Bob Talbot wrote:
>
> Severi
> > What am I missing:
> What are we missing?
>
> > The magnification of Canon's EF 2.8/100mm macro at 31cm (the closest
> focus
> > distance) is 0.48, or 1:2.1
> No, it is 1:1
>
> Where did you get the 1:2.1 from?
> Out of a book or did you calculate it yourself?
> If you calculated it, what focal length did you use?
> Where did you find the internodal distance (D) information?
>
> (Remember, 31cm if that is a correct quote is from the film plane. It
> includes U, V and a bit "D"in between ...)
I suspect he used a formula that needs the distance from the front nodal
point and inserted f=100mm and distance=0.31m. This would give a
maginfication of 1:2.1 (=100mm / (310mm-100mm) = f/(d-f) )
So the error is twofold:
1) The focal length is nowhere near 100mm at the closest focusing distance,
it's much smaller, maybe around 70-75 mm.
2) The 31 cm of closest focusing distance in the specs are measured
*from the film plane*, not from the front nodal point, as you already
said.
Thomas Bantel
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