B4 Lens Adapter
A practical approach to bring B4 broadcast lenses back into a more correct and usable cinema workflow.
What this adapter is for
B4 lenses are fascinating because of their huge zoom range and the practical flexibility they still offer today. They can cover situations that would otherwise require multiple cinema lenses and much slower lens changes.
That said, a corrective optical adapter is not an optional accessory if the goal is to see the lens behaving correctly. These lenses were designed for 3-sensor broadcast cameras that used a prism block to split light into red, green and blue paths, with the related image planes positioned differently from a modern single-sensor camera. In simple terms, the B4 standard was created around a different optical system, not around today's digital cinema sensors.
For this reason, what some TikTokers or YouTubers describe as "character" in reality are aberrations or glowing that are the consequence of using the lens outside the system it was designed for. The real character of a B4 lens becomes much more evident when the lens is used with a proper adapter that includes corrective optics.
Why we use the Blackmagic b4 mount
To keep costs lower, we use the Blackmagic B4 mount made for Ursa cameras. These mounts already include the corrective glass and they are built very well in steel and aluminium.
This gives us a solid and reliable base part instead of reinventing a component that already exists and works properly. It also helps us focus our effort where it matters more: adaptation, fit, mechanical integration and custom solutions built around real user needs.
What we achieved, a bit of mathematics
Originally, B4 lenses were designed to cover 8.8 x 6.6 mm sensors in a 4:3 aspect ratio, with an 11 mm diagonal. Over time they were also used on 16:9 sensors, meaning 9.6 x 5.4 mm while keeping the same diagonal.
The corrective optics of the B4 mount have a 1.1x magnification, so coverage becomes 10.56 x 5.94 mm, which corresponds to Full HD on the Ursa Mini Pro sensor. But we want to cover larger sensors, at least Micro Four Thirds. For this reason we recommend lenses that already include the integrated 2x extender, because this makes it possible to cover a sensor area of (10.56 x 5.94) x 2 = 21.12 x 11.88.
That is enough to cover the entire sensor of the Blackmagic Studio Camera 4K Pro and Plus. In the case of the Blackmagic Studio Camera 6K Pro, this adapter cannot be used because of the EF mount, see below.
What happens on other cameras: on Sony cameras, in APS-C mode, and by enabling Clear Zoom at about 1.1x to 1.2x OR by enabling SteadyShot/Active, the dark corners disappear with many lenses, especially those labelled Tele.
Also, from experience, on the Blackmagic Pocket 4K, without the extender enabled, B4 lenses cover the 2.6K windowed area.
From this we can say that B4 lenses with the 2x extender enabled can cover a sensor of about 22.22 x 12.5 mm. This information is useful for those who do not have Sony-like modes available.
Another thing to know: because of the construction of the BMD B4 mount, it is not possible to use it on other PL or EF mounts, only on mirrorless mounts. For PL or EF there are already products online, but they cost a few thousand euros.
Many of you asked us: if I enable the extender, do I lose the extra focal length range? That is true, but let's reason it out.
By enabling the extender, or by using an adapter with an integrated 2x extender, you always lose 2 stops of light. Let's take an example: a Canon 40x f/2.8 with one of these two extenders becomes a lens with a minimum aperture of f/5.6, and we can all agree that this can still work well. But if we then enable the second extender we lose another 2 stops of light and this lens becomes f/11. That creates the need to raise ISO or gain, which brings electronic noise into the image. So a 4x extender becomes pointless for many people, and this is why our initial advice is to use lenses that already have the extender together with our adapter.
How it works
If such an adapter may interest you, there are a few things you should know. We do not usually keep stock because the BMD B4 mounts are expensive, around 420 EUR, and we usually buy them from an authorised dealer in Rome. We proceed with small orders of 5 to 10 adapters every 2 to 3 months, so if you need one please consider a waiting time of 1 to 2 months.
Also, since we buy the B4 mounts from a dealer, we will ask for a deposit to cover that cost, or a little more. The payment methods we accept are bank transfer, PayPal and Revolut. The total cost of the adapter is 650 EUR plus shipping, and possibly some extra accessories, such as an additional mount (E-mount, M4/3, R-mount, X-mount, L-mount) or the support component used to vary the height relative to the rods.