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