Why does RED need to characterize a Canon mount lens before it will work?

[Les C.]

I was going to get the new Canon 24-70, but ... as that's been delayed for months, I was looking at the new Tamron 24-70.

It occurred to me that I'd have no way of knowing if it will work on the Scarlet (and if it would void the RED warranty, as it's not a Canon lens)?

But, if I put that lens on my Canon DSLR, it would work with all the features.

Why is that?

I guess I assumed that a chip in the lens would identify it's feature set in a manner that any compatible mount could detect.

Why does RED need to characterize a lens before it will work?

And, is this true in all cases?

[Daniel Kunz]

I put on the Tamron 17-50 2.8 VC, and it worked without problems. it even stepped down to f:2.7 =)

[Scott Crawley]

I don't think putting a lens on is going to void your warranty. I haven't seen that anywhere.

Lenses that have not been characterized yet may work, but not necessarily with accuracy. It's a high precision thing and each company is different, each lens is different.

[Alexander Ibrahim]

Its a difference in the use case between the DSLR and the cinema market.

Remember that DSLR lenses are not meant for continuous focus tracking. They are meant to focus at discrete instances and snap off a shot.

As a result they have different operating characteristics, many of which are non-linear. In english, the position of the AF servo doesn't have a direct correspondence to the focal distance.

DSLR's dont "care" because they have time to hunt for maximum AF sharpness between frames. Motion imaging cameras don't have that liberty ... they have to continuously be in focus.

My understanding is that Red cameras address this in one of two modes, which could be called "knowing" and "guessing."

With a fully characterized lens, the camera can make the most accurate corrections straight away, because it knows which lens motor position corresponds to a given distance.

In the guessing mode, the autofocus still works, but it hunts more. So as a subject moves ... the camera doesn't know how much to move the servo to make it be focused at a certain distance. So, it moves the servo, and then evaluates the image for sharpness at the AF point. (i.e. where you touch the monitor) Then it adjusts the servo again ... repeating this until the camera decides the image is in focus.

This guessing mode is ALWAYS how DSLR cameras operate. Thus they overshoot focus and come back to sharp, sometimes several times. We call that "hunting."

This is all compounded by the fact that DSLR lens manufacturers send metadata about focus and focal length back to the camera that isn't quite right. Photographers don't use that data the same way cinematographers and motion imaging post artists do, so they don't need accuracy. They effectively just need a gross reminder.

Red has to fix all that for these lenses to work using their built in features for cinematography.

DSLR cinematography almost always depends on a spectacularly powerful computer with a fantastic bit of software to manage all that. Its called the first assistant camera's brain. As a 1st AC on DSLR projects, you learn what lenses will do, and you start to move non-linearly in response it works out. (By the way ... this is a fantastic reason to spring for some manual Nikkor AI or Zeiss primes as opposed to cheap modern AF lenses.)

Red has to recreate that, but with a lot less processing power and flexibility than a human brain.

One thing they must do is empirically measure the actual FOV and focal distances at each FOV of DSLR lenses. Primes aren't so bad ... a Canon 50mm prime is pretty reliably 50mm, so you only have to calibrate the entire focal range in the smallest steps the motor can give. I don't actually know the precision of DSLR lenses, but a 14 bit 2/3" ENG lens has 16384 positions. For each position Red must map the actual focused distance against the reported distance in metadata. Then Red has to use mathematical regression to figure out a curve that can be calculated quickly that accurately represents how motor position corresponds to actual and reported focal distance.

For a zoom lens ... that becomes hugely more completed. Taking the 24-70, you have to divide the focal length range and divide it into a large number of steps. (I'd pick the steps available in typical cine zoom motors, which I believe is 16 bit or 65536 positions, then for each of them plot true focal distance versus motor position and reported focal distance. That could be over a billion data points which have to be accurately regressed into a polynomial.

That also makes clear why you'd want to calculate this data as opposed to looking it up. One focal position needs 16 bits for focal length, 32 bits for true focal distance (INT in mm), 16 bits for motor position, 8 bits for aperture. That means a billion position table would be 9 Gigabytes. For every zoom lens.

Now, my numbers here are just estimates. I didn't look up the precision of any of these digital motors ... I also didn't account for the possibility that analog motors are in use. I also didn't account for lenses whose focus characteristics are affected by aperture. (A cine lens or even a good manual SLR lens wouldn't be, but in search of lower costs some DSLR lenses are affected by aperture and compensate with AF.) I also didn't account for breathing, which also affects focus accuracy.

So, that's an outsiders guess at why they characterize lenses, and why it takes so damn long.

If you think what Red delivers is amazing from a user perspective, try thinking about how to accomplish it from an engineers perspective. To me at least, that multiplies their achievements from merely amazing to legendary.

[John Bannister]

Great posst Alexander!

[Les C.]

Alexander - Thanks a lot! You totally cleared that question up for me!

[Jacek Zakowicz]

Nice try Alexander
One question: how does the camera "know" where to move the lens focus in auto focus scenario.
Another words how does the camera know how far the subject is so it tells the lens to go there.. That's the missing piece in your logic.
Maybe next try...

[paulherrin]

If your lens is not on the list, it's ok, you still will get rudimentary support, such as IRIS control and Stabilization if the lens has it.

I'm assuming the basic support applies to Canon lenses as well as 3rd party offerings... You may even get lucky with simple autofocus functions...

[Phil Holland]

Anyways, back to logical-ville. When a lens isn't "characterized" by Red it "may" have issues when you attempt to use it's autofocus abilities and adjusting the electronic aperture. When connecting unsupported lenses that I've had the iris blades very quickly open and shut, which actually can induce terror in your heart. If you leave that lens on in that state for too long you could potentially damage your lens.

EF lenses work in steps. Some lenses have more steps than others to achieve close focus to infinity focus. It's been a few years since I built my last smart mount, but I remember the 85mm f/1.2L having a lot more steps than the 50mm f/1.4. I believe when you first reconnect an EF lenses to the Al or Ti mount it's either resetting the lens or calibrating to find out how many steps there are total in the focus throw.

Some Sigma, Tamron, Tokina, and Voigtlander lenses will work fine even without officially being characterized. If they do not, it's best to lock the aperture on a Canon DSLR and then partially engage the lens on your Red EF lens mount. You will be limited to manual focus however.

Hope that helps.

[Jacek Zakowicz]

Have you read Alexander's post?
Do you understand it?
I don't think you understand the question.
The link is a good read for anyone interested BTW- thanks....