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Re: [OM] external metering with extension tubes

Subject: Re: [OM] external metering with extension tubes
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
Date: Wed, 30 Jul 2003 22:37:55 -0500
At 07:33 PM 7/30/03, Lama wrote (in part):

If FL= focal length in mm, and E is extension in mm, and FF is the f-stop factor, here's the formula:

FF = (FL + E)/FL

Note:
This presumes focus setting on lens focus helical is at "infinity." If it's focused closer than that, the lens is extended further by its internal helical, in addition to the extension tube, and that extension must be added as well. The equation I use is similar, but also takes into account extension of the lens with its own focusing helical, FE:

FF = (FL + LE + E)/FL

-- John


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