# How does an afocal lens produce an image on a screen?

I'm really confused about it. If such a lens is necessarily focused at infinity and does not have a plan of focus, then, does it mean it can produce a clear picture on a screen, no matter how close it is to the lens ? Eg., I want to build a telephoto with two positive meniscus (which creates an afocal system), each one having 4000mm of focal length. Then, I would have two questions:

1. How should I calculate what is the resulting focal length ? Would it be halved as it is expected in any focal system with two identical positive lenses ? Like this:

F = (f1 * f2) / (f1 + f2 -d)

Where f1 and f2 are the two lenses' respective focal lengths and D is the space separating them.

1. At what distance should I put my screen from the lens in order to produce a clear picture ? Would there be a certain back focal length ?

Sorry if I can be unclear. I'm an amateur that got really interested in the optics of lenses, and english isn't my native language.