The blockbuster “Avatar” set the new standard for 3D movies and film-goers have been very enthusiastic.
Part of its success is due to the very good 3D camera work but the bulk of the 3D cameras has been a major disadvantage. The set-ups demand that the paired cameras either employ prisms or partially reflecting mirrors to permit a controllable separation of the two lenses, to capture the left and right image pairs.
Bwana Devil is a 1952 drama based on the true story of the Tsavo maneaters. It started the 3-D boom in the US film- making industry from 1952 to 1954.
The average human eye separation is around 65mm, so image capture is best served by the camera lens’ interocular distance (IOD) set at about 70mm for most subject material. For close-ups, a smaller IOD is preferred.
In “Dial M for Murder”, often recognised as one of the best of the 1950s’ 3D movies, Director Alfred Hitchock was forced to use a large and inflexible camera rig. In one key dramatic scene he used a scaled up telephone to provide an extreme close-up. The reason: it was physically impossible to rig the lenses to give a closer IOD.
A recent Australian innovation, the SpeedWedge, could make things much easier. It was developed by physicist and stereographer Leonard Coster. The rig consists of a housing that holds a pair of gen-locked Silicon Imaging SI-3D digital cameras. One camera is placed on top, its lens pointing downwards and aimed onto a partially-silvered mirror with 50% reflectance. This camera captures the left eye image.
Beneath it is another, matching camera installed horizontally within the rig, its lens pointing ahead and looking through the same partially-silvered mirror. This camera captures the right eye image. The complete rig is mounted onto a television camera tracking pedestal.
Realising that the 3D camera setup would not capture macro shots, Director Alfred Hitchcock organised a scaled-up phone for a key scene in the 1950s movie "Dial M for Murder".
Fig.1 shows the general concept. The partially silvered mirror is the key, with each camera receiving half the light from the scene. By having the cameras mounted at rightangles to each other, their effective lens separation can be varied from zero to as wide as is desired, without any mechanical interference between them.
In practice, if the scene involves action in the foreground, the IOD is set to a small value. Conversely, if the scene or subject is more distant, the IOD is set to a large value. While any video cameras could have been used in the SpeedWedge, the Silicon Imaging cameras were chosen because they use the SiliconDVR recording software which solves a major post production problem.
SiliconDVR records the two camera data streams in one go so, in terms of the capture workflow, a major task is handled elegantly. As Coster says: “If you can’t synchronise your cameras and record the two data streams easily on set, you’re in a lot of trouble!”
The SI camera heads have a significant advantage with their small size. This leads to a complete rig that can be picked up by one person. If you wanted to strap two big film cameras into the housing, you could do it but the weight and final size would be impractical for hand-held operation.
In practice, the Speedwedge rig allows the IOD (inter-ocular distance) to be varied from zero (for macro shots) to 70mm, covering mid-range and telephoto shots. In practice, the IOD needs to be set differently for each and every scene and the actual setting depends on how strong the director wants the 3D effect to be.