>>> i'm trying to render a simple scene using a cylindrical camera like the 
>>> following one. It uses the cylinder type 1 (vertical cylinder with fixed 
>>> viewpoint) and a full 360° viewing angle.....
>>
>> Have you tried to translate the camera up or down?
>>
> I did, but this translates the whole camera (viewpoint and image plane). 
> What I need is to keep the cylindrical image plane in its position and 
> change the viewpoint's relative position.

Have you tried playing around with camera type cylinder 3 (vertical 
cylinder, viewpoint moves along the cylinder's axis)?

plane {y,0
  pigment {checker}
  finish {ambient 1}
  scale 0.5
}
camera {
   cylinder 3
   angle 360
   up y
   location <0,0.5,0>
   look_at  <0,0.4,1>
}

The docs seem a little light on an explanation of this and I don't know if 
this is exactly the sort of distortion you're trying to achieve.

If this fails, you may be able to achieve what you want in two passes:
If you are trying to map the scene to a cylinder and be able to place the 
camera so that it's looking down through the cylinder, you could first 
render the scene using a cylinder 1 camera, then use that image as an 
image_map that you map to a cylinder (ambient 1) and render the result. 
You'll then be able to place the camera wherever you want relative to the 
cylinder.
Likewise, if you want to just add a perspective distortion to the result you 
could map the image from the first render to a plane and move the camera 
around for the second render.

Otherwise, a few more words describing what you're trying to achieve would 
be helpful, or a link to an image that uses the sort of projection you seek.

Regards,
Chris B.

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