To Change the Lens Focal Length by Varying the Eye to Target Distance in 100 (Unit) Steps

Select the Define Camera tool.
In the tool settings window's icon bar, click the Lens Focal Length icon.
Turn off Continuous View Updates.
If necessary, click the More button to reveal the settings options.
Expand the Controlled Movement section to display the fields for Increments in Distance and Angle.
In the Distance field, type 100.
In the camera view enter a data point in the top or bottom of the view.
The distance between the Eye Point (camera) and Target is increased, or reduced, by 100 units. If the data point is in the top half of the view, it is reduced, and vice-versa.
Further data points change the distance in 100 unit increments.