The Closest Object Method : Contolling the Parallax Range

In some shots (out-of-screen effects, for exemple), one may want to control precisely the parallax range by playing with the interaxial. There is two main methods to do so:

First, let's make sure the angulation, or post-production shift, is already set (which means: the parallax of the farhtest object is set), for exemple at +1%.

Let's set the interxial at 0cm, then progressively increase it while watching the stereo 3d compositing monitor , until the closest object is at the parallax we want.

For exemple, if I want a 3% parallax range, since my far parallax is +1%, let's increase the interaxial until the parallax of the closest object is -2%.

Let's not forget that when shooting indoors, the farthest object will tend to move closer to the audience, because the farthest object is not at optical infinity.

So you may want to go back to the First Step and re-adjust the Far Parallax by shifting or by angulating.

One may, naturally, want to use a stereo calculator (for exemple the RealD calculator, for Iphone, or any other mean, like a simple Excel spreadsheet), an effective solution if you double-check the 3D on your Stereo Compositing Monitor.

But one can also use a method that does not require a Stereo 3D compositing monitor, and is quite intuitive:

- We mesure the frame's width at the closest object distance. Two assitants use a mesuring tape at the level of the closest object, perpendicularly to the camera's optical axis, while the cameraman guide them to the very edges of the frame. They get the frame's width at the closest object's distance, in meters or in INCHES (it's very important to use inches and not feets, here, so you may avoid a conversion process). Exemple: 2m or 80in.

- Then we multiply that number by the desired parallax range, for exemple : 3%

- And we get the interaxial we want : for exemple: 2m x 3% = 6cm ; 80in x 3% = 2.4in

- And if we wanted 1% parallax range : 2m x 1% = 2cm ; 80in x 1% = 0,8in.

To sum things up:

(FRAME WIDTH AT THE CLOSEST OBJECT'S DISTANCE, IN METERS OR INCHES) x (DESIRED PARALLAX RANGE) = INTERAXIAL

This very straight-forward formula exists because the parallax range is the total amount of parallax that exists in an image

This amount is actually how much (in percentage) of the frame's width the interaxial represent at the level of the closest object (when the farthest object is at infinity - which means, when the farthest object is at 0%).

Let's look into that: Let's imagine our cameras are strictly parallel, no angulation, no post-production shift. So everything is in negative space, but the infinity, which is at 0%.

If my frame is 1 meter (40in) wide at the level of the closest object, and my interaxial is 10 cm (or 4in), then the interaxial equals 10% of the width of the frame at the closest object distance - we know that's a negative parallax, so the parallax of the closest object is -10%, while my background is at 0%. So the total amount of parallax in my scene is 0% - -10% = 10%. That's my parallax range, by definition.

If the interaxial is now only 1cm, or 0.4in, and the frame's width at the closest object disantce, still 1m (or 40in), then the interaxial is only 1% of the frame's width - so my parallax range is 1%, since I still have 0% on the background, and -1% on the closest object.

Of course, this works perfectly only when the background is at optical infinity. Indoors, beacause a wall “masks” the optical infinity (which is at 0%), this wall (the farthest visible object) is not at 0%, but closer, for exemple at -0.5%.

So when I will calculate my 3% parallax range, I'm actually calculating the parallax range between the closest object and infinity. So if I've got a wall at -0.5% masking the optical infinity, what I'll actually get is a parallax range of 2.5%.

If one wants to do things very accurately, he should first calculate the total parallax range (from the closest object to infinity), then substract from it the parallax of the wall (calculated by the same mean: using a mesuring tape to know the frame's width at the wall's distance). In that case, it's probably better to start from here to get the exact parallax range you want; and THEN to set your far parallax by shifting or angulating.

But let's note that in most cases, that last point can be ignored, the standard formula giving good results anyway, even if you're shooting indoors.