<<Oh, yeah, working distance... will the M.Zuiko 90mm macro have internal focus?
<<So 2:1 magnification is 4x in 35mm terms?
Undoubtedly the internal focusing will result in focal length shortening which
leads to less light being lost relative to extension but loss of working
distance.
As chuck had mentioned I was trying to tease out the effect of the MFT crop
alone on working distance. So this pretends the lens is symmetrical and rather
like the OM 135 macro where all mag is achieved by extension, but it is still a
very valuable exercise to understand working distance. The crop does not
"really" affect magnification as that is only object and image size but as
mentioned even Oly surrendered on that and uses effective mag.
So effectively 1:1 mag on FF can be achieved by 1:2 on MFT and 1:2 on FF at 1:4
on MFT.
So M=F/(f-d) where F=focal length and d is the object distance to the lens (or
rear nodal point in more complex lens). The "d" reflects working distance
after subtracting out the lens length from rear nodal to front. This is easily
derived from the thin lens formula.
The tricksy thing is for a real image the Mag at 1:1 is -1. +1 is for a
virtual image.
So at Mag -1, d=2f or twice the focal length. (the 4f in you link reflects
object to image distance, not to the lens)
Solving at -1/2 mag (1:2) d=3f
Solving at -1/4 mag (1:4) d=5f
Soooo, at 1:1 on FF (full frame) the MFT only needs 1:2 with the crop. So for
the new 90mm macro on MFT d=3*90=270mm. That is like using a 270/2 or 135 mm
lens on FF-- mag=1.
Let's do that at 1:2 on FF equivalent to to 1:4 (-1/4) on MFT. At 1:4 on MFT,
d=5f or 450mm. So on FF need 1:2 mag where d=3f so the FF equivalent lens is
150 mm.
So you see the advantage of MFT at any given mag for working distance
diminishes as the mag increases. Still the working distance of a 135mm lens is
not terrible on FF but the issue is the FL shortening used.
At 2:1 FF the equivalent working distance lens is 120mm on MFT for 1:1. I did
not show the math for that.
Hope that is clearer, Mike
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