How to shift a focal point af an objective

[martim]

Hi, i have a simple optical system composed of:

1. objective with 50 mm focal lenght

2. CCD camera that can not be mounted behind the objective, for mechanical reasons... it can only be mounted 1-2 cm far behind the objective.

So i need to "translate" the focal point of the objective 1-2 cm behind the ojcetive original focal point.

How can i do this?? Have i two use one or at least two lenses?

Thanks in advance!

 

[schip666!]

You can add a negative lens in "series" to increase the focal length. See http://en.wikipedia.org/wiki/Lens_(optics ) "compound lenses". But this will change the image coverage, just like putting a telephoto lens on your camera, and -- maybe you don't care -- likely reduce the sharpness a bit.

 

[martim]

Hi, thanks for your answer... a reduction of the sharpness is not a problem...

A question: why a negative lens? I though about two solutions (but maybe the are not the best choice...)

1. a positive lens (diam. 6 - 10 mm) with a focal as shot as possible... few millimeters, posizioned symmetrycally between the focus of the objective and the CCD... to maintain the 1:1 magnification

2. a system comped by two piano convex lenses

what is, in your opinion, the best solution?

bye!