Changing focal lengths

[P3D Sam Smith <sam@xxxxxxxxxxxx>]

I'm still a little unclear regarding a post a while back about changing the
focal length of an unmatched pair of lenses by shimming the distance between
the first two elements of a triplet. Does front cell focusing only move the
front optic, or does it move the first two? How will this conversion effect
focus? How far can you go with this before image quality suffers? I've lost
the original post, so forgive me if I've misquoted.
If this does indeed work, I'd appreciate a more detailed description as to
how you accomplish this. Is it better to lengthen or shorten the focal
length? Does the shimming increase or shorten the focal length? If the
problem is simply a case of misalignment instead of focal length, how would
you determine this?

Many thanks,

Sam


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