Pulling IO+convergence

[Johnny Johnson]

Quick questions to those in the know:

Is it generally a good idea to pull both IO and convergence in a shot, or do people stick to stick with one or the other to avoid complications? I'm wondering if I need a dual wireless remote control (ie cmotion) for both or whether I can get away with a single remote and move the motor as needed for convergence or IO pulls.

Also, is it a good idea to slave the convergence to focus or just judge per shot?

Many thanks

[Pedro Guimaraes]

For speed sake, yes. getting thru your day etc....

Good idea? Ask your stereographer. It's his decision.

IO descisions are not always linked to convergence. Since you will be changing IO on a shot per shot basis having it automated is a big help.

Convergence is up to the stereographer. This is where there is a varying degree of opinions. There is no wrong or right. Just personal styles/artistic choices. Some converge prior to the shot and don't pull. Others like J.Cameron will "pull" convergence during a scene(often linked to focus). Also if you shooting live you will want to pull convergence. The ET rig will allow you to hold a convergence point AND change you IO, which can be a useful feature. There definately are situations when you NEED to adjust IO during a shot....many times this needs to happen remotely. Much like focus, IO is not one of those things you change in post.

That said many others (me included) belive in not converging during acquisition. Converging in post. Sure, when you shift the images you lose some pixels but you have much better control of any "pulls" due to keyframes and also you can experiment. Also when you toe a camera in to converge you do introduce some keystone distortion as well as possibly introduce some color issues in relation to the optical path thru the beam splitter. All said I see more benefit in converging in post. Naturally some situations call for it on set.

Slaving convergence to focus is a purely artistic choice.....some like it some hate it.......the battle rages on.....take your pick.


So if your shooting with many rigs out there you will have 2 c-motion hand controllers(cooperates). One for FIZ of both lenses and one for IO and convergence. So 2 first AC's usually......or one 1stAC and the stereographer controlling the IO/convergence.

If your shooting with ET rig they have there own hand controller "THC" it controls IO/convergence on thier rig and usually is included in the rental price.

[Leonard Coster]

Hi Johnny,

Good questions all.
There are arguments either way which means there is no wrong answer! - it is a subjective choice. To my mind you can use these additional tools to aid the narrative.

The way to decide is to look at the effect of each parameter.

IOD controls the perceived depth in the scene. If you change it during a shot you are screwing with peoples perception of the space. This may be exactly what you want - or exactly what you want to avoid !

There may be divergence limit reasons why you want to change IO in a shot - say you start extremely close on a subject - and you need a small IO to make that image coherent but you then pan off onto more distant objects and at that original low IO everything looks too flat.
What do you do?
one option is to change the IOD. You can try hide it in a camera move or make a feature of it - creative choice.

BTW this is by no means an exhaustive description of the effects - just a start for a long caffeine fueled conversation!

If you do want to pull IOD you will need smooth access to this control and a remote motor is a good way. As Pedro says you can't really tweak IOD in post so you need to try and get it correct and the move smooth in camera.
There are tools like ocular (nuke plugin) with will let you fiddle with IOD a bit but its not the same as shooting it right... no offense to ocular - it could save your ass one day...

Convergence controls where the audience will see objects relative to the screen plane. That object on which you converge will appear at the screen plane and those in front or behind it, proportionally in front or behind the screen for the audience.

So do you converge in camera?
You can, or you can shoot parallel and converge in post.
There are again good, situation dependent reasons why you would choose either one and you may run a mixture during your shoot.
Including finding a happy medium of convergence which you leave set an then trim in post.
Even if you do converge in camera you might keyframe tweak it in post to get it perfect or smooth out the moves...

If during a shot you pull convergence you will effectively be racking the set towards or away from the audience. This can be a great effect! Say running down a hallway but converging away so it feels like your hero player is making less progress than they should - maybe a bad example - you get the idea - it's a tool.

Or you can mimic the way your eye works - a technique employed for the majority of shots in avatar - where the primary subject is in focus and converged.
If during the shot focus pulls to another subject in the frame, you match it with a convergence pull so that this new subject arrives into convergence and focus at the same time - just like you eyes normally do.

You can of course deviate from all these states for subjective narrative reasons.

This is a BIG topic and one that you really need to experiment with. If you can, set up a stereo rig with live monitoring - anything anaglyph is ok if thats all thats available - and try these things. Get a stereographer to help you out for a few hours and explore the effects so you know what is possible.

If the question is more about expediency on set or in post then your stereographer will help you make a decision in concert with your operators and post people. There is a knock on effect from everything you do so make sure you're over the whole workflow and all the considerations particular to your shoot before you jump to any conclusions....

Happy shooting !

[Johnny Johnson]

thanks guys for the answers

Clearly there is as much artistic flair as focus pulling (if not more)

Ill likely be working more often wtih the ET freestyle rig which has built in motors for IO+convergence so ill just use one cmotion CAMIN box for this.

As an aside, is it important to lock the focal length on a pair of zooms with motors or is setting them by hand okay? It may sound daft, but I'm just thinking if the matching lenses arent calibrated electronically, they might be slightly off and then there's an extra bit of zooming in in post

[Pedro Guimaraes]

thanks guys for the answers

Clearly there is as much artistic flair as focus pulling (if not more)

Ill likely be working more often wtih the ET freestyle rig which has built in motors for IO+convergence so ill just use one cmotion CAMIN box for this.

As an aside, is it important to lock the focal length on a pair of zooms with motors or is setting them by hand okay? It may sound daft, but I'm just thinking if the matching lenses arent calibrated electronically, they might be slightly off and then there's an extra bit of zooming in in post

freestyle rig is made by p+s, ET rig is called Quasar, 21st century3D rig is called BX3.

Also for focus, Iriz Zoom control on any rig you will need 2 camins, 6 motors, special 3D control cable between camin's, start stop cables and one controler (cooperate).

Hand is okay.....but why leave for post when you can do on set? So for speed and accuracy we use motors on all lens controls (FIZ).

With c-motion you can adjust one lens at a time to get proper matching. Preston does not have this feature.

If you must, for budget reasons have just focus control then you are correct. One camin only. Whenever possible it's better to have FIZ controls....you do not want to go thru 2 months of long days on set always matching by hand.....slow and inacurate and some lenses can move!