Cinematography Mailing List - CML

IA as calculated and then as used


The illustration below show some calculations that I did for a film that I shot late last year.

Equipment was a pair of Panasonic 3700's in a Freestyle rig

It was a low budget job that had to be shot very quickly, in the end we only used the 7mm & 14mm DigiPrimes.

Offline was to be done in 3D so any jumps in artists positioning had to be smoothed out whilst shooting. The female artist in these tests is really there as a placeholder for the screen plane as action would generally be staged so that the main action was between the male and female actors distances from camera with anything forward of the actresses position coming into negative space.

We shot knowing that we intended to put the screen plane at approximately 1mtr and staged the action so that on a limited number of occasions objects like flaming torches came much closer and therefore into the audience.


In this first instance all settings are as recommended by the rig-solver in Frame Forge.

Screen plane was set by clicking between the eyes of our female lead, unfortunately rig solver would not allow 1mtr screen plane with a 7mm lens and insisted on setting it to 1.19mtr.

The figures at the top right of each shot are the percentage disparity of each subject.

As you can see with everything except the 7mm the disparity of the elements remains pretty constant but there is a huge jump in the disparity of the lead male as we move from 7mm to 14mm and these were the lenses we intended to use most and in fact were the only lenses used.

I wasn't happy with the way this caused the male actor to jump position relative to the female and/or the screen plane as I knew that we had little post time and I wanted to ease the transitions between shots as we actually shot.


I then calculated the IA and convergence settings that I would need to use to keep the relationship between the male and female leads constant, or as close as I could!

I wanted to keep the male lead in a fixed position behind the screen, the female artist is really just a placeholder for screen position

This resulted in the overall depth decreasing as we went to longer lenses but as we stuck to the 7mm and 14mm it gave us a much smoother ride when going between shots.

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