calculating MFD with extension tubes?

[GooniesNeverSayDie11]

Does anyone know of a simple way to determine your new MFD [ minimum focusing distance ] if you add extension tubes? Like lets say I have a 5ft MFD on the prime lens and I add a 12mm tube. What would my new MFD be? Thanks.

 

[Overread35]

This should have the answers you're after:
http://www.thephotoforum.com/forum/...ication-extension-tubes-well-other-maths.html

 

[GooniesNeverSayDie11]

Thanks Overread.