By Steve Shaw, Light Illusion

Last month we looked at Interocular distance, miniaturisation, convergence and focus. I concluded that too great an interocular distance causes eye strain, elongation and miniaturisation, whereas too small a distance causes objects to appear unnaturally large. I also looked at the effects of different convergence and focus settings. This month, I will discuss the cut-out effect, scene focus and viewing distance.

The cut-out effect

While there is a certain amount of disagreement on the issue of the cut-out effect – mainly to do with terminology and the individual’s concept of the image that is being looked at, in general terms, the cut-out effect is when planes of objects seem unnaturally separated, as if they appear in separate layers like cardboard cut-outs stuck at different distances.

One argument is that using a long lens foreshortens the image (as it does in 2D cinematography), so producing the cut-out effect. Therefore increasing the interocular distance adds more volume back into the scene, thus lessening the cut-out effect.

In my view, the problem with this is that adding more ‘artificial 3D volume’ makes the objects seem more ‘cut-out’ as it ‘enhances’ their apparent separation. See what you think…

The effect of using exaggerated interocular distances to compensate for compressed objects when using long lenses can be seen to a greater extent in the following panoramic images, where distance between objects is far greater.

In the second image, the cut-out effect, or more accurately a ‘planar’ effect, can clearly be seen.

If we now add a long lens to the camera and shoot with a small interocular distance the effect is exaggerated further as the effect is to shrink (foreshorten) apparent distance between objects, as shown in the image below:

If we now increase the interocular distance, the planar effect becomes a lot more apparent, because although the larger interocular distance has increased the ‘stereo volume’, it has done so as a percentage of the original object sizes and distances. What this means is that while the apparent depth of the objects has increased, the distance between them has also increased by the same percentage, making the objects (that may now appear less cut-out) to be positioned on very separate planes within the now stretched depth space.

And all of this is in conjunction with the miniaturisation of the scene caused by the excessive amount stereo. Having said that, the fact that there are normally few foreground objects within such shots means the effect of miniaturisation is reduced.

What do you think? Does the use of exaggerated interocular distance improve or worsen the cut-out effect? You can leave a reply at the end of this article.

Scene focus

In many situations, for the image to appear realistic requires a virtually infinite depth of field – or a hyper-focal image. This is because the 3D nature of the image tells the eye/brain that it can look and focus where it chooses, but an out-of-focus area of the image obviously prevents this, causing the suspension of disbelief, as well as causing headaches due to the brain being told one thing, while the eye sees something else.

Foreground objects being out of focus are worse than the background in this respect.

Viewing distance

(Please note that the image below has been shot with a very excessive interocular distance, so don’t go straining your eyes by looking at it too hard!)

The issues surrounding viewing distance go back to the first part of this series and those ‘rules’. One of the key things for viewing distance/screen size is to make sure you don’t make the audience do a Marty Feldman (i.e. go wall-eyed).

The appearance of a stereoscopic image on small screens is different to the same image on a big screen, so this is where you may need to whip out your spreadsheets for the rules, or make sure you are checking the images with care – or better still, download the SGO Mistika Android Stereo 3D app from the Android Market. It is also worth remembering that this is more tricky with converged shooting than with parallel shooting. It is hard to capture images wrongly using parallel (with sensible interocular values), whereas if you converge too much the background can easily end up with way too much positive parallax, that cannot be fixed in post without a lot of hard (and expensive) work.

However, thanks to the exaggerated convergence of the image below, we can easily see the effect on the distance between objects and the lengthening of the objects themselves that occurs when large interocular distance are used. It also shows well what happens if you get close to, or further away from, the screen.

Starting at a normal viewing distance, with your anaglyph glasses, move closer to the screen, and then further away. Note the apparent shortening and lengthening of the distances (and objects) within the image.

Conclusion

Using long lenses to provide close-ups on distant objects is often unavoidable, but it will result in image foreshortening. However, while it is tempting to increase the interocular distance in order to compensate for this, as I have shown in the above examples, this can in fact make the problem worse.

The issue of focus is even more important with objects in the foreground, as these are closer to the viewer, and the brain knows that these should be clearer than images further away. However, focus can be used to guide the audience to specific locations within the image, as demonstrated on a number of occasions in Martin Scorsese’s film Hugo.

As also shown above, the ratio of the viewer from the screen, based on the screen size, needs to be understood in determining the amount of stereoscopic effect that will be apparent. Again, there are all sorts of calculations for this, but as was stated at the beginning of this series, our industry is about the image, not about strict mathematics!

In my final part of this series next month, I will discuss projection and displays, and headache-inducing problems.

Steve Shaw is a Partner in Light Illusion, a top consulting service for the digital film market, with offices in the UK and India.

www.lightillusion.com