Hi,
looking around there are many interesting options for mount adaptors that can add to the camera abilities like ND, Manual IRIS, or even boost speed (by concentrating picture) like the Metabone booster.
The thing that I don't like is that all this adapters are made to adapt lenses to different cameras and not for the camera they are made for !
I explain myself better. I have a Canon 5D Mark II that I use for video with a 24-105mm lens that I use 90% of the time.
I would love to have an ND filter at the tips of my fingers where the lens mount.. or (or maybe.. at the same time)... I would love to have manual iris control (Guess you mount the lens normally once, open all iris electronically, switch off the camera, and re-mount the lens this time with no electronic connection)

These are the kind of items I am referring too.. Why don't these exist form Canon TO CANON (or from Nikon to Nikon) ... for example.. but just from CANON to SONY, or from CANON to MICRO 4/3 etc.... Why all these nice features only on conversion adapters?

EF adapter with ND filters FotodioX Vizelex ND Throttle Adapter VZLX-THRTL-EOS-NEX-PRO B&H

EF adapter with Iris: FotodioX Canon EF Pro Lens Adapter with Built-In EOS(IRIS)-FX-P

Is there some reason that has to do with optics (inevitably distance changes) ? And .. can it be solved with some new optics inside the adapter (like the Metabones for example) ?

Simple, adapters have to take up SPACE - they add at least SOME thickness between lens and body.

The adapters take advantage of the short flange distance on the smaller cameras, and stick an additionally useful function into that space.

Think of it this way, on your full size system, there's a mirror in between the lens and the sensor - meaning the lens HAS to be further away from the sensor. "Mirrorless" by nature can be smaller because... NO MIRROR!

Now you see systems which can be more compact! BUT, it also means that lenses intended for a larger flange offset can be adapted... and still focus on the same plane where the sensor is, once you add the thickness of the adapter.

Ok.. but.. help me here.
I own a Canon 5D Mark II and I am waiting for a Blackmagic 4K to be delivered (hopefully) soon.
I asked for the BM4K with Canon mount so I expect to put on it the same 24-105mm lenses I use daily on my 5D.
On the BM4K the lenses will be mounted much more near to the focus plane (the sensor) because.. as you said.. there is no mirror. And I will be getting Super35mm and not full frame with a 1,5 (or so) ratio.. that means.. less wide angle.. the 24mm will look like a 36mm on my 5D... correct?

So.. in theory.. If I mount them farer from the camera (few cm.. like on the 5D.. maybe not so far because the BM4K sensor is smaller...) ideally I could get full 24mm wide angle no? Maybe with the need of extra lenses too (like metabones) or maybe just with passive mechanics adapter.. but there is plenty of space to do so no?

In fact Metabones (Metabones® (http://www.metabones.com/article/of/metabones-nikon-g-to-bmcc-bmpcc-speed-boosters-available-starting-today)) makes such lenses adapters but only for the Micro 4/3 mount version of the BM4K and not for the CanonEF mount version.. and it's the same sensor and (almost) the same distance! Do you see my point here?

I really don't understand what is the technical problem in doing the same with a Canon mount on both sides.

If you ordered a camera with Canon mount, the mount already compensates for the extra distance from the sensor required by the lens. The mount projects out from the body enough to keep the lens to sensor distance the same as always. This means no adapter can be mounted between lens and camera.

So I made a mistake getting the Canon mount version?
I just as well could have got the Micro 4/3 version and buy something like this for small money?:
Pro Canon EOS EF to Micro 4/3rds Lens Mount Adapter with Aperture Adjustment M43 | eBay (http://www.ebay.it/itm/Pro-Canon-EOS-EF-to-Micro-4-3rds-Lens-Mount-Adapter-with-Aperture-Adjustment-M43-/121283002064?pt=UK_Photography_CameraLenses_Lens_caps_hoods_adaptors_ET&hash=item1c3d07bed0&_uhb=1)
At least I would have had more options on the table for future upgrades/adaptations?

hey.. maybe I am still in time to change my order?

M

Apperently I am mistaken as it seems only the 2,5K camera has double option (EF/MFT) while the 4K only has EF mount

I wasn't sure about the 4k since I have the 2.5k BMCC with EF mount. I probably should have gotten the MFT mount but since I have a lot of Canon lenses (from using film and digital EOS cameras for a long time) I thought the EF would be the logical choice.

The good news is that I also have a lot of old Nikon lenses for the original Nikon F and the old Nikon lenses had a longer back focus so I can use adapters to mount them on the BMCC as well. They also had manual aperture control so they're easy to use.