About the Perspective View Settings
In the View dialog box, on the Perspective sub-tab you can modify the settings of the perspective view.
Focal Length (mm)—You can use a lens to change the angle of the view. The focal length of the lens defines the angle of the view. As the focal length increases, the angle of the view becomes narrower and the model appears larger. The greater the focal length, the closer the model appears to the eye. The angle of the view refers to the visible expanse of the background, or the expanse of the scene that the lens covers. A wide angle lens makes more of the background visible, while a telephoto lens makes the angle of view narrower, and less of the background is visible. You can choose a lens with one of the predefined focal lengths or usrede a custom focal length to change the angle of the view.
Maintain Field of View—You can click this check box to maintain the field of model while changing the focal length. When this check box is selected, any modification in the focal length changes the eye distance as well to maintain the field of view.
Eye Distance—Eye distance from the model determines the amount of perspective distortion. The eye distance value is a percentage value of the model’s diameter. For example, a value of 200 means the eye is 2 model diameters from the model center. The lesser the eye distance, the closer the model appears to the eye, and greater is the distortion of the model.
In the Perspective mode, you may find it difficult to view the model if the eye is close to the model. If the eye is very close to the model and view angle is large with a high zoom value, the model can become so distorted that it may not be recognizable. Surfaces may also not be rendered correctly if the model is complex and contains many surfaces that are close to each other.
Depth of Field—You can modify the depth of a field by adjusting the Focus and Blur. The Focus slider allows you to adjust distance between the eye point and the point at which the scene is in focus. The Blur slider allows you to determine how quickly the scene is blurred outside the focal plane.