However, there is one issue that cannot be overcome by different shooting techniques, or through post-production:

Accommodation and Convergence
Accommodation is where the muscles in the eye distort the eye's lens to focus on a given object at a given distance, while Convergence is where additional muscles in the eye rotate the eyeball so that the left and right eye images align to produce a single image (with parallax differences due to the inter ocular distance between the two eyes).

This muscular adjustment for Accommodation and Convergence are, in the real world, linked as the point of focus for the eye's lens and the point of convergence for the eyeballs occur in the same place. This is known as Fusion; where the point of focus and convergence are the same, and the eye's muscles are used to working this way in the real world.

However, when looking at a stereoscopic 3-D image Accommodation and Convergence do not work like this.

The focus (Accommodation point) for the image is always at the screen plane; while the convergence point varies with the perceived depth of the image.

The only point at which Accommodation and Convergence are natural, and in agreement, is when the object being viewed is on the screen plane.

This is far more of a problem when viewing images on a small screen, such as the image son the website via a PC monitor, than viewing the same image on a large theatrical screen, as the eyes focus point (Accommodation and Convergence) is much nearer to infinity. This is one reason that stereoscopic image tend to look (and feel) better when viewed further away; from the back row.

It is also a reason why positive parallax images (images placed behind the screen plane) can be easier to look at than negative parallax (such as IMAX) images, where the image is all in front of the screen plane, as the positive parallax images are less stressful on Accommodation and Convergence.

Could this be the problem that stops Stereoscopic 3-D going mainstream - especially on DVDs and tv transmission, where the tv screen is small?

Divergence
Divergence (the need for the eyes to rotate outward to fuse the two separate left and right eye images into one) is a problem that can be easily overcome when shooting, but is impossible to fix in post (at least with a realistic budget!).

It is a problem most associated with shooting wide inter ocular distances and using camera convergence to set the zero parallax point, with background objects some way away from the convergence point.

In such a set-up objects behind the screen plane (zero parallax point) can become too far separated, requiring the viewer's eyes to diverge to mesh, or fuse, the image into a single occurrence.

For many people this is impossible to achieve if divergence is required at all, and for those that can accept some divergence a value of no more that 1°, or 1.5° degrees from the eye's convergence angle at zero parallax (the screen plane). However, even 1° divergence is a huge value!

This is yet another reason why shooting parallel can be preferable, as it is all but impossible to cause a divergent image, except when shooting very wide inter ocular distances and horizontally re-positioning the images in post to push most of the image behind the screen plane.

Contact steve@lightillusion.com for more info.