Cinematography-Theory-And-Practice

Figure 14.11. (top) In both of theseframes, the focal length and distancefrom camera to subject are the samebut the F/stop changes. In the topframe, the lens is wide open and thedepth-of-field is very small; only onecard is sharp.Figure 14.12. (bottom) The lens isstopped down to f/11 and almostall the cards are in apparent focus —meaning that they only appear tobe in focus because they are withinthe depth-of-field. Critical focus, thepoint at which the lens is actuallyfocused,is still on the red king.may actually shift. With some zooms the nodal point actually endsup 5 inches behind the film plane at its long end. With the old Cooke20-100 the range was about 10.5 inches in front of the film plane at20mm, and 5 inches in front of the film plane at 100mm.Motion control rigs that keep the nodal point over the pan centeruse a long worm gear that moves the camera/zoom back and forthover the center point as you zoom. There are nodal point cameraheads for tripods, dollies, and cranes. In practical depth-of-fieldapplications with zoom lenses, the only thing that is really of anyconsequence to us is at the wide end, where the nodal point is typicallyaround 10 inches in front of the film plane. Thus, if you areshooting a close-up at the wide end of a zoom it’s as if you were 10inches closer to your subject matter, which also reduces your depthof-field.Being closer you of course have less depth-of-field. This isone of the reasons that zooms are seldom used in macro, table-top,and other situations where critical focus is important. In averageshooting situations, nodal point is not critical, but on more technicalshoots, it is important to be aware of the implications.cinematography280