I have downloaded the demo version of Image Modeler. I thought that I should begin with a simple object and took four shots from four different angles of a cornflakes box - just like the images of the IM box in the user guide. I loaded all four images into IM and positioned markers in all four views where applicable. When I got close to 100 markers and still didn´t get any camera calibration I gave up. Then I put markers in just two of the images and after just eight (the minimum due to the docs) I got a very decent calibration. Then I thought that I just had to add markers in the other shots as well afterwards but no - the new markers won´t get included into the solution. The new markers are greyed out in the scene browser even if I manually press the calibration button. The modeling tools refuse to lock on these markers. How do you use more than two shots when modeling an object? It is possible, right?
 
EDIT: Okay, found out the reason - it´s definately the lens distortion. No matter how many markers or constraints I add, nor how much I tweak the camera values or the calibration settings - the images are useless and won´t work. I modeled a box in a 3d package and rendered out 4 views corresponding to my photographs of the cornflakes box. IM was able to calibrate almost perfectly with a minimum of markers. I wish it was that easy in real life...

Hello,
 
  I don't remember how the demo version is set by default, but distortion can be estimated by the engine.
Check the camera device associated to the images, and "free" the distortion parameter:
- "known" + value, the distortion is known and set to the specified value
- "constant", the distortion will be estimated and is supposed constant for all images that shares this device.
- "variable", the distortion will be computed and is variable  
 
Generally I set distortion to "constant" when I know that all shots have the same focal length,  
"variable" if not, and "known" with value 0.16 for my camera as I pre-calibrated it with a simple project.
 
Best,
 
Stef

The following comes from my experience in doing photosurveys of very large structures (such as stadium interiors) where my distance to subject can vary from a 10 or 20 to 150 meters or more.
 
I find that with most lenses, even very high end wide primes, that your distortion will be variable if your focus depth is changing notably between shots.
 
I only set the distortion level for "constant" when I know the shots have both the same focal length and the same focus depth.  For many applications (say, multiple views of a cereal box) this focus depth difference would be irrelevant and thus "constant" would be perfectly fine.

I am getting the exact same problem, using the FULL version of ImageModeler 4, and I have my camera device set to Variable focal length and Variable distortion.  I set up my initial set of markers on 3 images, and they calibrated great.  But any additional markers I place just stay gray, as the original post describes.  Freeing the camera parameters does not help.  I can't get any more images to calibrate!  It's as if the program is somehow satisfied with its job on the first three images, and doesn't want to play with me anymore.