Originally posted by Stiv ...

The only thing I'm not sure of is how you worked out the 1:1 working distance. Isn't 208mm the working distance for x1.67 not x1.0?

Thanks for pointing that out. It was a misleading, big error on my part.

Actually, the 208mm working distance I quoted is what one would expect when a 208mm lens is added to an existing lens

* focused at infinity*.

In general, when a lens of focal length f' is added to a lens with focal length f"

*already focused at infinity*, the "new" lens has a magnification of m'=f"/f' and a working distance of f'.

When this "new" lens is extended to increase the magnification, the actual working distance decreases, so an

*add-on close-up lens' focal length is the ***maximum** expected working distance.
A useful formula describing the ratio of working distances when a lens is extended to magnification m from m' is:

w/w' = (1+1/m)/(1+1/m')

For example, when a 208mm Raynox 150 is added to your 70mm macro lens focused at infinity, the magnification is 70/208 ~ .33x and the working distance is about 208mm.

When the 70mm lens is fully extended (to 1x) and the close-up lens is added, the magnification is (1+1)(1+.33)-1=1.66x and the ratio of the working distances is:

w/w'=(1+1/1.66)/(1+1/.33)=0.4

Wow! I was way off in my earlier statements about working distance with the combined lens at a total mag of 1:1, which would be

w/w'=(1+1/1)/(1+1/.33) = 0.5 or a working distance of 104mm,

**less** than that of the 70mm at 1:1 (140mm).

I apologize for my error and hope I've not lead too many people astray,

Dave

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Last edited by newarts; 01-05-2011 at 07:40 AM.
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