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When shooting strictly parallel, without a post-production shift nor angulation, we only get negative parallaxes, or 0% parallax for objects at optical infinity. Everything is in “out-of-screen effect”, but infinity, which is on the screen plane
For that reason, on screens smaller than IMAX, it is necessary, in post-production, to shift on image in relation to the other, to create positive parallaxes. If one shifts all the left image to the left by 1% of the width of the frame (that is 19 pixels in Full HD, or 20 pixels in 2K), then the objects at infinity, that were at 0% parallax, goes at +1%. And the closest object, that was, for exemple, at -2%, will go to -1%.
This means that we've pushed all of the 3D, in one block, further away from the audience, without modifying the quantity of depth: everything appears to be farther.
This technique is used, for exemple, to adapt the 3D image to different screen sizes: we will shift one image in relation to the other until our far parallax is at the artifical horizon of each screen size; that is:
- About +2% for small screens
- +1% for 6,5m/250in screens (that's 21ft)
- +0,5% for 13m/500in screens (42ft)
- +0,25% for a 26m/1000in IMAX screen. (84ft)
Because all of these values lead to a maximum positive parallax of 6.5cm/2.5in.
It is very useful to be able to see directly on set the 3D effect of a post-production shift (also called H.I.T., Horizontal Image Translation), by pre-visualizing it on a Stereo 3D Compositing Monitor, such as the Transvidéo CineMonitor HD 3D. By turning a knob, you can shift one image in relation to the other, pixel-wise.
When we shoot, this is the FIRST STEP: ADJUSTING THE FAR PARALLAX
So, when we shoot parallel with a post-production shift:
- Before touching the interaxial (the distance between the cameras) one should look for the farthest object in the scene, and turn the pre-visualization knob of the monitor until the two images of that object have the wanted parallax - for example +1% (that's 19 pixels in Full HD, 20 pixels in 2K).
- Be careful: if that object isn't at infinity (which is often the case when shooting indoor), when I will increase my interaxial, the far objects will tend to come slightly closer. So after changing the interaxial, we may re-adjust, if we want to, the far parallax a second time to push it back to the Artificial Horizon.
