Field of View > Focal Length Constraints

Field of View/Focal Length Constraints

You can create constraints on the camera's field of view and focal length in a similar fashion to path and orientation constraints. Field of view constraints are enabled (and make sense) only when the camera lens is set to Zoom.

Warning 1: This topic is for experts. Do not use field of view constraints on a shot unless you have a specific need encountered on that shot. Do not use them just because focal length values were recorded during shooting. FOV/FL values calculated by SynthEyes are more accurate by definition than recorded values.

Warning 2: Do not use focal length values unless you have measured and entered a very good value for the plate width. Use field of view values instead.

The Known lens mode can also be viewed as a simple form of field of view constraint: one that allows arbitrary animation of the field of view, but that requires that the exact field of view be known and keyed in for the entire length of the shot. We will not discuss this mode further, except to note that the same effect, and many more, can also be achieved with field of view constraints.

As with path constraints, field of view constraints are created with a seed field of view track, animated lock enable, and lock weight. See the Lens panel , Solver panel , and lock control dialog.

Both hard and soft locks operate at full effect all the time, regardless of the state of the Constrain checkbox on the solver panel.

As with path constraints, field of view constraints affect the solution as a whole. If you have a spike in the field of view track on a particular frame, adding a constraint on that single frame will not do what you probably expect. All the trackers locations will be affected, and you will have the same spike, but in a slightly different location. This is not a bug. Instead, you need to also key surrounding frames. In all cases, identifying and correcting the cause of the spike will be a better approach if possible.

If the lens zooms intermittently, you can determine an average zoom value for each stationary portion of the shot, and lock the field of view to that value. You can repeat this for each stationary portion, producing a smoother field of view track.

Sometimes you may have a marginal zoom shot where you are given the starting and ending zoom values (field of view or focal length), but you do not know the exact details of the zoom in between. SynthEyes might report a zoom from 60 to 120mm, but you know the actual values were 50 to 100mm. You can address this by entering a one frame field of view constraint at the beginning and end of the shot with the correct values. As long as your values are reasonably correct in reality, the overall zoom curve should alter to match your values. 

If only the endpoints change, but the interior remains at other values, then SynthEyes has significant evidence to the contrary from your values, which most likely indicates the values are wrong, the plate width is wrong, or that there is substantial uncorrected lens distortion.