The objective was to shoot these scenes exclusively by candlelight - that is, without a boost from any artificial light whatsoever. As I mentioned earlier, Stanley Kubrick and I had been discussing this possibility for years, but had not been able to find sufficiently fast lenses to do it. Stanley finally discovered three 50mm f/0.7 Zeiss still-camera lenses which were left over from a batch made for use by NASA in their Apollo moon-landing program. [...] But when you looked through this lens it appeared to have a fantastic range of focus, quite unbelievable. However, when you did a photographic test you discovered that it had no depth of field at all - which one expected anyway. So we literally had to scale this lens by doing hand tests from about 200 feet down to about 4 feet, marking every distance that would lead up to the 10-foot range. We had to literally get it down to inches on the actual scaling.
[...]
The point of focus was so critical and there was hardly any depth of field with that f/0.7 lens. My focus operator, Doug Milsome, used a closed-circuit video camera as the only way to keep track of the distances with any degree of accuracy. The video camera was placed at a 90-degree angle to the film camera position and was monitored by means of a TV screen mounted above the camera lens scale. A grid was placed over the TV screen and by taping the various artists' positions, the distances could be transferred to the TV grid to allow the artists a certain flexibility of movement, while keeping them in focus.
