Re: Realist vs. 4"x5"

[P3D John Bercovitz <bercov@xxxxxxxxxx>]

One always thinks of a few more things to say later.  I should 
have said that for a lens, ideally you'd have an MTF curve for 
each f/stop.  That way you can tell if a lens is doing better at 
f/8 than at f/5.6.  Hey!  A new use for 3D: The 3D MTF curve!  
Axes are transfer, distance off the optical axis, and f number. 
Of course you need a 3-space straightedge to go with it to read 
the graph.  8-)  Seriously, if I'm not boring you, a friend of 
mine did do this for the interaction of the solar wind and the 
solar magnetic field and it's a wonderful pair.  I'll modestly 
mention that I suggested it to him when I saw his flat pictures 
and could make no sense of them.  Subsequently he bought a pair of 
projectors, a silver screen, and throw away glasses, and showed 
the pairs at several conferences.  But I digress.
 
I should also mention that under certain circumstances you can 
have a transfer factor greater than 100%.  It's easy to see why 
in film with its somewhat non-linear response to light.  The same 
thing can also happen in lenses, though.
 
Lastly, now that I'm home, I looked up the films I use.  This data 
is freely available from the Great Yellow Father.  I'll quote you 
a couple of points off each chart.
 
Kodachrome 25, 64, 200
100%+ out to 20 cycles/mm
30% at 50 ~/mm
(The difference in the K-chromes is primarily in their grain.)
 
Vericolor III   (VPS)
100%+ out to 15 ~/mm
50% at 50 ~/mm
VPS is rated ASA 160 but I shoot it at ASA 100.  I don't know what 
that does to the MTF curve.
 
John B
 


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