. Like most Metabones adapters, the Speed Booster has a solid tripod mount shoe with a built-in Arca-compatible plate. This shoe may interfere with the use of Arca-compatible plates mounted underneath the camera body. On my Olympus E-M5 and E-M1, it is impossible to use a camera plate when the Speed Booster for Olympus OM lenses is mounted on the camera, and vice versa. There is no collision, instead, between camera plates and tripod shoe of the Metabones Olympus OM lens adapter, which sits a few mm farther away from the camera.

The shoe of the Speed Booster can be removed by unscrewing two bolts. This leaves the camera body to support the lens weight, which is a less desirable solution but still possible if the lens is not very heavy. It should be possible to redesign the tripod shoe of the Speed Booster to avoid this problem by making the foot bend obliquely forward. It may be possible to file away the rear part of the plate of the tripod shoe if you are inclined to do so, but this reduces the length of the built-in Arca plate to a bare minimum and requires a larger plate to be attached at the bottom of the tripod foot with a standard 1/4-20 bolt.

Two rubber grommets to plug the screw holes and a hex key (of a size different than the two hex keys used for the 1/4-20 mounting bolt of normal Arca plates and their small safety bolts) are provided with the Speed Booster. So now I may have to carry three different sizes of hex keys in my camera bag. The grommets are a nice touch, but only a half-baked effort: removing the tripod shoe leaves four holes (two screw holes and two guide holes for cylindrical brass aligning pegs), but there are only two grommets in the box. All four holes are closed at their bottom by a brass sleeve containing the optical elements, so no light or dust can enter through them, even if left open.

Although the tripod shoe of Metabones lens adapters is also removable, grommets and key are not included with them. The grommets would be more useful than with a Speed Booster, because removing the tripod shoe in some adapters leaves screw holes open all the way to the interior of the adapter, which may result in dirt and stray light passing through.

Unlike Metabones lens adapters, which are sold without any accessories, the Speed Booster comes with Metabones-branded front and rear plastic caps. Both caps are very difficult to remove (especially the rear one). These caps are just as fiddly when used on other equipment, and you will need to replace them with third-party or original caps of better quality. My Speed Booster has an Olympus OM mount at its front, and an original Olympus OM body cap sits just fine there. Fake Olympus-branded Micro 4/3 caps from China sit very well at its rear, while the "Micro" branded third-party caps prevalent on eBay sit just as poorly on Metabones items as they do on everything else.

The bayonet mounts of Metabones adapters and Speed Booster couple smoothly and tightly with other equipment. These bayonets, however, are not exact copies of the original bayonets used by Micro 4/3 cameras and by the makers of legacy lenses. The angles and thickness of the bayonet lugs are often slightly different, and Metabones bayonets often have sharp corners where rounded corners are present in the original equipment. In some cases, mounting and dismounting Metabones-made bayonets from other equipment requires more care and time than ordinarily needed by original equipment. In about half a dozen Metabones adapters that I own, I found this to be the case with Olympus OM, Nikon F, Micro 4/3 and other bayonets made by Metabones. M42, M39 and C threaded mounts made by Metabones seem instead to be problem-free. In conclusion, the lens and camera mounts of Metabones Speed Booster and lens adapters are better than those ordinarily seen on cheap no-name items from China, but not as good as those found on the cameras and lenses these Metabones items are used with.

The Speed Booster is currently available in a broad variety of lens and camera attachments. Some of them provide autofocus (AF) couplings, but AF with legacy lenses that support it is said to be very slow on current mirrorless cameras. Metabones Micro 4/3 adapters and Speed Boosters do not provide AF. Versions of the Metabones Speed Booster are available for most current mirrorless systems. Two different versions especially designed for Blackmagic videocameras with Micro 4/3 lens mounts are also available. They cannot be used on ordinary Micro 4/3 cameras.

Pricewise, the Metabones Speed Booster costs four times a corresponding Metabones adapter without optics, or up to 20 times a no-brand Micro 4/3 adapter from China. Therefore, this is a purchase that should be carefully considered.

How does it work?

Since digital compact cameras became popular in the early 2000s, optical add-on accessories capable of changing the focal length of a camera lens have been commonly offered. Besides close-up attachments, these accessories are mainly of two types: a "telephoto add-on", which increases the focal length of the lens, and a "wideangle add-on", which has the opposite effect. Both accessories mount in front of the camera lens. These add-on accessories are, optically speaking, a Galileian telescope and an inverted Galileian telescope, respectively. The image quality of these accessories is quite limited because of their simple optical construction and wide diameter. In particular, geometric distortion is evident. The general effect is similar to that of focal length multipliers and dividers, but the latter devices mount at the rear of an interchangeable lens and often provide a better image quality.

The concept of a focal length divider to mount at the rear of a camera lens has been discussed on and off for at least thirty years, but perhaps never commercially implemented until now. Comparable optics are commonly in use on astronomical telescopes. Most photographers who have been around for a while have owned a focal length multiplier at one time or another. This is a device that mounts between a lens and camera body and increases the focal length of the lens by a fixed factor. As a consequence, only the center of the image circle is used, the effective focal length of the lens becomes 1.6 or 2 times higher, and the lens loses between one and two stops of speed. An unavoidable result is that the focal length multiplier, in addition to magnifying the central portion of the image circle, also magnifies its optical aberrations. A cheap focal length multiplier often adds its own aberrations as well, and additionally lowers the image contrast. Therefore, focal length multipliers are frowned upon by photographers who can afford "real" telephoto lenses, and are typically regarded as "poor-man's" accessories.

Although the general criticism of focal length multipliers is justified, especially when they are used on cheap lenses, it must be acknowledged that focal length multipliers of good quality (e.g., the Nikon ones, although Kenko ones are also surprisingly good when matched to certain lenses) can give excellent results when combined with professional-quality telephoto lenses. If the lens is fast and extremely good to start with (typically, a 300 mm f/2.8 or another telephoto lens with a diameter of the front lens element of at least 100 mm, and a price tag to match), it may tolerate the addition of even a 2x focal length multiplier with only a modest loss of image quality. 1.4x or 1.6x focal length multipliers are less demanding, and can often be used with good f/3.5-f/4 telephoto lenses starting from 200-300 mm. Certain lenses, like the Sigma 200-500 mm f/2.8, are equipped with a specially matched focal length multiplier that introduces virtually no additional aberrations (but still magnify the aberrations of the lens, which is unavoidable). For a short time, 3x focal length multipliers were available in the past, but provided an unsatisfactory image quality even when matched to the best available lenses.

In the film SLR world, there was no good reason to design a focal length reducer. Such a device unavoidably shrinks the diameter of the image circle, which means a lens designed for a 24 x 36 mm frame no longer covers entirely this format. In the best case, it would also result in a better image resolution in the center of the frame, measured in line-pairs/mm. The total number of resolved line pairs across the image circle, however, at best would remain the same. However, the much smaller image circle made this type of accessory essentially useless to most photographers.

A focal length reducer, on the other hand, could make sense for using a full-frame lens on a mirrorless camera with a smaller sensor. The advantage provided by the Speed Booster on Micro 4/3 is a reduction in focal length (by a factor of 0.71) and an increase of one stop in lens speed. The smaller image circle fully covers the smaller sensor. A more detailed discussion of the Speed Booster, together with test results, can be read at LensRentals.

Lenses designed for the APS-C format are broadly available, and are also cheaper to produce than equivalent full-frame ones. Therefore, the incentive to produce focal length dividers for the APS-C format are not as great as for mirrorless cameras. Possibly, in the past, patents were an additional obstacle to the commercial development of focal length dividers. I have no direct information, but it was sometimes stated that a certain company owned a patent for a focal length divider and sat on it without doing anything, thereby blocking other companies as well.

With the introduction of mirrorless cameras, there was suddenly a use for hundreds of models of legacy lenses, at the same time as native lenses for mirrorless cameras were not too varied, and their quality not too exciting. Things are better now in the Micro 4/3 world, but there is still a use for some of the legacy lenses available on the second-hand market, especially if their focal length can be reduced and their speed increased by a stop. The idea of a focal length divider that sits between a legacy lens and a mirrorless body is, optically, similar to the idea of a focal length reducer designed to sit between a camera and a lens originally designed to sit directly on the same camera. However, the purpose of the original focal length divider patent and of the Speed Booster is different (the patented idea was likely to use the same lens on the same camera, much like a focal length multiplier, while the Speed Booster is meant to use the same lens on a different camera). This might be enough to legally circumvent the original focal length divider patent, if one exists and has not expired.

Evaluating focal length reducers

Tests and reviews available on the Internet show that the Metabones Speed Booster largely lives up to its promises in terms of resolution, contrast and general image quality. When it doesn't, it is in borderline cases with peculiar legacy lenses. Cheaper copycats are now available, and their tests show that, basically, you get what you pay for. Image quality of these focal length dividers largely correlates with price. Just like there are good focal length multipliers and bad ones, as well as good lenses and bad ones, there are now good focal length dividers and bad ones. The Metabones Speed Booster is so far the most expensive and the best of the lot.

I feel no inclination to repeat the reviews and tests already available on the Internet. Instead, I intend to discuss why one would want to purchase and use a Metabones Speed Booster, and give a few practical examples. In other words, what are the economical justifications in this purchase, and the practical uses of this device?

I am not going to discuss the cheaper alternatives to the Speed Booster. The already published results indicate that the image quality provided by lower-quality focal length dividers is not acceptable to me, so I will not waste money on them. I am also discussing only the Micro 4/3 mirrorless system. The other mirrorless systems have drastically smaller numbers and diversity of native lenses, and therefore, conclusions that are applicable to Micro 4/3 do not necessarily hold also for the other systems.

Why use a Speed Booster?

A focal length divider has two effects. One is providing a lens with a shorter focal length (in this specific case, by 0.71x). The other is providing a gain in lens speed (in this case, one stop). We need to consider the combination of these two effects. Finally, we need to consider whether it makes financial sense to purchase a Metabones Speed Booster and one or more legacy lenses, instead of equivalent native Micro 4/3 lenses. A single Speed Booster can be used on multiple legacy lenses, and this should be factored into the decision. However, I don't think it is realistic to consider a scenario where a photographer frequently uses a Speed Booster with more than two, or possibly three, legacy lenses.

Speed Booster on wideangle or fisheye

The Micro 4/3 format suffers no shortage of wideangle and fisheye lenses. The current extreme wideangles are not very fast, but Olympus will start selling next year a high-quality 7-14 mm f/2.8, and already sells an optically excellent, small and relatively cheap Olympus 12 mm f/2 with AF and MF. The price of a Speed Booster alone reaches half the price of this 12 mm f/2. On top of this, you need to add a legacy full-frame 18 mm f/2.8 to get an equivalent adapted lens. A second-hand AF Nikkor 18 mm f/2.8 ED weighs three times more and costs at least as much as a new Olympus 12 mm f/2, without providing AF and automatic aperture. A second-hand Tokina 17mm f/3.5 SL is a cheaper choice, but so is the Olympus 12 mm f/2, and the latter does not need a Speed Booster.

Samyang markets its version of a Micro 4/3 12 mm f/2 without AF, which costs slightly less than a Speed Booster and provides a good optical quality. Its actual focal length is higher than 12 mm (more like 13-14-mm), and it is larger and heavier than the Olympus 12 mm. If you have money to burn, Zeiss makes a 12 mm f/2.8 poorer than the other two 12 mm lenses in vignetting and speed, but about as good in all other respects except price. Panasonic makes a good 12-35 mm f/2.8 zoom and Olympus a likewise good 12-40 mm f/2.8 that might be alternatives if you prefer a zoom in order to carry around fewer lenses.

If you need a shorter focal length, cannot wait for the Olympus 7-14 mm and cannot use a fisheye de-fished in post-processing, there are already a Panasonic Micro 4/3 7-14 mm f/4, an Olympus 9-18 mm f/4-5.6 and an Olympus Four-Thirds 7-14 mm f/4.

The Samyang 7.5 mm Micro 4/3 fisheye is optically very good, cheaper than a Speed Booster, and much smaller than any legacy fisheye. This makes it unattractive to use a legacy fisheye on Micro 4/3, with or without Speed Booster. There is simply nothing to win in practice by using an adapted f/2.8 (effective f/2) fisheye instead of a native f/3.5.

In conclusion, I can see no economically justifiable reason to buy a Metabones Speed booster and a legacy wideangle or fisheye, as opposed to a native Micro 4/3 wideangle. Regardless of price, it is hard to think of a good reason for choosing the second alternative, except possibly if you already have suitable legacy lenses and only need to buy a Speed Booster. In this case, perhaps you can save some money, but you are forced to carry around a lot more weight than with native Micro 4/3 lenses.

Speed Booster on medium focal lengths

There is no shortage of medium focal lengths in the Micro 4/3 format, including the fastest currently produced commercial camera lenses (Ibelux 40 mm f/0.85). The only niche where the Speed Booster is cost-competitive is when used with a f/1.8 or f/1.4 "normal" (50-55 mm) SLR or DSLR lens. These are the cheapest lenses on the second-hand market, and often among the best in terms of image quality. In this case, the result is a quite good 35-39 mm f/1.0 or f/1.2 at a price significantly lower than an equivalent native Micro 4/3 lens.

Most super-fast Micro 4/3 lenses have manual focus and manual aperture, so the use of a legacy lens with Speed Booster does not require the photographer to give up any functionality available with these native lenses. It is of course possible to use the Speed Booster with an f/1.2 legacy lens and in theory achieve f/0.9, but f/1.2 lenses are quite expensive on the second-hand market. Very fast legacy lenses are also more likely than others to produce corner vignetting, fringing and chromatic aberration when used fully open on the Speed Booster. This is especially likely if these lenses have a very large diameter of the rear element, and if this large rear element is close to the lens mount. In these cases, the Speed Booster can also give less than one full stop of speed increase.

In summary, purchasing a Speed Booster and a legacy f/1.8 or f/1.4 lens of "normal" focal length (50-55 mm) is a cost-effective way to obtain an f/1.2 portrait lens for a Micro 4/3 camera. A legacy 50-55 mm f/1.2 on Speed Booster, instead, is a questionable proposition, and not likely to provide an increase in image quality and lens speed commensurate to the much higher price.

Speed Booster on telephoto lens

Native fast super-telephoto lenses are sorely lacking in the Micro 4/3 format. Except for an expensive Olympus 4/3 300 mm f/2.8 and a couple of likewise expensive 4/3 zooms, which at present autofocus satisfactorily with only one Micro 4/3 camera model (Olympus E-M1), there is no Micro 4/3 AF long telephoto lens of professional level. Olympus is developing a professional-quality Micro 4/3 300 mm f/4, which will be very useful but by no means enough to satisfy professional photographers. Panasonic is developing a presumably good 150 mm f/2.8, and Olympus will soon start distributing a high-quality 40-150 mm f/2.8.

What happens if we couple a legacy, long but not-too-heavy telephoto lens of good optical quality and not excessively high price, like the Olympus Zuiko Reflex 500 mm f/8, to a Speed Booster? In this specific case, we obtain a 350 mm f/5.6, which is short enough and fast enough to be used hand-held in good light and can make good use of the image stabilization built into Olympus and some Panasonic bodies. The already good image resolution improves even further, with respect to a lens adapted devoid of optics. Corner vignetting is slightly detectable on full-frame but not a problem on Micro 4/3, since the Speed Booster does not use the whole image circle. Other characteristics of the lens, including donut bokeh and difficult manual focusing, remain. However, we get a lens that is optically far better than the existing Panasonic 100-300 and Olympus 75-300 Micro 4/3 consumer zooms at 300 mm, which are currently the only choices besides heavy and expensive Olympus 4/3 lenses.

A relatively commonplace 300 mm f/2.8 plus Speed Booster turns into an even better 215 mm f/2, hardly usable hand-held but certainly interesting for tripod work. This lens weighs around 3 kg, fits in a medium-large camera backpack and is slightly better than a 400 mm f/2 on full frame in terms of exposure time and field of view. Such a full frame lens simply does not exist, and if it did, would require two sherpas to carry.

In conclusion, effective focal lengths and speeds up to 150 mm f/2.8 will soon be covered more advantageously by native lenses. Therefore, legacy telephoto lenses up to 200 mm f/4 are not optically interesting for use with a Speed Booster, although still cheaper than a native lens. On the other hand, legacy long telephoto lenses from 300 mm and above, especially if fast and sharp to start with, remain interesting for use with a Speed Booster in the foreseeable future.

Metabones Speed Booster versus Metabones adapter

Starting with a Metabones Speed Booster and adding a Metabones Micro 4/3 adapter (without internal optics) for an extra 100 €, you can have two focal lengths and lens speeds for each legacy lens. In the above example of legacy 50 mm f/1.8 and 500 mm f/8, this combination gives the following effective Micro 4/3 focal lengths, speeds and fields of view:

50 mm f/1.8 + Speed Booster (=35 mm f/1.2), whole image, reduced.
50 mm f/1.8, whole image, reduced.
50 mm f/1.8 + Speed Booster (=35 mm f/1.2), 1:1 center crop.
50 mm f/1.8, 1:1 center crop.
500 mm f/8 + Speed Booster (=350 mm f/5.6), whole image, reduced.
500 mm f/8, whole image, reduced.

The focal lengths and speeds with Speed Booster versus adapter are sufficiently different from each other to be interesting, considering their limited additional cost and weight.

Depth of field, diffraction and all the rest

The above discussion does not explicitly take into account the differences in DOF in connection with different sensor sizes. Therefore, while it is legitimate to compare a 200 mm f/2 on Micro 4/3 with a 400 mm f/2 on full frame for what concerns exposure time and field of view, the two lenses give a different DOF at the same aperture on the different formats (the 200 mm f/2 on Micro 4/3 has a DOF similar to a 400 mm f/4 on full frame). This is because the circle of confusion on a Micro 4/3 sensor has a much smaller absolute diameter. The Speed Booster only increases speed by one stop, while you would need an increase of two stops to get the same DOF of full frame on Micro 4/3. In other words, the Speed Booster is a step in the "right" direction toward achieving the same DOF of the full-frame format, but you would need a second step to really get there.

A lens that achieves a 33 mm focal length and an f/1 speed with a Speed Booster is in no way different from a lens designed to provide natively a 33 mm focal length and f/1 speed. DOF, speed, loss of resolution to diffraction etc. are the same, no matter how optically obtained. Therefore, the recommendation of not exceeding f/8 to f/11 with Micro 4/3 lenses in order not to loose image detail to diffraction still holds for lenses coupled to a Speed Booster. In the latter case, however, the aperture not to exceed, as indicated on the aperture ring of the lens, is f/11 to f/16 (which is converted into effective f/8-f/11 by the Speed Booster).

Choice of lens attachment

The Metabones Speed Booster is available with a variety of front lens attachments. If you already have a few legacy lenses, then the choice of attachment type for the Speed Booster is easy: just get the one that fits your legacy lenses. If you are planning to purchase legacy lenses for use with a Speed Booster, then it pays off to choose carefully the type of lens attachment. It makes sense to use legacy lenses of good optical quality and with modern multi-coated surfaces for maximum contrast. At the same time, it is useless to choose modern autofocus lenses, because they must be operated in manual focus and aperture anyway. This includes, for instance, Nikon AF-S or AF-I lenses, which are quite expensive on the second-hand market because they can still be used on modern Nikon DSLRs.

My personal choice for use with the Speed Booster is Olympus Zuiko lenses for the OM system, which are good and relatively modern optical designs, solidly built and typically compact, and relatively cheap on the second-hand market. A couple of other lens systems may also be suitable, but you should probably pass up on most of the third-party legacy lenses and all the lenses with M42 mounts, unless you already have a lot of them and want to test them.

Is a 0.5x 2-stops Speed Booster possible?

In theory, it should be possible to build a 0.5x Speed Booster for using full frame (not APS-C) lenses on Micro 4/3 sensors. In practice, its design may be too difficult or expensive to be practical, or the results in the corners may be significantly poorer than with the current 0.71x Speed Booster. I don't think a 0.5x Speed Booster for Micro 4/3 would be able to turn an f/1.4 lens into an effective f/0.7. The limit seems to be around f/0.9. It might be easier to design a true 0.5x Speed Booster that covers a smaller sensor than Micro 4/3, for example for the Nikon mirrorless format. But we can still wait and hope. With a kit consisting of a lens adapter, a 0.71x Speed Booster and a 0.5x Speed Booster, we would get three significantly different focal lengths and lens speeds from each legacy lens.

Updated There is now a 0.65x Speed Booster for Micro 4/3, with a speed limit of f/0.8. So Metabones is still at work pushing the limits.


With respect to purchasing a native Micro 4/3 lens, it is not desirable to purchase a combination of Speed Booster and legacy wideangle lens or fisheye. The combination of Speed Booster and legacy 50-55 mm f/1.4 or f/1.8 lens is instead unbeatable price-wise. The combination of Speed Booster and good legacy telephoto lens of focal length 300 mm or higher may also be interesting in terms of price.