The unit settings of a model can often get you into trouble. Understanding some of the common problems can help you avoid them. This section addresses the two most common problems that modelers encounter.
This situation can happen when you model things on a very tiny scale and then have to get very close to them in a Camera or Perspective viewport. Architectural walkthrough animations are notorious for this kind of behavior. You've got a camera moving along a path and at some point it gets too close to a wall and you're suddenly able to see through to a room on the other side.
You can turn on the manual viewport clipping and adjust it to see the entire object. Viewport clipping has a Near and Far range setting, if a camera gets closer to an object than the Near Clip value, you will see through that object. Likewise, objects that are located beyond the Far Clip value, will be invisible to the camera.
If you need to work in real world units, such as inches or meters, you should set the scale of the scene before you begin modeling by changing the System Unit Scale value from the default of 1 unit=1 inch to something like 1 unit=0.1 inch or even 0.01 inch.
If zooming and panning are too fast or too slow, the most likely cause is the System Unit Scale. 3ds Max can exhibit round-off errors when dealing with extremely large or small distances. These round-off errors can also cause normals to be flipped or strange viewport clipping. 3ds Max does not have the numerical resolution to zoom infinitely from the some remote corner of the solar system down to an ant on your doorstep.
If you're going to change the System Unit Scale, you should change it before beginning any modeling. If you do have to set it later, it's best to rescale the entire scene with Rescale World Units. For example, if working on a tiny scale, like modeling coins, you might change the System Unit Scale from the default of 1 unit=1 inch to something like 1 unit=0.1 or 0.01 inch. For larger scaled scenes, like an airport, increase the System Unit Scale.
As a rule of thumb, keep the scale such that the smallest detail is not less than one generic unit. If this makes the scene too big to work with comfortably and efficiently, you can create separate scenes for models that include cameras for "close" and "far" shots.