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I would like to purchase a Blackmagic camera but it doesn't have any follow focus system. When searching further, I noticed that many professional cinema cameras doesn't have an integrated follow focus system (Arri, Red...), while a basic Sony A7sii does have one.

I would like to know please how do these cameras work. I saw that they are used with a separate follow focus system (be it wired or wireless) but technically when an object is moving, should the operator grope and try to match the focus on the moving object or is there any automatic recognition system ?

Thanks, Cheers

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    Follow focus usually means a manual focussing rig designed to be ergonomic for the operator. You're mixing it up with "auto focus". – stib Feb 15 at 3:58
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On professio al sets with a camera crew, there is an extra person which pulls the focus. We don't really use automatic focus. This needs some training, but it's not so hard to pull off.

Normally, you attach 15mm rods to the camera body, on which you then mount the FF system, "wired" systems start at around 20$, useable ones at around 100$.

Wireless you could take a Tilta Nucleus Nano for 230$

  • Thanks for you answer. And is it precise when done manually ? – Thomas Carlton Feb 18 at 13:20
  • Well, this depends on you. You can get really accurate focus with it, but this needs training. You will get the best focus, when you have a dedicated focus puller. – Timothy Lukas H. Feb 18 at 13:26
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    For documentary work, where you don't get multiple takes, or the ability to measure focus before the shot, a high definition external monitor with focus peaking is a very useful piece of kit. Focus peaking highlights areas that are in focus with a coloured outline. – stib Feb 20 at 4:44
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Follow Focus is used in professional Cinematography.

There is a specific role created for the purpose of getting the right focus called "Focus Puller" - 1 person that controls the follow focus.

How accurate Your focus is mostly depends on the brightness of Your surroundings. While shooting on F2.8 You have really low depth of field (so it is hard to pull the focus).

Then You can sacrifice some of that light hitting the sensor and go into F4.0 or higher but then there is the risk that if You don't have enough light and Your scene is dark.

Most of the cameras have a marker on the body stating where the sensor is placed and in professional environement it is used to measure the distance between sensor and the object being filmed. Then focus puller can mark the focus whell or try hitting it by any other mark that might be used for that.

But to directly answer the question - There is a system that is allowing You to mimick follow focus, it is called Auto Focus. But then You need to use it consciously. If there is no auto focus allowed (either body or lens) then You're left with manual or wireless follow focus servo systems.

I think it also depends heavily on the sensor being used but that is for another thread.

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The feature you are describing on your a7 is "Auto Focus" the problem with auto-focus is that focus is actually a relatively hard problem to solve. An automated system has to figure out what you want to focus on and then has to achieve focus on it. There are two major problems with this.

First, for video, having a computer guess at what you want to be focused on is very dangerous. Having things very mildly out of focus is much less distracting than having the focus jump around because the computer gets confused about what it should be focusing on. Auto focus is generally just as likely to ruin a video shot as it is to save it.

Second, there are two main ways to handle auto focus. The first is contrast based detection, which doesn't require altering the sensor, but requires going past the point of focus and then returning after things start being less sharp rather than more sharp. This results in the focus "breathing" which is, again, very distracting. The other option is phase detect auto-focus. This can tell if something is in focus or not, but requires that the focus point be on a particular pixel which has to be altered to be able to both capture the visual information as well as capture phase information to determine if things are in focus. This complicates the sensor and means there is less room for light gathering, so these hybrid sensors aren't really super desirable due to their limited areas of being able to focus and cost to the consistency of the sensor.

For these reasons, professional camera bodies don't generally have auto-focus while shooting. Many of the Black Magic cameras actually do have Auto-focus, but it's contrast based and intended to be used before you begin shooting, not while the shot is going.

That doesn't mean these cameras don't give you tools to help with focusing. The main aid they give is a feature called "focus peaking". Focus peaking analyses the image and looks for the areas of the image that are the sharpest. It then highlights the edged detected for the top x% of sharpest lines and/or points in the image. This is typically configurable on better camera bodies. This visual information lets the operator know where the focal plain in the image is and easily make manual adjustments on the fly without overshooting.

It's possible to do this as a single camera operator with a little bit of practice, even for relatively complicated shots, but on higher end productions, 2-3 people actually operate the camera to simplify things. One person would be the "follow focus" operator. A follow focus is simply a system of gears that rig the focus ring of the lens to a dial that can be controlled by another operator. This ring can normally be marked with a dry erase marker to label what spots they need to hit and they'll also typically have a monitor with focus peaking to allow them to see what they are doing. This operator can then remotely adjust focus for the camera without taking the main camera operator's attention away from framing and executing the shot itself.

Sometimes these follow focus controls may actually be used by a single camera operator as well since they can pivot the orientation of the ring to be more logically following in and out and to give a finer grained control over the focus ring by not gearing 1:1.

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