Okay, my problem solved. You can access the inspector of ARCamera->Camera->Clipping Planes to set near and far. 

Hi, 

Facing similar problem. Anyone has any solution yet?

Here is a post in 2012. https://developer.vuforia.com/forum/qcar-api/when-i-zoom-out-3d-object-disappear

Being a newbie, I don't know if it works anymore or how Render:setARProjection should be used.

Can anyone help? thanks!

Hi
I have this problem too and I can't increase my image target size and can't use multi targets too. You wrote about a way to employ dead reckoning to ... . can you explain it? thank you.

Have you evaluated the use of a MultiTarget? This would enable you to define a target that has elements both on the wall and floor, and could support a greater range of perspectives.

My advice is to use a larger target image if at all possible, perhaps by combining the A4 prints. As a rule of thumb, the maximum reliable tracking distance is approximately 4 to 5 times the diagonal length of the target image. Also tracking quality tends to degrade as you move the model further from the center of the target - so having the model positioned high above the target may be a problem.

In these cases, it's best to experiment to determine what size and configuration of targets works best.

Any idea on how to do that ? 

Ok, thanks for confirming that. 

Well, my app is supposed to show suspended-chimney in rooms so the client can see how it'll look like before buying one. 

For now I was fixing the image target on a wall and then rendering the object between the target and me.

What I was thinking is to put the target on the ground, behind the place where the chimney will be suspended and then once the object is renderred, forget about the image target and let the object in the space so I can move anywhere with my iPad and see it even if the image target is out of the camera. Is that what you're calling dead reckoning ? If yes I think this is what I need !

Do you have any advices on how to do that ? Or any code samples ? 

(I think you've already realized but I'm not english so.. sorry if I'm not always understandable)

You're necessarily working w/ a limited distance then.

What are your requirements? Can you use the target to identify the context and then employ dead reckoning to infer an approximate position based on accelerometer and compass data?

That's in fact a part of my problem, the target has to be a A4 paper, it can't be larger.

I found that topic this morning but wasn't really helping as I'm really far from the target :

https://developer.vuforia.com/forum/unity-3-extension-technical-discussion/filter-camera-position-and-rotation-avoid-jitter

You'll most likely need a larger target, or to use a flat MultiTarget. The shaking indicates that the apparent features of the target aren't sufficient to stabilize tracking at those distances.