3D Lens attachment (w Mirrors)

Started Jun 29, 2015 | Discussions thread
Cullings Regular Member • Posts: 200
Re: 3D Lens attachment (w Mirrors) * Back-to-Basics *

Interocular (io) distance vs Interaxial (ia) distance:

The separation between our human eyes is called the Interocular distance.
As a rule of thumb this distance is 65mm.

Interaxial (ia) distance is the separation of the stereo camera pair.
ia should vary shot to shot and changes our impression of scale.

The Depth Budget:
Each shot has a maximum amount of usable depth within which to create effective 3D.

Maximum Deviation:
The maximum deviation on screen is a measurement of Parallax.
It limits what is safe for the viewer. Avoids eye strain, and in turn headaches
and sickness. Too much deviation must be avoided, especially positive Parallax which could (depending upon screen size and seating position) cause our eyes to diverge. It is possible to ‘break the rules’. But not, continuously or, by large amounts for long periods.

Guides for TV (as defined by Sky):
Positive Parallax (appears behind the viewing plane): +2%
Negative Parallax (appears in front of the viewing plane): ­-1%
Total Parallax: 3%

The illusion of stereoscopic vision:
It’s important to note that the illusion of S3D created on the screen is
not the same as our perception of depth in real life. Our eyes have a fixed
FOV. Cameras do not. We have brains designed to fill in the gaps.
Cameras do not.

Interaxial (ia) and Convergence (cv) distances should vary shot to shot.

There is no ‘real life’ value to dictate Interaxial distance. Especially when close ups may place someones head 10 foot high on the cinema
screen. Fixing Interaxial distance is hugely restrictive, and ia should
in fact vary shot to shot.

Setting ia and cv is a creative decision to support the story. Together they define the scale of the viewer and the scene. We can view the shot as gods or insects - it’s up to the Director of Photography and the Director to decide.

1/30th rule of thumb:
Interaxial distance will generally be around 1/30th of the distance
to the convergence point. So if you're converging 30m away then the
camera pair should be 1m apart.  (!!!)

When shooting native S3D we’d ideally need two camera rigs:

A mirror rig. For action close to the camera.
( where ia is less the width of the camera )
A side by side rig. For action far from the camera

downloaded 8 July 2015, from:

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my note: this is from the viewpoint of a cinematographer.   Varying interaxial distance can be very difficult.   I think the "cha-cha" method may be the easiest.  Or perhaps "cha-cha-cha-cha".     - John S

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