Starguide - A Practical Guide to Astro-Photography

It is important that is always slightly larger than the actual focal length of the projection eyepiece and can also be varied within narrow limits, which then has a very large influence on the image distance . 

In the extreme case - when approaches , i.e. the projection eyepiece is used practically like a "visual" eyepiece (dashed lines), the image distance approaches infinity and with it the effective focal length of the entire system. In this way, the magnification of the entire system can be continuously varied as long as no vignetting (edge darkening) takes place.

see also : Practical Prime Focus Eyepiece Projection

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