As discussed previously, there are two different approaches to setting convergence. The first is to 'toe-in', or converge, the cameras such that both cameras point at the object to be placed on the screen plane.

However, this approach can also mean it may be necessary to rack convergence dynamically through a shot while shooting to maintain the point of interest on the screen plane, similar to racking focus in 2D cinematography.

Additionally, changing convergence, either dynamically, or on a shot-by-shot basis during production, can add a lot of time to a shoot.

There are additional potential issues if the camera or object is moving close or further away from the camera as racking convergence can have some very unexpected effects on the viewer's perception of the image, as the object can appear to get larger or smaller relative to its original perceived size due to the amount of stereo effect changing (see miniaturisation).

A further issue is that each camera will suffer different keystone convergence, or vertical parallax distortion, making the images hard for a viewer to 'realise' visually if these keystoning effects are not fixed during post-production.

And this keystoning distortion will increase with larger camera separations (inter-ocular distances - see later).

The alternative is to shoot parallel and set the convergence point in post...

This first set of images shows a scene shot with convergence; the second set the same image shot parallel.

If you hover your mouse over the above and blow left eye image the matched right eye image will be seen.

If you look closely you can see more 'rotation' on the back wall in the images shot with convergence (top images), which will give rise to vertical parallax offsets as the side of the wall that rotates away from the camera gets smaller, while the side that rotates towards the camera gets larger - but the fact you have to look very closely to see this shows the level of error we are talking about - and I deliberately shot a very wide inter ocular distance for these images!

But, as stated, if this keystoning problem is severe, as it can be, it will require correction via post-production, and can be very time consuming to fix.

However, the error is usually small (as seen above), and is often not noticeable in the real world - see following Anaglyph image examples.

The alternative method to setting convergence in-camera is to shoot parallel, and set the convergence point in post.

This obviously has the benefit of speeding shooting during production, and combined with the usual need to shoot a deep depth of field (hyper-focus - see the focus page), can improve shot throughput on-set dramatically, leaving just inter ocular distance to be changed on a shot-by-shot basis.

However, the parallax of each camera relative one to the other is fixed (the amount of difference in an object the left eye/camera sees compared to the right) and changing the convergence point in post is making an unnatural change as the relative parallax isn't also changed.

With small changes this isn't a problem, but if the change is large it can lead to an unnatural image, which again will break the suspension of disbelief.

There is also the issue that re-aligning the left and right eye images means bringing them closer together, or further apart, so causing blanking problems at the edges that requires re-sizing and cropping of the images.

This can be seen in the following images where no attempt has been made to mask the blanking problems caused by changing convergence in post.

However, allowing for such post alignment when shooting is not difficult, and can often be easier and more cost effective than the time taken to set-up camera convergence on-set.

Weighing one option against the other – looking at the relative pros and cons – is the only way to make a selective judgment on the best way to proceed.

The following images show different examples of the same basic scene shot using two cameras set to converge on a single point, and again using a parallel camera set-up.

The above image had convergence set by aligning the toe-in of the cameras to the first newel post, with some slight adjustment in post to overcome small shoot inaccuracies.

See later for discussion on the issues with the close-up banister rail.

This following image shows parallel shooting with convergence set in post.

Allowing for alignment differences the actual appearance of the two images are very similar, with difficulty in seeing any real variations in convergence or parallax.

All things considered, I prefer the parallel approach as the positives outweigh the negatives – especially when the potential issues of keystoning and time lost during shooting are taken into consideration.

But if there was time to toe-in the cameras on-set it would probably be worth considering it...

Next Page - Inter Ocular Distance

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

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