To Change the Lens Focal Length of the Camera View

Select the Define Camera tool.

In the tool settings window's icon bar, click the Lens Focal Length icon.

Turn on Continuous View Updates and Display View Cone.

With the pointer in the camera view, enter a data point.

Move the pointer:

Up to reduce the focal length.

Down to increase focal length.

As the pointer moves, the camera view updates dynamically, as does the view cone in other open views.

Enter a data point to accept the new view orientation.

With Reference Point set to Target, the camera moves, while the target remains fixed.

With Reference Point set to Eye, the target moves, while the camera remains fixed.

Note: You can move the camera closer to or away from the (fixed) target, or vice-versa. This can be done interactively, or with Controlled Movement. Whether the camera or the target moves is controlled by the setting for Reference Point. When it is set to:

Eye — The target moves relative to a fixed camera.
Target — The camera moves relative to a fixed target.

As you change the lens focal length in the camera view, either the Eye Pt or Target fields adjust automatically to reflect their new positions. That is, the one that is not selected as the (fixed) Reference Point.

Note: As well, the View Angle fields change.

Tip: To simply control the movement for focal length setting you should use the Controlled Movement setting. You can change the focal length in predefined steps. To do this, you can use the Distance value of the Controlled Movement Increment fields.