Long image distance requires long focal length which leads to tiny FOV

[js05]

Hi there

I need to build an optical system that captures two pictures of a scene simultaneously using two CCD cameras. It are requirements that the two pictures are taken from exactly the same viewpoint but differ in blur through varying image distance (distance from lens to image plane).

I achieve this by using a cubical beam splitter that splits the light rays coming from a lens onto the two cameras. I position one camera a bit further away from the beam splitter and thus obtain the desired difference in blur.

So far so good, but the problem now is: The beam splitter increases the image distance by nearly 51mm which results in a total image distance of approximately 94mm. Which means that I need a lens having a focal length of 85-90mm for a working distance of ~1000mm. As the CCD sensor chips of my cameras are 3.2mm x 2.4mm in size the horizontal angle of view calculated by 2*arctan(3.2/(2*85)) is 2 degrees only. The horizontal field of view at a working distance of 1000mm is 73mm only.

Is there a way to increase the angle of view? I know that my problem is the long image distance but I just don't know how to reduce it.

Thanks for any help.
Jonas

 

[drawoh]

js05,

Why do the cube beamsplitters increase your focal distance? Select the focal length you want. Position the cube as close to your cameras as possible, and select the smallest cube you can. I find it hard to believe you need 51mm space for your beamsplitters.

Alternately, get a big cube beamsplitter and two lenses. Place the beamsplitter ahead of the lenses. This might be a good idea anyway. If you shoot through a fast lens and a cube, you are going to get a prism effect. You want light passing through a cube to be approximately collimated.

If your system is not moving or vibrating, there is such a thing as a membrane beamsplitter. If you can tolerate a slight ghost image, you can use a regular beamsplitter. You will have to account for refraction through the beamsplitter.

JHG