The first thing to understand is the term 'Stereo Budget', which is used to define the total parallax value within a given scene, from maximum negative (in front of the screen plane) to maximum positive (behind the screen). If the total depth budget is too great the scene can and will be difficult to view, and different size screen have different budget allowances.

Maximum positive parallax should, ideally, be equal to the standard human eye separation. In this way the eyes are not forced into divergence, which is what will happen if the positive parallax is greater than the human eye separation. At positive parallax equal to the human eye the object being viewed will appear to be at infinity.

Note: the lines appear to converge as parallel lines converge at infinity...

Negative parallax is slightly different, but should ideally be equal to the maximum positive parallax, which will place the object being viewed half way to the screen.

However, negative parallax is actually more flexible as the viewer can cope with larger changes in negative parallax than for positive, and in reality it is possible to go to 2 or even 3 (or more!) times the suggested value if the story warrants it, and if all other limitations are taken into account.

However, measurements of parallax are relative to the screen size and viewing distance...

But what exactly does this all mean?

What it means is stereo images that work well on a small screen, may not work well when viewed on a large screen.

As a rule of thumb, the following calculation shows the maximum positive parallax for a given screen size:

Human Interocular (2.5 inches approx) divided by the screen width, with the result multiplied by the number of horizontal pixels.

So, for a small 24 inch wide HD monitor the maximum positive parallax would be:

2.5/24x1920=200 pixels

But, on a 30ft screen also showing HD the calculation would be:

2.5/360x1920=13.3 pixels

The smaller allowed number of pixels is because the pixels on-screen are a lot bigger than on the smaller monitor, but the human eye separation remains the same... Logical, isn't it!

What this means is that the final display plays a big part in what will work, and what wont. One size does not fit all.

Next Page - Convergence

As always, test before committing to a particular style of stereoscopic 3-D shooting.

Or better still, contact steve@lightillusion.com for more info.