From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


Actually what you measured is the Back Focal Length which is different than
the Focal Length.
Take and put the lens on a rotating table so that you can look through it
and see the field without having to use a EP to see something.  You may use
a small telescope that doesn't move to inspect the image if so desired but
the telescope can't move with the lens under investigation. For the test,
rotate the lens back and forth by about 5-10 degrees and inspect the image
and move the center of the lens forward or back until the image doesn't
move back and forth as you rotate the lens.  The center of rotation is the
center of the lens and is where the Focal Length is measured from.  I think
that you will find that the FL of the lens is going to be a bit longer than
that 30mm that you measured the BFL to. Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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